May 1, 2012

Simple Delay Timer Circuits Explained

In many electronic circuit applications a delay of a few seconds or minutes becomes a crucial requirement for ensuring correct operation of the circuit. Without the specified delay the circuit could malfunction or even get damaged.

Here we discuss how we can make simple delay timers using very ordinary components like transistors, capacitors and diodes.

Let's analyze the various configurations in details.

The first circuit diagram shows how a transistors and a few other passive components may be connected for acquiring the intended delay timing outputs.

The transistor has been provided with the usual base resistor for the current limiting functions.

A LED which is used here just indication purposes behaves like the collector load of the circuit.

A capacitor, which is the crucial part of the circuit gets the specific position in the circuit, we can see that it's been placed at the other end of the base resistor and not directly to the base of the transistor.

A push button is used to initiate the circuit.

On depressing the button momentarily, a positive voltage from the supply line enters the base resistor and switches ON the transistor and subsequently the LED.

However in the course of the above action, the capacitor also gets charged fully.

On releasing the push button, though the power to the base gets disconnected, the transistor continues to conduct with the aid of the stored energy in the capacitor which now starts discharging its stored charge via the transistor.

The LED also stays switched ON until the capacitor gets fully discharged.

Te value of the capacitor determines the time delay or for how long the transistor stays in the conducting mode.

Along with the capacitor, the value of the base resistor also plays an important role in determining the timing for which the transistor remains switched ON after the push button is released.

However the circuit using just one transistor will be able to produce time delays which may range only for a few seconds.

By adding one more transistor stage (next figure) the above time delay range can be increased significantly.

The addition of another transistor stage increases the sensitivity of the circuit, which enables the use of larger values of the timing resistor thereby enhancing the time delay range of the circuit.

The following circuit shows how the associated push button may be rendered inactive as soon as it's pressed and while the delay timer is in the activated state.

During this time any further pressing of the push button has no impact on the timer as long as the output is active or until the timer has finished its delay operation.








Two Step Sequential Timer

The above circuit can be modified to produce a two step sequential delay generator. This circuit was requested by one of the avid readers of this blog, Mr.Marco.




A simple delay OFF alarm circuit is shown in the following diagram. 

The circuit was requested by Dmats.

The following circuit was requested by Fastshack3

"I am looking to build a circuit that would control an output relay. This would be done in 12V and the sequence will be initiated by a manual switch. I will need an adjustable time delay (possibly displayed time) after the switch is released, then the output would go on for an adjustable time (also possibly displayed) before shutting off. The sequence would not restart until the button was pressed and released again. The time after the button release would be from 250 milliseconds to 5 seconds. The "on" time for the output to turn on the relay would be from 500 milliseconds to 30 seconds. Let me know if you can offer any insight. Thanks!"


217 comments:

  1. Good day, can please make me a circuit similar to above but in reverse. A circuit that will switch off the relay in 2 minutes after giving a power.

    thanks

    ReplyDelete
    Replies
    1. Good day to you, just remove the capacitor from its position and put it in place of the switch, positive will to the positive, and the negative will go the resistor 2m2.

      Delete
    2. Good day, I think it can be done by replace NC and NO wiring on the relay.

      Delete
    3. Hello,

      I have a similar requirement, but I need to reset a 5V touch alarm (by cutting power to it just momentarily) after it trips and starts buzzing a piezo buzzer for about five seconds. Thanks very much!

      Delete
    4. ...the solution is also the same as explained by me in the above comment.

      Delete
    5. Regarding the Simple Delay Timer Circuit. I'm new, can I simply remove the push button to allow the circuit to run on its own?

      Delete
    6. you mean, delay on power switch ON?

      for that you will need to replace the switch points with the emitter/collector of a BC557 transistor, then connect a 0.22uF capacitor across its base and ground line, and make sure to connect two 10k resistors on either side of the 0.22uF capacitor with their free ends connecting with the positive line.

      Delete
    7. I went to Radio Shack and bought some parts, a breadboard, resistors, transistors, a 555 timer, etc. I am studying now, and will need time to understand your advice. I almost get it. Can you email me at hotmail, celso7?
      Thank you for your time. This is for my sons school project. He is doing the artwork part.
      Celso Gutierrez

      Delete
    8. if you can interpret my advise through a drawing then I could check it and suggest you the needful.

      you could send it to my email ID which is provided in my "contact" page.

      Delete
    9. Sorry Swagatam I didn't understand your answer to my question above about the 5V alarm circuit. I have a signal coming from an alarm to a buzzer. I want the buzzer to go for 5-10 seconds, then momentarily cut off power to the alarm unit so it resets. Since there is continuous power to the buzzer I don't know how to cut power to the alarm after the delay.

      Delete
    10. Hi Glen, you can use an IC 555 monostable between the signal and the buzzer. This can be set for triggering for the desired length of time....but I could not understand why the alarm needs to be reset? It should have its own resetting system right?...what kind of alarm circuit are you using by the way?

      Delete
    11. The "alarm" is actually a capacitance touch board from Adafruit and I have attached it to a metal clock to be used in a sculpture. It goes off when anyone touches the clock, but at the moment it is locking on after the touch.
      I could combine your timer circuit with your transistor latch circuit and use the NC relay contacts to cut off power for a fraction of a second. But I wondered if there is a way to use a PNP transistor as an NC contact and when it is turned on after the delay it could reset the alarm? (The alarm takes very little power.)

      Delete
  2. success, adjusted some resistors too. thank you very much

    ReplyDelete
  3. hi,
    pls help me to make some modifications to this ckt. i used a relay in this ckt with relay drive trasistor and is working. i need this ckt as, Once the relay get on, it should remains ON condition even we off the push switch.ie, when we on the switch, relay should get ON after the set delay and later on ,relay should remains ON condition for ever until total supply is disconnected.and push swich should not have any effect after the initial ON of relay.hope my requirement is clear. kindly help with a simpe circuit.

    ReplyDelete
    Replies
    1. Hi,

      Though I couldn't clearly understand your question, I guess you are asking for a latch circuit.
      Connect a 100K resistor across the collector of BC557 and the base of BC547, remove the capacitor entirely, the latch is ready.

      Delete
    2. Hi, I'm from Viet Nam.
      Can you help me to to make some modifications to this circuit?
      Instead of use a push button, we use a normal switch. When we turn on the switch, relay is active. After 5s, relay get off and we turn off the switch.
      We can turn on/off the switch many time without affect to circuit.
      I think it's very interesting for you, so I hope you will help me, please. Thanks very much!
      My email: master_pro2510@yahoo.com
      P/s: Sorry, I have English trauma! My language is realy terrible!

      Delete
    3. Hi Hoang,

      I'll help you to make this circuit, in return you will also have to help me with a 300 word article.
      Explain the above requirement in your own words to me, I'll publish it as a new article with your name and also the required circuit diagram.

      You can correspond with me at hitman2008@live.in

      Regards.

      Delete
  4. Nice and useful circuit. How much time will pass before capacitor is discharged if resistor is 2m2? How much time is needed for capacitor to fully charge? If I add 12V relay at place of LED would I need to change BC557 for a more powerful one? (it would be also helpful to leave LED as indicator of circuit working)

    ReplyDelete
    Replies
    1. Thanks!
      Actually the formula is little complicated and therefore I did not mention it here, it's better and easier to check the results through practical experimentation.

      BC557 is OK for operating a relay.

      Delete
  5. Hello there,

    Many thanks for that. This could solve me a big problem. I do require some more help - I hope it's not too much trouble.

    1. I'm bound to a 1.5V DC power for various reasons. Would your circuit still work? If not, can you please suggest modifications so it does?

    2. I require a delay of half a second roughly. What values of R / C should I use for that?

    Kind regards,
    josef.

    ReplyDelete
    Replies
    1. Hello Josef,

      Yes the circuit will work with 1.5v supply.

      The first circuit will be OK, just reduce the base resistor of BC 547 to 1K. I am not sure about the capacitor which would produce 1/2 sec delay, you can try different values, start with a 0.1uF then either increase or decrease it, depending on the results.

      I wonder how you would measure the 1/2 sec timing.

      Regards.

      Delete
    2. Hello,

      Thanks a lot for your prompt response - I will certainly give it a try.

      I will not measure the 1/2 sec - I'm trying to start 5-6 clocks with a single switch but I want them not to tick at the same time but rather have "distributed" ticking sound. Therefore, for each clock I plan to add your circuit and change the capacitor (which from your answer I understand sets the time) so overall I will get a delay in the range of roughly 0.1-0.9 seconds (different for each clock).

      You suggested 0.1uF for 0.5 second delay. If I want to play in the range of 0.1-0.9 seconds, do you have a rough estimate of what range of capacitors I should play with?

      All the best,
      Josef.

      Delete
    3. The capacitor and the 33K resistor shown in the figure, both together decide the timing delay period, either increasing the capacitor or the resistor value will result in an increase in the delay period.

      Actually I don't know what value would produce a 1/2 sec delay, a 1K and 0.1uF will certainly not give you this delay, I just told it randomly.

      I think with a 1K, you would require a 1uF cap, which would probably give delay in seconds range, you will have to adjust either the resistor or the capacitor value for achieving the exact timing. You will have to find it by trial and error.

      Thanks and Regards

      Delete
    4. Dear
      Can you make a timer which on for 5 or 10second and then off for 5second and then repeat again on for 5 or 10 second and so on continuously until power cut off.

      Delete
  6. My situation might require a different type of circuit altogether, I need a timer that is going to turn a 12vdc fan on for about 3 minutes. Is this type of circuit practical for such a long period of time?

    ReplyDelete
    Replies
    1. Please refer to the following article, this would fulfill your need, with little modifications:

      http://homemadecircuitsandschematics.blogspot.in/2012/01/how-to-make-simple-timer-circuit-using.html

      Regards.

      Delete
  7. Hi. Very good article. I want to set up a time delay for turn on and practically no delay at turn-off to eliminate pops in the speakers. The system is running about 200watts of power. Will the above circuit with the one transistor work?? If so,do I need to use different value of components?? I want a 3 or 4 second turn on delay with no turn off delay.Any advice greatly appreciated....
    Cheers,
    Bernie

    ReplyDelete
    Replies
    1. Hi,
      This circuit will need to be changed significantly for the required application..it cannot be explained through words, I will have to draw it practically.
      I'll possibly do it soon and present it here.

      Regards.

      Delete
  8. Hi, this is an excellent article, thank you.

    I would like to use such a circuit to effectively 'hold down' a key on an older mobile telephone (a Nokia 3310) so I would solder two wires onto the key terminals (let's say it is the '2' key) then have these two wires go to the delay circuit where I can hard-wire another button.

    When this new button is pressed then the original button's circuit is closed for just over 1 second and this activates the phones speed dial for that number.

    Would you be able to give any advice as to how I would do this without blowing up the phone :)

    Also to be really clever it would be great if the pressing of this second hard-wired button actually energises the delay circuit so that it can sit there using zero power until use, then I could use the phone's battery to power it without the phone going flat all the time :)

    ReplyDelete
    Replies
    1. Hi Ian,

      Thanks, and welcome to my blog.

      Yes it's very much possible without blowing anything.

      You just have to connect two wires to the relevant cell phone key pads over the PCB of the cell phone very carefully using smd soldering iron.

      The other ends of these wires may now be just connected with the relay's pole and the N/O contact, polarity does not matter.

      The relay circuit will require an external 12V supply, it may be provided from a small battery.

      The relay circuit consumes current only until the relay is switched, the moment relay is deenergized, the current consumption become almost zero. The relay cannot be operated using the cell phone battery unless it's some kind of a specialized mini low voltage type relay;)

      Delete
  9. Thank you Swagatam!

    Do you think I might be able to use an optocoupler instead of a relay?

    If so then perhaps I might be able to use the first, simpler circuit and maybe power it from the phone?

    I agree that I will have to do some very careful soldering! :-/
    Hope I can do that without destroying the phone :)

    I'll keep you posted.

    ReplyDelete
    Replies
    1. Hi Ian,

      I don't think an opto coupler will work here, because a cell phone will require complete switching of its buttons for functioning, just as it is done through manual switching of the phone's mechanical keypads.
      Opto couplers incorporate semiconductor switches which involve polarity, therefore it might pose problems.
      So I think a relay would work better here.

      Regards.

      Delete
  10. Hello

    I think this is circuit I need for a project I am working on.
    Can you tell me if it would work using a relay instead of a switch all using a 9v battery.

    Thank you

    ReplyDelete
    Replies
    1. Yes it will work only if the relay switches momentarily.

      Delete
    2. Thank you for the quick reply
      Sorry I have one last question

      The relay I am using is 5v so I will now be using 4.5v on the circuit, will I need to change the values of the components if so do you what to or how to work it out

      Thank you for your help

      Delete
    3. I don't think any changes would be required, you may use the same circuit configuration with 4.5V also....

      Delete
  11. Hello! I need a circuit that will turn on an LED light 30 minutes after the switch/button is switched ON. can you explain this please?

    ReplyDelete
    Replies
    1. Hi,

      You can make the circuit shown in the following link:

      http://homemadecircuitsandschematics.blogspot.in/2012/01/how-to-make-simple-versatile-timer.html

      Just replace the buzzer with LEDs.

      Delete
  12. Hi! Excellent article! I think your first diagram can make my project works, however, is it possible to create a circuit such that when a 12VDC is supplied to the + pole, the motor would turn clockwise after a 2 sec delay and when vise versa a 12VDC is supplied to the - pole, the motor would turn anticlockwise after a 2 sec delay? Very much appreciated

    ReplyDelete
    Replies
    1. Thanks very much!

      I am afraid the above circuit cannot be applied as per your suggestion. I think your application will require a set/reset kind of circuit. I'll try to figure it out.

      Regards.

      Delete
    2. Very much appreciated! Thank you!

      Delete
  13. Gud afternoon sir, Excellent article...
    But i need to use this timer with dark sensor. i.e. when lights off, then this timer get activated for few minutes (Without using Relay).
    *Dark sensor is constructed based on 2NPN + LDR.
    Could you please suggest me how to link with Dark sensor...?
    Plz sir...
    Thanks in Advance...

    ReplyDelete
    Replies
    1. Thanks!

      Alight sensor circuit is shown here:

      http://homemadecircuitsandschematics.blogspot.in/2012/01/how-to-make-light-activated-day-night.html

      The delay circuits supply terminals should be connected at the points where the relay is connected in the light sensor circuit, and the relay should be removed as it's not required.

      In the delay circuit the switch should be replaced with a 0.22 capacitor and a resistor of 100K should be connected across the supply terminals.

      The above mods should make the entire circuit work as per your requirements.

      Delete
    2. Thank you sir, Thanks you very much.
      It is working with small modification.
      Sir, can u give me any software name to design circuits like above (for PCB also).
      Plz.....
      Thanks in advance...

      Delete
    3. Thanks Vinay,

      Actually I have never used any software for designing PCBs, so can't provide any useful info about them, you can probably Google a bit and you might find many of them online.

      Delete
    4. Sir, can u tell me how to flow limited current for mobile/something by using Resistors..
      I need 200mA - 800mA current. So, can u say few words about current limiting (When u r free. I can wait sir)
      Thanx in advance...

      Delete
    5. Hi Vinay,

      Please tell me about the exact application that you are looking for, I'll try to explain you then.

      Delete
    6. Sir, i'm planning to recharge my mobile (Nokia) with my bike battery in both conditions (Bike Engine is ON & OFF).
      By using 7805 IC, i'm unable to recharge my mobile,doesn't show any indication about charging. When using IC 7806, it shows "Charger not Supported".

      Can u help me sir...? (I can wait for long time sir.Plz)

      Delete
    7. One more question sir.
      When i'm using above 2Transistor circuit, It works fine in Breadboard, but on PCB, it's not working properly.I soldered components on PCB properly.I tried once again on bread board, it works. But on PCB it is not working properly.
      I think, the quality of PCB is poor.I'm using MATRIXBOARD & connections are done by soldering copper wire.
      NOTE: All connections on PCB are in correct(No doubt).
      Can u give me suggestion sir...?

      Delete
    8. Use a different battery and check the result, or use an AC DC adapter and the 7805 circuit and check at home with mains voltage, confirm if the circuit is OK or not...

      Delete
    9. There may be some kind of short circuit or leakage between the leads of the transistors, or else there cannot be any other reason for it not to work, provided the connections are absolutely correct.

      you may clean the PCB with thinner and check again...

      Delete
  14. Hi Sir Swagatam. This is really nice blog. May I please ask your help on my project. I want to have a time delaying circuit.

    Desired setup:
    After closing the circuit, the first LED light should switch on for few seconds then turns off. After few seconds, the second LED will turn on for few second then the switches off. The cycle DO not need to be repeated.

    ReplyDelete
    Replies
    1. Thanks very much Marcos!

      I'll try to update your request soon here, if I forget please do remind me....

      Delete
    2. Thanks Swagatam. This request comes from the Philippines. Actually, I've been searching this problem over the net for couple of month already but can't find anything specific. I even resorted to mechanical solutions already but not so much satisfied because I believe electronics is more efficient and effective and less bulky. Not until I came across with your blog. Though it is not yet exact as what Im longing to find but I have observed you're so accomodating and patient in answering queries as reflected in this long thread. Besides, this thread is active and updated. Im not so expert in electronics but an avid learner and trying to solve simple problems. Thanks so much. Im am so hopeful now and know now a site where to wait everyday. I will surely wait for your answer and be eager for it. Please take time and I wish to find answer now to my simple yet became serious desire.

      Mabuhay ka kaibigan!

      Delete
    3. Thank you very much Marcos! I appreciate your kind words.

      I've updated the required circuit, please check it.

      Delete
  15. Hi Swagatam,

    Glad to see today your quick response to my query. Thanks so much for your time and effort. I will make this and share you the results after.

    By the way, let me just clarify few things on the diagram:

    1. Is the capacitor value read as 33uf 25V?(cant figure out clearly:)
    2. The two vertical resistors (lowest in the diagram), are these 1K ohms too?
    3. Can I use a regular 12V adaptor as source?

    Thanks to your patience.

    Best regards,

    Marco Luis

    ReplyDelete
    Replies
    1. Thanks Marco!

      If you click the diagram, you would get an enlarged view of it.

      Yes they are 33uF/25V, the LED resistors can be 1K 1/4 watt.

      Any 12V DC supply can be used for this circuit.

      Thanks and Regards.

      Delete
  16. Hi Swagatam,
    Thanks for your time and energy for us non-professional electronics enthusiasts!

    My circuit sounds an alarm if the refrigerator door has been left open too long. My microswitch attached to the door makes the circuit 'open' when door is closed, and closes the circuit, providing VDD, when the door is open.

    Is there a version of your timer that will STOP counting in the event the refrigerator door is closed (VDD is removed from circuit) before the timer sets off the alarm? I assume most one-shot circuits involve charging a capacitor, in which case removing the VDD doesn't stop the timer....

    Thanks
    DM

    Thanks

    ReplyDelete
    Replies
    1. Hi DMat,

      You are most welcome!

      If the VDD is cut OFF everything immediately stops for the above circuits.

      So you can definitely employ the circuit shown in the above article for your application, the second one is the most appropriate.

      The alarm will never sound once the door is closed, because it will not be getting the supply voltage for generating the sound anyway.

      Regards.

      Delete
    2. I am sorry the second circuit is not the one for your application, I'll update the article soon with an appropriate one.

      Delete
    3. Thanks a lot, Swagatam. I'll give it a go!

      Regards,
      DMAT

      Delete
    4. My pleasure!

      I have made some changes in the circuit, please check it.

      Regards.

      Delete
  17. how can I make this circuit to delay for 3-6 hours
    ? thanks
    Ally From Tanzania

    ReplyDelete
    Replies
    1. This circuit is not suitable for long duration applications, you may try the following circuit:

      http://homemadecircuitsandschematics.blogspot.in/2012/01/how-to-make-simple-versatile-timer.html

      Delete
  18. Hi Swagatam,
    Looks like you know your stuff when it comes to these timer circuits, this one is a little out there but dont believe it is out of your knowledge.
    This is a replacement part for an old Bryant furnace 822 relay. What is needed is a circuit that will get a 24VAC supply when the thermostat kicks in, it will have to have a 45 second delay before triggering a relay that powers the 1/3HP blower motor, the motor needs to run for 45 seconds after the voltage is shut off via the thermostat. I'm sure that there is a more efficient circuit other than the 822 relay to do the job, especially when you take cost into the equation.

    doug@evlllc.com

    ReplyDelete
    Replies
    1. Thanks Doug,

      So it should be like this in short:

      thermostat kicks in ....>..... the timer starts counting and starts the motor after 45 seconds.....>....thermostat shuts off, but the timer keeps the motor ON for 45 seconds and then shuts it OFF.

      The process repeats after the motor halts....right?

      Delete
    2. Thank you for the quick reply, yes you have it correct. Once the thermostat kicks in it sends 24VAC thru the limit switch(as long as it's not tripped from an overheat), then thru the pilot lights thermo coupler (providing that the pilot is lit)then applies it to the timer/relay. Once the thermostat kicks out the voltage goes to zero across all components.
      Yes, the process would have to repeat each time the thermostat kicks the furnace on.
      I was orginaly looking at the 556 timer chip to see if it would be able to serve the dual delay, but looking to you for the best way to get it done.

      Delete
    3. OK, I'll try to design the circuit soon, when it's done will post it here for you to see.

      Regards.

      Delete
    4. Hi Doug,

      I have tried to design the circuit correctly, please see it here:

      http://homemadecircuitsandschematics.blogspot.in/2012/12/thermostat-delay-relay-timer-circuit.html

      Delete
  19. Hi Swagatam,

    great job :)

    I am looking for a circuit that turn off a LED after 20 minutes.
    can you please help me, I am a biginer :)

    thank you

    mihai

    ReplyDelete
    Replies
    1. Hi Mihai,

      Thank You!

      You can try the following circuit:

      http://homemadecircuitsandschematics.blogspot.in/2012/01/how-to-make-simple-versatile-timer.html

      Just replace the buzzer with a LED/1K series connection

      Delete
  20. Hi,

    Please help me enhance any existing circuit that is using any timer or ic. It is for my project and our professor wanted us to add some functions to any of it.

    Thanks,
    Maye

    ReplyDelete
    Replies
    1. you may try the following circuit:

      http://homemadecircuitsandschematics.blogspot.in/2012/01/how-to-make-simple-versatile-timer.html

      Delete
  21. Hello,

    Looking at the 2nd picture, is there a way to get the delay up to 1.5 minutes? I've constructed the thing, but I can't seem to find a good combination of resistor/capacitor to get it up to 1.5 minutes delay.

    Also, I assume that the effects are similar if used with a 9V instead of 12V battery? I'm currently running a 9V, since that is all I have access too.

    Help would be greatly appreciated :) Thanks.

    ReplyDelete
    Replies
    1. Hello,

      You can try different values for the 2m2 resistor or the 1000uF cap.

      Try replacing the 1000uF with a 10uF, it might produce closer results, then you can refine the outcome by tailoring the 2m2 resistor.

      Yes 9V can be used here.

      Regards.

      Delete
  22. Hello,

    For my fridge door alarm project, I need an LED to turn on when the fridge door is opened and after a delay of 60-90s a buzzer would sound. Can I use any of the above circuits?

    Regards,
    Hakeem

    ReplyDelete
    Replies
    1. hello,

      the following circuit would be appropriate, with some further modifications though:



      http://homemadecircuitsandschematics.blogspot.in/2012/01/how-to-make-simple-timer-circuit-using.html

      Delete
    2. Hi,

      One of the requirements is that I can't use integrated circuits, but can I use the third circuit?

      Delete
  23. Hello. I am a glass specialist, pretty good with DIY etc, but not too hot on electrics. I am planning a build for a 10mm Toughened Glass Watch display box, however our client would like this to be a watch winder box. I have established that we require 4 x 10rpm 24V AC Motors to run simultaneously. They need to start turning every hour, for 15-20 minutes. However, they need to turn clockwise then anti clockwise hour to hour. It would be good if I could also have a manual option to start and stop the motors when I wanted.
    Is this something someone can help with? Please? Thanks!Milton

    ReplyDelete
    Replies
    1. Thank you for the request, I'll try to do it soon and publish it in my blog.

      Delete
  24. Hello, I am a broiler poultry farmer. I have no electronic knowledge or skills. I am looking to make some kind of relay, switch, vibration sensor that will alarm when the feed lines run for a certain amount of time which indicates that I am out of feed. This is causing damage to my equipment. I need something that will trigger a timer to restart every time the automatic feed line comes on. Basically I need something that will alarm if the feed line runs over 20 minutes. Could you post a schematic for something like this?

    ReplyDelete
    Replies
    1. I'll try to post it in this blog...it might take a few days though....by the way how do the feed lines run, using DC motors or AC motors? because my circuit would need a trigger from it to begin the counting process so it's important to know regarding its voltage level..

      Delete
  25. how can i replace push button to simple ON/OFF switch.........
    plzz help me out

    ReplyDelete
    Replies
    1. what do you intend to make with this circuit?? the switch condition will depend on that.

      Delete
  26. I have constructed a simple 12 VDC circuit that includes a power supply and a linear actuator. The actuator has internal limit switches that open the circuit when it is fully extended or fully retracted. After one of the limit switches opens the circuit, the input polarity to the motor must be reversed to restart the motor to extend or retract. I need to modify the circuit so that when the 12 VDC power supply is energized the actuator will extend or retract, depending upon its position when the 12 VDC power was last de-energized. When a limit switch opens ( ie the actuator is fully extended or retracted), there needs to a time delay of 30 seconds to 3 minutes and then the polarity to the motor needs to be reversed to start the actuator. This process needs to be repeated without any manual input other than Initially energizing the 12 VDC power supply. Can this be accomplished with relays and timers or is a PLC the best method? Thanks for any advise you can offer.

    ReplyDelete
    Replies
    1. I guess your need matches the one explained in the following article:

      http://homemadecircuitsandschematics.blogspot.in/2013/06/programmable-bidirectional-motor-timer.html

      Though it's quite complicated, probably it's the only way of doing it through discrete components.

      Delete
  27. hi there, can u help me to construct a timer circuit for me?
    i use the timer to control the delay of the rotation of 3 stepper motors in sequence.
    which means when the first motor move back and forth for 180 degree,it will delayed for 2 seconds before the second motor starts to rotate,the same process goes to the third motor.and i need the motor to completely stop and reset so that when i pushed the push button ,the first motor will repeat the step ..

    ReplyDelete
    Replies
    1. The sequential timer can be surely designed, in fact I have already posted one such design in this blog, however I could not understand the connection between the motor rotation and the timing, if you could clarify that part then I could possibly suggest you an appropriate design.

      Delete
  28. Hi,

    Can your make me a circuit using a tilt switch instead of a push to on, a 3-16v buzzer and a key switch that has a 10 second delay

    Thanks a lot

    ReplyDelete
    Replies
    1. Replace the push-switch in the second circuit with the tilt switch terminals with a series 1uF/25V capacitor.

      Delete
  29. Hi guys!
    I need transistorized repeating timer, I need to have timer that will work without triggering switch but should work for two minutes and be off for two hours then after two hours repeat as first stage without pressing a switch again.
    Please any one can help me.

    ReplyDelete
    Replies
    1. I can provide a 4060 IC circuit, not a transistorized one.

      Delete
  30. Good day,
    I have a 24vdc door activation circuit that I need a 5 second timer for. When the push paddles is depressed, I need to activate the latch relay for 3-5 seconds (to hold the latch open long enough for the operator to activate). Can one of your 12v circuits be modified to work with the 24vdc power supply?

    ReplyDelete
    Replies
    1. Good day!
      you can try the second circuit presented in the above article, replace the LED/resistor with the latch relay with protection diode.
      The push button may be replaced with the paddle switch.

      Delete
    2. ...the 1000uF cap should be tried with different values ranging from 10uF to 100uF for acquiring the required delay.

      Delete
  31. Hi
    I am after a circuit which will turn a 12 V light on with a switch and then will turn it off a fixed amount of time (about 5 seconds) after a tilt switch has been tilted. It is a project to help out my cousin and I can't get it to work and I would be really grateful if you could help.
    Rich

    ReplyDelete
    Replies
    1. The 12V light should illuminate and timing initiated when the tilt switch is tilted the first time or after its tilted back to its original position??? please clarify this point.

      Delete
  32. I want to create a small, variable delay. Changing the resister on the transistor base will allow the delay to be adjusted?
    Using a 12v supply, can you recommend what resistance and capacitance values I should experiment with to induce a delay between 0.1 and 0.5 seconds?

    ReplyDelete
    Replies
    1. you can try the second circuit given in the above article, use a 1M pot in series with a 1K resistor at the base of the transistor and a 10uF capacitor

      Delete
  33. Hey everyone,
    I am looking for a time delay circuit that I can use for some drop photography. I am looking to conenct a light gate to it so that when a drop breaks the gate it starts the time delay circuit then triggers my flash gun. (flash gun has a 2 wire cable to connect to circuit). Does anyone know of a circuit which can help me? If you could include the light gate into the circuit would be a bonus.

    Thanks :)

    ReplyDelete
  34. Swagatam... i need circuit, that after power on the circuit , output should be 5 or 10 sec, delay...Please help.

    ReplyDelete
    Replies
    1. you can try this:

      http://homemadecircuitsandschematics.blogspot.in/2013/02/make-this-simple-delay-on-circuit.html

      Delete
  35. Good morning sir,
    I am totally confused with so many timer/delay ckts. Please refer me a ckt.
    as per my requirements.
    1) I want to start a 12v/500mA (max)
    d.c gear motor after 40-60sec of switching on and will run untill it is switched off.
    2) Another previously related problem.please refer any 12-0-12 a.c to 12v d.c rectifier ckt.with component
    list and diagram......thanks.....
    With regards,
    K. Kausik

    ReplyDelete
    Replies
    1. Good morning kaushik,

      you can try the following circuit, the values of C2, R2, D1 will decide the length of the ON time delay.

      http://homemadecircuitsandschematics.blogspot.in/2013/02/make-this-simple-delay-on-circuit.html

      please repeat your previous request, I am not able to remember it.

      Delete
  36. hi can you suggest to me a simple circuit with 15 components for me as an electronics student ............pls i know u can help me...

    ReplyDelete
    Replies
    1. hi, you can try the following design:

      http://homemadecircuitsandschematics.blogspot.in/2012/04/semi-automatic-water-level.html

      Delete
  37. i want to make ten LED light using time delay feature having fed one by one after specific time day.

    ReplyDelete
  38. Hi Swagatam,
    I wanted to make a timer circuit which cuts off the supply to a circuit after 5-6 seconds after being triggered , even when the trigger is still on

    ReplyDelete
    Replies
    1. Hi Shyam,

      you can try the following design:

      http://3.bp.blogspot.com/-FHAn8wSZwQ0/UrFIJJpYjgI/AAAAAAAAF7I/M8L5IqLxCwA/s1600/automatic+choke+solenoid+circuit.png

      ignore the relay contact capacitors and wiring set up, it was drawn for a different application.

      Delete
  39. Hi Swagatam,
    I Tried the circuit which you gave , but the relay never turns off... I have changed the capacitor values ,but also no change.... can you please tell me what could be the problem.

    ReplyDelete
    Replies
    1. Hi Shyam, I think I misread your requirement, the given circuit is good but is not applicable as per your need.

      I'll give you a suitable one but let me know whether it should be a relay operated or without a relay, meaning the current requirement of the circuit is relatively higher or nominal??

      Delete
  40. Hi Swagatam, I wish to operate a 12v relay with this circuit. the current requirement is nominal.

    ReplyDelete
    Replies
    1. Hi Shyam, I think the previous 555 based circuit which I recommended is perfect for your application and it'll surely work. Connect the relay between pin3 and ground, and make sure the line connecting pin4/8 of the IC gets the (+) 12V supply....remove the entire relay connections that's shown in the original diagram....your relay must be between pin3 and ground....let the 1N4007 diode be there with the coil, its anode will go to ground and cathode to pin3.

      I thought that you wanted the entire circuit to get cut off after 5/6 sec but later realized that you wanted to execute the cut for another circuit via this timer circuit, therefore the suggested 555 circuit is right for the application.

      Delete
    2. Dear Sir ,it still doesn't work because, as I had mentioned earlier the switch remains on . Only when I turn off the switch the relay gets off immediately,there is no 5 sec delay even then.

      Sorry Sir if I am bothering you too much..

      Delete
    3. Dear Shyam,

      It seems there's something seriously wrong in the connections or the IC may be faulty, because the circuit is extremely standard and straightforward and should start working immediately.
      pressing the push button should trigger the relay, and switch it off after the predetermined delay decided by the values of the 47K res and 10uF cap, irrespective of the switch position.

      http://3.bp.blogspot.com/-FHAn8wSZwQ0/UrFIJJpYjgI/AAAAAAAAF7I/M8L5IqLxCwA/s1600/automatic+choke+solenoid+circuit.png

      Delete
  41. Hi Swagatam

    Hoping you will be able to suggest a suitable circuit for a problem we are having. We need a circuit that can be fed via an engine ignition switch which will output a 12v feed to a solenoid for a short time, about a second, then power down once the feed from the ignition switch is turned off. Basically we have a piece of equipment that no longer turns off with the ignition switch, our current best option is to directly wire in a switch to the solenoid but if the circuit I describe is feasible then that would be a preferred option.
    Thanks in advance.

    ReplyDelete
    Replies
    1. Hi Hex,

      There are a few confusions:

      Do you want the solenoid to stay ON for 1 second after the ignition is turned OFF or should it be in response with the ignition switching, or is it regardless of the ignition ON time? Meaning once the ignition is switch ON, the solenoid will switch OFF after a second regardless of the ignition ON/OFF situation??

      What kind of solenoid are you using, a spring loaded one or the one which requires a push-pull opposite supplies for locking and unlocking.

      Delete
  42. hello sir . i need a similar circuit but with 1 hour delay. What changes should i do?
    Thankyou

    ReplyDelete
    Replies
    1. Priyanka, you can try the following design:

      http://homemadecircuitsandschematics.blogspot.in/2014/11/long-duration-timer-circuit-using.html

      the 1000uF/2m2 components will need to selected by some trial and error for achieving the desired delay response.

      Delete
  43. how would i do a circuit like this that turn off 5 seconds after being turned on with a 12v source?

    ReplyDelete
    Replies
    1. use the first circuit, connect the load or the circuit in place of the LED/resistor

      Delete
    2. Hi Swagatam

      I have a circuit what is exactly like the one in "simple delay timer circuit". The only minor difference is +5V, and the major one is that it has to be approximately 1 sec "Delay ON" and immediately off again.

      How would I do that?
      Thank you

      Delete
    3. Hi Detlef,

      In the first circuit, you can remove the 1000uF capacitor and replace the push switch points with a 100uF capacitor, now it'll behave in the way you have suggested, you may need to tweak the value of the 33k resistor in order to achieve the required 1 sec timing.

      Delete
  44. Hai, I need to place three LEDs L1, L2, L3 in a single circuit. So when I power ON the circuit means, the LED L1 should glow after 30 mins and the LED L2 should glow after 45 mins and the LED L3 should glow after 60 mins. Could u pls provide a circuit for that???

    ReplyDelete
    Replies
    1. Hai, you can use a IC 555 and IC 4017 "chaser" circuit, set the 555 as a 15 minute timer astable and then use the outputs of the 4017 IC for getting the required sequences.

      for 30 minutes you can just skip one output of the 4017.

      make sure to connect pin15 with pin10 of the IC 4017

      Delete
  45. Thanx. I tried an 555 astable calculator to obtain 1 sec time high and 15 mins time low. But not able to obtain a correct value for R1, R2, C. Could u pls help me by giving the value for R1, R2 and C for the 555 astable circuit for 1 sec time high and 15 mins time low? Thanx...

    ReplyDelete
    Replies
    1. 15 minutes high, and 1 sec low will also do.

      R1 = 1.3 Meg

      R2 = 1.5K

      C = 1000uF

      Delete
  46. Hi, Swagatam,

    i need a delayed off timer working on 3v dc power supply , kindly share your idea if you have.

    Best Regards,

    Obaid

    ReplyDelete
    Replies
    1. Hi Obaidullah, you can try the second or the third design from the above article

      Delete
  47. Dear Sir Please help me ....

    I Want A Circuit Wich can Relay On 6v For 20 Second from get A Puch....


    After 20 Second It Will Automatically Goes off

    Again push then get on for 20 second

    Thanx Sir

    ReplyDelete
    Replies
    1. Dear Sharoj,

      You can make the first circuit from the above article, just replace the LED assembly with your relay

      Delete
  48. Hello,
    Do you know if it is possible to use instead a 12V supply a 9V supply?
    Thanks

    ReplyDelete
    Replies
    1. Hi, for the shown circuits you can use any voltage between 3V and 24V

      Delete
  49. Hi, i need a circuit that has 1 push to make switch but can control 2 things (in this case a water pump and fuel injector) i want the water pump to be going before the injector does its thing to build up enough pressure in the hose and all this needs to work on 12V. Can you help?

    ReplyDelete
    Replies
    1. Hi you can use the second circuit from top and do the following modifications:

      replace the LED/resistor with a relay coil (connect a diode across the coil).

      Now connect the pole of the relay with the 12v positive, N/O with (+) of the water pump and the N/C with the (+) of the injector...the (-) wires of the two units may be joined together and connected with the circuit negative.

      The above mods will allow you to implement the actions as specified by you.....reduce the 1000uF value for setting the required time delay between the pump and the injector activation.

      Delete
  50. hi there, i need a 15v circuit with a push-button that when i press and hold for 3 seconds then release the circuit LED will remain on for 10-15 seconds without dimming effects. Is it possible? Thanks for your help.

    ReplyDelete
    Replies
    1. hi,

      the first circuit in the above article will fulfill your requirement, but you won't need 3 seconds to initiate it, just a fraction of a second press will be enough to trigger the LED for the desired length of time...

      Delete
    2. hi mr swagatam,

      is it possible i add one more LED to the circuit but it will delay on around 2seconds after the 1st LED light up and both LED will off at the same time.
      Thanks for your help.

      Delete
    3. Hi Babusan,

      I will have to figure it out, can't able to simulate it quickly...if it's possible I'll produce it in the above article soon...

      Delete
    4. hi swagatam,

      sure, hope to hear from you soon. Thank you in advance.

      Delete
  51. This comment has been removed by the author.

    ReplyDelete
  52. This comment has been removed by the author.

    ReplyDelete
  53. Hi Swagatam,

    I am impressed with the simple design you manage to came out to create a time delay without using the common 555 timer IC which most users would have used.

    I am interested in the formulas on how did you manage to determine the values of the resistors and capacitors and I believe one does not simply just random pick the values and try it out to determine it. It would be great if you can provide me a rough explanation on the formulas part and also how does the circuitry works

    For instance, in the section where you mentioned about adding an additional transistor to the circuit would increase the time delay but why and how does it affect the time delay? Does it have anything to do with the transistor switching on/off and meanwhile does it mean the more transistors the circuit has, the longer the delay it can provide?

    Hope to hear from you soon. Thanks:)

    ReplyDelete
    Replies
    1. Thanks John,

      Definitely there should be a universal formula for determining the RC timing values, however determining the same using a practical trial and error method is much simpler than solving the formula.

      If you are interested to know regarding the formulas, you can refer to the following wikipedia article:

      https://en.wikipedia.org/wiki/RC_time_constant

      By adding more transistors we are making the circuit more sensitive to the voltage rise across the charging capacitor...so if one transistor requires 0.6V to trigger, with two transistors we could reduce it to 0.3V with three transistors it can become more sensitive and trigger at 0.1V..this improvement allows smaller capacitors to be used and generate longer time delays...however making the circuit more sensitive can make it prone to false triggering from external spurious signals, that's the drawback, although this can be tackled by adding caps across the base/emitters of the transistors.

      Delete
    2. Hi Swagatam,

      Thanks for the link and explaination on how the side effects it will cause for having more transistors and it really give me a better idea of how the circuit work out. Currently I have another quiry which is why when do you use a pnp transistor and connect in that format instead of using the NPN transistor and connect back the similar connection like in the first circuit?

      Does it have any differences? (like lesser power consumption? neater? lesser voltage required?)

      Hope to hear from you soon:)

      Delete
    3. Hi John, the PNP transistors activates in response to a negative (-) pulse at its base...the in the discussed design the NPN is supposed to produce a negative pulse when activated which is intended to be responded by the next stage, and therefore a PNP stage is used in that position.

      together they make the circuit more sensitive than a single NPN

      Delete
  54. Hi,
    I need a circuit which has two out on and off.
    I mean one on and the other off and reverse with transistor or mosfet.

    Thanks

    ReplyDelete
  55. Hi,
    I need a circuit which has two out on and off.
    I mean one on and the other off and reverse with transistor or mosfet.
    So i can pulse it with 555.
    Thanks

    ReplyDelete
    Replies
    1. you can use a 4047 IC...a 555 will not be needed as it has it's own oscillator.

      Delete
  56. Hey, I need a circuit that can delay a signal output to a electric motor. I would be using a 9v battery to
    power the circuit and a contact switch to turn it of and on. I need the motor to turn on about 0.5s after the contact switch is pressed and to stay on until the contact switch is no longer pressed.

    ReplyDelete
    Replies
    1. you can use the following circuit:

      http://www.homemade-circuits.com/2013/02/make-this-simple-delay-on-circuit.html

      adjust C2/R2 appropriately for the intended delay period....you can eliminate the emitter zener diode D1, since it's required only for longer delays.

      Delete
  57. Hi!
    I need a circuit with auto stop when the dc motor stops.

    ReplyDelete
  58. Thank you so much for the article! I am working on a analog circuit that controls the glowing time period of a bulb.
    I'm using a relay,2 resistors and 2 capacitors,one is electrolytic and the other one is ceramic. I understood the role played by electrolytic capacitor i;e it stores enerygy but I'm unable to figure out why on earth is the other capacitor used for. PLEASE check the link for circuit

    ReplyDelete
  59. Hi Swagatam Majumdar

    I find that circuit requested by Fastshack3 is useful to my case, well.... almost, since it require 12V signal to activate. Is there any way to activate the circuit with ground or 0V signal?. Thanks in advance

    ReplyDelete
    Replies
    1. Hi balakutak,

      you can do it by eliminating the BC547 stage and replacing the collector/emitter position with the push button and the capacitor.

      the base resistor of the BC557 could require an increase to may be upto 100k

      Delete
  60. Hello. I am trying to build this circuit -
    http://s768.photobucket.com/user/dpbayly1/media/BMW%20X5%20Folder%20%202/autofoldmirrors_zps53079bfd.png.html - but the problem is, the time delay relays that I am using, does not reset, when the SPDT relay changes its state from NO to NC, as mechanical switch of the central pin is too fast. Is there a way to "slow" down the flow of electricity, when the pin switches? I was thinking about two coils, one at pin 87A and the other at pin 87. what do you think?

    ReplyDelete
    Replies
    1. you can reduce the current to the relay coils for reducing the electromagnetic pull, but still that would create a difference in milliseconds only....

      Delete
  61. Good day

    i would like a circuit that counts up in hours maybe to a maximum of 5 hours continuously when every time it is switched ON.i have been having a problem with this for a long time now.

    Thank you

    ReplyDelete
    Replies
    1. you can use a 4060 and 4017 timer circuit as given here:

      http://www.homemade-circuits.com/2012/04/how-to-make-long-duration-timer-circuit.html

      Delete
  62. Is is possible to make a circuit that lights up a bulb but then a momentary switch can cut the power to turn the led light off? The tough part would be to keep the light off for longer than when you release the switch. I want the bulb to stay off for 5 seconds before it getting power again.

    ReplyDelete
    Replies
    1. you can use the second circuit from the above article, and replace the BC557 with another BC547 transistor..after this you can tweak values of the 1000uF and the 2M2 for getting the desired off time delay

      Delete
  63. Hi, good job on these. Extremely useful, so thanks! Is it possible that when switch is pressed it only charges the capacitor but does not power on the light? The LED is only required on for 5 seconds after the switch is released.

    ReplyDelete
    Replies
    1. you can try connecting a 220uF capacitor in series with the LED with a 1M resistor in parallel with this capacitor, this will probably take care of the mentioned situation.

      Delete
  64. I see this is a rather old article, but I have a small project that I need help with. I'm building a "Groundhog day" clock that will play "I got you babe" when the alarm goes off. So I need a circuit that will activate a relay after about 5 seconds and only last about 500 milliseconds. I have both SSD and mechanical relays of different voltages, so I'm pretty flexible with the design.
    The reason for the delays is that the MP3 player takes a few seconds to turn on, then I have to bridge the play button to get it to play the song. I have tapped into the alarm clocks pre-amp, so the volume can be controlled by the original knob.
    I will post a link to my project, once I have it completed.
    Thanks for any assistance.

    ReplyDelete
    Replies
    1. Instead of the above circuits the following would suit your application better:

      http://www.homemade-circuits.com/2013/02/make-this-simple-delay-on-circuit.html

      just make sure to add a 0.22uF (arbitrarily selected) capacitor between the base of T2 and the junction of R3/R4, this will take care of the 500ms activation of the relay

      Delete
  65. I saw that to design a latch you told to use a 100K resistor across the collector of BC557 and the base of BC547 removing the capacitor entirely. Could you please suggest a modification to this design that would de-latch the power to the same led upon click of another button.

    ReplyDelete
    Replies
    1. you can add a push button across the base of BC547 and ground for the required de-latching feature

      Delete
    2. Thank you! That was perfect... but I have another question... how do I de-latch by pulling a GPIO pin from low to high? Could you please help me with this..

      Delete
    3. you can do it by adding another BC547 to the design.

      collector to the existing BC547 base, emitter to ground, and the base to the high logic via a 10K resistor

      Delete
    4. Thanks for this ...very helpful..:) With the above latching/delatching circuit, I am driving a microcontroller board instead of the LED. Before connecting the board, the supplied voltage comes across the points. But just after connecting the board, the voltage at the same points goes down by nearly 30% and is not sufficient to turn the board on. Is there a way we can reduce the resistor values in the circuit you provided to avoid the voltage drop? Or any other recommendations?

      Delete
    5. please try increasing the 1K resistors to 10K and see the response :)

      Delete
  66. what could be written while i submit synopsis for this project ??

    ReplyDelete
  67. Hi Mate,

    I was looking for something like this. Can this be modified such that the delay time is less than a second. like 0.1 to 0.9.


    Thanks

    ReplyDelete
  68. hi Mate,

    Sorry if this is a resubmission. i want to delay the timer to less than a second. I am guessing it can be done by reducing the farad of the capacitor. Could you please advise.

    Seen several modules in ebay but did not find anything that could meet my requirement.

    ReplyDelete
    Replies
    1. you are right mate, just reduce the 1000uF capacitor to something like 1uF, or even less to get the intended small delay effect

      Delete
  69. Thank you mate. I don't see the 1000uf capacitor that your referring to on the last circuit. Well that is the circuit am after. What if i need to delay the energizing once i hit the switch to about a second or two. i am guessing i need to add a resistor in series to the positive end of the capacitor.

    I've also tested a circuit where i just add a 10v 2200uf capacitor in parallel with a 5v Relay. I seem to get what i want, the relay stays energized for less than a second. Would that be ok to operate an ac transformer for a DIY spot welder.

    ReplyDelete
    Replies
    1. I was referring to the first circuit which looks more appropriate for your requirement....simply replace the LED/resistor with a relay/diode, and adjust the 1000uF capacitor value to some lower level, and you will get exactly what you are looking for.

      Connecting a capacitor in parallel to the relay coil will delay the switch OFF as well as the switch ON of the relay which probably we don't want...

      Delete
    2. Thanks A lot buddy. Your the best.

      Delete
  70. Hi,

    I'd like to ask if some of these circuits would fit for alarm system? A few months ago my basement was robbed. Now I made very simple alarm system (opened door - ON, Closed door - OFF) and additionally installed "jail" doors. I would like to have delayed-OFF alarm (for one or few minutes), Can anyone suggest some proper and functioning circuit? [Siren properties: 12V; 150mA]

    Thank You in advance

    ReplyDelete
    Replies
    1. Hi, yes definitely you can use any of the first two circuits and use it for the mentioned purpose in conjunction with a reed switch and magnet assembly...I hope you know how to configure the proposed idea.

      Delete
  71. Thank you. Well I hope too that I know how to carry out. The problem is delay time, if it will be too short what should I change in the circuit to prolong alarm?

    ReplyDelete
    Replies
    1. the timing can be varied by altering the input capacitor and resistor at the BC547 base individually or together. If you have problems let me know about it, I'll create the design for you.

      Delete
    2. Hi, So I've done the circuit according the second scheme, but the delay OFF time was very short (~0.5 sec). Then I changed capacitor from 1000 uF to 2200uF and resistor from 2m2 to 3m3, but delay off time extended maybe up to one second. Do you have any suggestion what should I do to prolong alarm? Thank you.

      Delete
    3. Hi, In the second diagram, using a 1000uF and a 2M2 resistor at the base of the NPN should produce a delay of over 2 minutes....if it's giving 0.5 seconds then there could be something seriously wrong with your circuit...
      similarly with a 2200uF and 3M3 the delay should be well over 5 minutes.

      I think you might have connected the transistors wrongly or something could be faulty in the circuit.

      Delete
    4. Hi again, might be I made mistakes, I'll check it later today. Thank you again.

      Delete
  72. Hi Swagatam. I have built the single transistor version of this for an application to hold a 3V relay on for å period after a pushbutton is pressed and the timing function works well - I can see the LED growing slowly dimmer. I have connected the relay's negative side (with a diode across to the positive) to the 1K resistor, but the relay does not latch and hold (it clicks though). I also tried reducing and then removing the 1K resistor and even the LED. Still no latch. What am I doing wrong?

    ReplyDelete
    Replies
    1. Hi Glen, a single transistor will not be enough to sustain the relay operation, you may either try the second circuit or configure another BC547 with the existing one in the "Darlington mode"....this will do the job for you.

      Delete
    2. A-ha. Thanks, I'll try that.

      Delete
  73. Thanks Swagatam. I have built a Darlington bridge with two 2N2222 transistors (that's what I had available) and the relay now latches. But I cannot get a delay of more than about 5 seconds (I need 10-15.) I have increased the caps to 3 x 6800uF but when I increase the timing resistor beyond 40K it stops working - the transistor will not turn on. Any suggestions?

    ReplyDelete
    Replies
    1. that's very strange, because 6800uF is huge and should easily produce above 5 to 10 minutes of delay...I think may be your relay has a low coil resistance which is causing quicker discharge of the cap.

      in that case you may need to build the second circuit, and still if it does not provide the required delay then you could probably go for an IC 555 monostable circuit

      Delete
    2. Since I am now using two transistors as a Darlington pair the circuit is almost the same as your circuit two. See the circuit diagram I sent via Google Drive. I just added your cap and timing resistor and used two 2N2222s.

      I agree that the coil resistance may be the problem since when I remove the relay and just use the LED the delay is much longer. The specs for the 3V relay say 60 ohms at .15W to 20 ohms at .45W.

      When I replace the relay with the LED I can also increase the timing resistor beyond 40K and the transistors still turn on - why does the relay affect how high the timing resistor can be?

      Does the choice of transistors have any effect?

      And finally, once we solve this problem, I need to get the final circuit as small as possible. I have read that there are Darlington pair transistors in a combined package. Would these work and can you recommend a particular model?

      Delete
    3. Yes I saw the diagram in my email, it's good.
      However now I understand the real cause of the problem....it's because of the 3V supply which is much nearer to the holding threshold 0.6V compared to 12V...and that's why the capacitor charge is able to reach this limit pretty quickly.

      there's one option that you can try to counter this issue....connect 4 nos of 1N4148 diodes in series with the base resistor of the transistor, this will help to increase the delay significantly even with smaller capacitor.

      Delete
    4. 2N2222 is OK, the problem is not with the transistor but the voltage level as explained above.

      Delete
    5. ...sorry correction: the 4 diode in series will not increase the delay in fact it will do the opposite....

      Delete
  74. Yes, I wondered how that would help. :-) But I still don't understand why with the relay connected I cannot increase the timing resistor beyond 40k ohms? (The transistor stops turning on.)

    ReplyDelete
    Replies
    1. The 4 diode idea would effectively work for a delay ON timer, but since here we are discussing a delay OFF timer, the idea would be disadvantageous.

      the base resistor directly influences the collector/emitter current specs. as it increases the collector/emitter current delivering capability proportionately decreases.

      you can refer to the following article for the formula:

      http://www.homemade-circuits.com/2012/01/how-to-make-relay-driver-stage-in.html

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Swagatam Majumdar
Swagatam MajumdarHi Friends, Welcome to my site, a place where you will discover a massive collection of electronic circuit ideas, mostly requested by the dedicated readers and exclusively designed by me for their customized application needs. I have posted more than 1100 circuit designs in this site, if you have a personalized circuit requirement you may feel free to request it through the comment box, if it seems feasible to me then surely you may find it published here with your credentials attached in the post, thanks and please keep reading



 
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