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You are here: Home / Mini Projects / Simple Bedroom Lamp Timer Circuit Using IC 741

Simple Bedroom Lamp Timer Circuit Using IC 741

Last Updated on February 11, 2021 by Swagatam 13 Comments

A simple automatic bedroom lamp timer circuit has been described here to switch off your bedroom lamp after a fixed predetermined time. The use of the reliable 741 IC makes the circuit very easy to build and moreover very accurate.

Introduction

The circuit of an automatic bedroom lamp timer presented here is super easy to understand and uses very few components to operate. The use of an integrated circuit 741 makes the circuit more accurate compared to the transistor ones.

In many houses, the light switch sometimes is situated at a distance from the bed and often, folks may have the habit of reading a book or so before dozing off.
In such an occasion one would desire they had a device that could be timed as required to automatically switch-off the light after a certain fixed period.

The proposed circuit of a bedroom lamp timer is designed exactly to satisfy the above need. The device will automatically switch off the connected bedroom light and itself after a particular period of time depending upon the setting.

A standard RC timing configuration has been employed and works satisfactorily for the present application.

The IC 741 which is wired as a monostable multivibrator forms the main active part of the circuit.

You already must be too familiar with this IC and we know that it’s basically an op-amp having huge applications in electronic circuit configurations.

In one of the standard applications the IC 741 may be used to compare voltages in between its inverting and non inverting inputs. On sensing the trigger threshold at one of its inputs, the IC toggles its output condition and activates the output parameters

Here as explained above, it is used as a comparator and is used to compare the charging voltage of a capacitor to a particular set level after which it switches the output.

Circuit Operation

Referring to the figure, we see that the inverting input is set to about 2/3 rd of the supply voltage. This voltage level in fact becomes the reference source to the IC.

The non inverting input pin of the IC is connected to the unction of a RC network, where the other end of the resistor R (variable if required) is connected to the positive supply and the negative pin of the capacitor C goes to the ground point of the circuit.

Powering the circuit is rather done in an interesting manner. Here the transistor T2 along with PB1 is so wired that on pressing PB1, T2 latches and holds the supply voltage to keep the circuit powered.

Now, initially the voltage at pin #3 is almost at the ground potential because of C, however as C charges the potential at this pin of the IC starts rising.

Depending upon the value of R, after a particular length of time C charges itself to a level which may be above 2/3 rd of the supply voltage as set at the inverting input.

The IC responds and immediately toggles its output to produce a logic high or a positive voltage.

T1 and the triac at the output deactivates and switches of the external load connected to it. The high logic at the output of the IC also inhibits T2 from conducting and breaks the latch to switch of the whole system.

The timing sequence of this bedroom lamp timer can be repeated or initiated by pressing PB1 whenever felt necessary.

Using Transistors

The simple bedroom lamp timer can be also implemented using a couple of BJTs as shown in the following diagram.

The idea looks even more compact due to the inclusion of a transformerless power supply.

The two BJTs are wired like a delay OFF timer stage.

As soon as the push button is pressed, the 100 uF capacitor is fully charged, and the transistors become activated, switching ON the triac and the bedroom lamp.

On releasing the push button the transistor and the triac along with the lamp continue to remain switched ON due to the presence of the stored charged inside the 100 uF capacitor, which holds the BJTs base forward biased.

However, the capacitor now also starts discharging though the 2M2 resistor and the NPN emitter pin.

When the voltage across the capacitor drops sufficiently below the holding level of the NPN, the transistors are unable to remain switched ON anymore and are switched OFF, turning off the triac the lamp.

The time for which the lamp can stay ON, is basically determined by the 100 uF capacitor and the 2M2 resistors.

Bedroom Lamp Timer Using IC 741 Circuit Diagram

WARNING: All the circuit concepts presented above are not isolated from the mains AC, and therefore touching the elements in the powered condition can be fatal. Necessary precautions are strictly advised.

Make sure to use a plastic enclosure for the circuit once the working of the finalized design is confirmed.

SHARING IS CARING!



Previous: Simple Analogue Weighing Scale Machine
Next: Making a Time Machine – Concept Explored

About Swagatam

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials.
If you have any circuit related query, you may interact through comments, I'll be most happy to help!

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  1. Search Related Posts for Commenting

  2. Nassim Sabaa says

    Dear Mr. Swagatam
    I hope you are fine and healthy. I would like to inform you of another problem in below link.
    Please let the visitors of your interesting site to see the content of tile “Voltage regulator circuits using transistor and Zener Diode”. it seems have been locked.
    https://www.homemade-circuits.com/?s=1.5+volt+output+power+supply
    best regards
    Nassim

    Reply
    • Swagatam says

      Thank you Dear Nassim, The link is working fine for me, you can check it here:

      Voltage Regulator Circuits using Transistor and Zener Diode

      Reply
  3. Nassime Sabaa says

    Thank you so much dear Swagatam
    truly yours
    nassime sabaa

    Reply
  4. Nassime Sabaa says

    Dear Mr Swagatam
    referring my few ours ago that I told you that I can not see the circuit ” Simple Bedroom Lamp Timer Circuit Using IC 741″ I would like to inform you that when I used tor browser from Torproject, I saw your interesting IC741 circuit board I mentioned above.
    I am so astonished why Google chrome browser does not show it.
    Please accept my excuse.
    God bless you
    Bye

    Reply
    • Swagatam says

      Thank you Dear Nassime for your kind concerns.

      Yes it is indeed strange, and difficult to understand how these different search engines actually work to judge the quality of the websites!
      I Appreciate your feedback, please keep up the good work!

      Reply
  5. Nassime Sabaa says

    Please accept my warmest greetings sir
    Hello
    Is there any problem in this page?
    I cant not see the circuit under title ” Simple Bedroom Lamp Timer Circuit Using IC 741″ .
    there is a 2 transistor circuit instead.
    sincerely yours

    Reply
    • Swagatam says

      Hi, thank you very much for pointing out the missing diagram, it seems the diagram was deleted by mistake.

      But No problem, I have somehow added the diagram, as per the given explanation. It should work almost like the original op amp concept

      Reply
  6. andrzej says

    Swagatam
    Hi, and greet you warmly
    I have one question about the OFF-ON switch
    I am looking for a simple schematic (transistors, etc.) to make it possible with one button
    1) switch on the device
    2 turn off the device
    on the same button ,,,, bistable
    I’m from Poland
    andrew

    I admire your devotion and dedication to people

    Reply
    • Swagatam says

      Thank you Andrew,
      You can try one of the concepts presented in the following article

      https://www.homemade-circuits.com/build-these-simple-flip-flop-circuits/

      Reply
  7. antony says

    HI,
    plz give me timer circuit diagram for exhaust fan,it should automatically turn off after few minutes when i switch it off.
    means it have to run a few more minutes after I switch it off

    Reply
    • Swagatam says

      Try the following circuit:

      https://homemade-circuits.com/wp-content/uploads/2012/05/delaytimerimprovedcircuit-2-1.png

      replaec the push button with an ON/OFF switch

      Reply
  8. Riclyn Jan Notarion says

    hi sir, we are having project which has the main application of diodes and i am thinking of presenting this in our class since i think it is easier and we can make this before 1 month.. so, may i ask for the components that you've been using together with the value and the value of the power supply that is needed to work this on.

    Please… Thank you in advance.

    really need your quick reply.

    Reply
    • Swagatam says

      Hi Riclyn,
      the last circuit is although not entirely diode based, you can make it if you wish to.
      All the the parts info is printed in the diagram itself, please note it down on a paper and procure the parts for assembly.

      Reply



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