Single Transistor LED Flasher Circuit

Can you imagine of making a formidable little LED flasher or blinker using just a single transistor and a couple of other passive parts.

That looks too good to be true, however the following diagram will simply prove that it's really possible to create a legal LED flasher circuit using just one general purpose transistor as the main component.

I came across this phenomena some eight years ago, accidentally, while trying to make a smallest possible motorcycle side indicator flasher, and was really amazed.
However, then I did have not any idea that it was happening due to the negative resistance characteristics of the transistor.

 The circuit actually exploits the negative resistance factor in transistors to produce the blinking effect.

 I'll be soon writing a comprehensive article on this and we'll see there how the concept can be modified in many different ways.

Parts List for the proposed single transistor LED flasher circuit

R1 = 2K7,
R2 = 100 Ohms,
T1 = BC 547,
C1 = 100 uF to 470 uF
LED = Any Type, any color


LED flasher circuit using single transistor

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62 comments

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December 13, 2011 at 4:51 PM delete

I actually wanted a circuit of water flow controller which will be fitted in the overhead tank only. When the level of water in this overhead tank goes below a certain level, the pump is switched on and it starts pumping water from the underground tank; when the overhead tank is full, it is switched off; plus motor protection in case there is no water in the underground tank. my email id is sanjay8040@gmail.com
thanks

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December 14, 2011 at 2:52 PM delete

Hi Sunjay, I've already given you the entire diagram in Bright Hub, you can check it out there.
Regards.

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December 15, 2011 at 8:59 AM delete

As i understand, the circuit in Bright Hub is of a level indicator and motor protector , to be installed in the overhead tank; the water level controller is to be installed in the underground tank. Now I want a controller cum motor protector which can be installed in the overhead tank itself. Our underground tank is very big so it may not be practical to put the controller there. Thanks

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December 15, 2011 at 9:32 AM delete

Hi Sunjay,
You can install the entire circuit in your overhead tank, only the points at "F" needs to be positioned in such a way that it comes in contact with the water that's entering the motor.
Thus, as long as water supply is present to the motor, it remains switched ON and the moment water supply stops, the motor also stops.

Regards.

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December 15, 2011 at 11:13 AM delete

I shud use the last circuit meant for the overhead tank, with IC 4049. When water level reaches point A, the motor will stop; and when water level goes below point E, motor will start. My doubt was that at point E, there are only 2 points from the IC. Will this work to start the motor? I hope my understanding is correct. Thanks a lot for your guidance. Regards

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December 15, 2011 at 11:27 AM delete

Hi Sunjay,

The ground point shown at the bottom, inside the tank is the common point for all the sensing terminals, it must be immersed in the water at the bottom of the tank permanently.
Here the motor does not have any connections with the sensors in the upper tank.
The motor will switch ON and OFF depending upon the water supply present to it, as long as there's water entering it, it remains switched ON, as soon as water supply ends, the motor stops, this is the only way through which we can protect the motor.
In simple words as long as water is present across point F, the motors is switched ON and vice versa.
So you can position this point anyway you want, whether in the upper tank or in the underground tank, it will depend on you how you want to switch the motor.
Regards.

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December 15, 2011 at 12:16 PM delete

hi swagatam..thanks for your quick reply. Since our underground tank is very big, the OHT will start overflowing with the system in question. I am looking for a system similar to the one for the UGT. Actually it will be reverse of the UGT system. At C, the motor starts and at A, the motor stops. And if u can provide F type protection, all the more better. I hope i am able to make myself clear. Best regards.

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December 15, 2011 at 12:27 PM delete

OK, it means you require motor control from both the tanks, it can be done simply by introducing the point F near the brim of the OHT.
May be today or by tomorrow I'll be posting a new article on this subject, I'll post the diagram there and inform you. You can check it once it gets published.
Regards.

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December 15, 2011 at 12:30 PM delete

Just a minor clarification. I want to put the controller only in the OHT; nothing to do with the UGT. Point F will be put at the entry point of water in OHT.

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December 15, 2011 at 12:59 PM delete

Hi Sunjay,
Yes, that makes the procedure much simpler, put the pair of F terminals at the point where the water enters the UHT.

Regards.

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December 15, 2011 at 1:01 PM delete

Sunjay, The relay diode is wrongly shown in the diagram, invert it in your circuit.

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December 18, 2011 at 10:40 AM delete

hi swagatam..ref ur msg of 14th..i was waiting for the circuit diagram of the water level 'CONTROLLER' for the overhead tank. The circuit given by you for tha OHT in Bright Hub, as i understand, is that of a level indicator. Thanks n regards

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December 18, 2011 at 10:47 AM delete

Hi Sunjay,
If you want a circuit only for controlling the motor pump, then you may refer the following diagram.
http://homemadecircuitsandschematics.blogspot.com/2011/12/how-to-make-simple-water-level.html

If you have any further doubts, feel free to comment.
Regards.

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Anonymous
June 9, 2012 at 7:35 AM delete

Hello Swagatam . . i want to ask u that i am making a joystick car project and i want 4 l.e.d blinking on and off . . at the corners of my car . . so can i use this circuit . .??

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October 22, 2012 at 3:02 PM delete

admin i m new to the electronics.....
pls tell me...
negative terminal battery ka kaha lagega??
diagram me to + terminal hi show kara hai na
pls tell me.......

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October 22, 2012 at 8:06 PM delete

At the junction of R2 and C1

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Anonymous
February 4, 2013 at 12:45 PM delete

Mr.Swagatam, the emitter of T1 in the circuit diagram is connected to R1 or to positive side of the LED? Just a clarification. Thanks

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February 4, 2013 at 2:29 PM delete

the arrow head of the transistor symbol is the emitter.

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Anonymous
February 8, 2013 at 6:16 PM delete

Sir, I have followed the circuit above and I have the right components needed but the LED is not flashing. It just lights up. What must gone wrong?

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February 8, 2013 at 9:01 PM delete

Not sure what's wrong, my circuit worked superbly with the shown components and supply.

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Anonymous
February 9, 2013 at 7:02 PM delete

is there a kind of LED that I should use, sir?

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Anonymous
February 10, 2013 at 8:47 AM delete

Mr. Swagatam, i have question from the above circuit. By how much voltage of LED should be use? another question is, is the supply voltage (12v) should be exact or can we increase or decrease it?

and lastly, what is the difference of using 100uF capacitor in using 470uF? what should be the voltage of the capacitor?

thanks for answering :)

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February 10, 2013 at 9:33 PM delete

any type led would work....I had used a 5mm green led, and 12V supply.

different capacitor values will produce different flash rates.

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March 7, 2013 at 8:51 PM delete

Hi swagatam,

Can we use 3 to 9v supply for this circuit ?

Regards,

Bibin Emdond

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March 8, 2013 at 8:17 AM delete

Hi Bibin,

A 9V supply might just work, but not 3V.

Regards.

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April 11, 2013 at 11:36 PM delete

Hi Swagatam
I tried the above circuit ,but it didn't worked
I used 4k resistor in place of R1

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April 12, 2013 at 8:00 AM delete

Hi Sirj,

this circuit is so simple, it should start working immediately... assemble everything exactly as given in the diagram, it will work.

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June 18, 2013 at 7:18 AM delete

Hi, i was wondering if you could change the blinking time playing with the RC values. It works with the simple RC constant or is it more complex to calculate this time?

Thanks in advance

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June 18, 2013 at 10:18 AM delete

Hi, yes it's dependent on the values of R1/C1, but only to an extent beyond which it just stops functioning.

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July 13, 2013 at 2:11 AM delete

i used all the components shown and it didn't work the first time. it works now because i changed out the transistor until the led started blinking

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Anonymous
August 19, 2013 at 7:12 PM delete

dear sir pls give two led alternate flasher with adjustable flash rate circuit

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August 20, 2013 at 8:08 PM delete

you can try this one:

http://homemadecircuitsandschematics.blogspot.in/2011/12/make-yourself-simple-led-flasher-at.html

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Anonymous
November 21, 2013 at 4:23 PM delete

You need to add an ic which is 555 to this circuit. You can get its datasheet and some good circuits if you search on google. Using this circuit as controlling timing is just wastage of time.

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February 1, 2014 at 7:46 PM delete

What are the formulas to calculate the timing of the flash?

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February 2, 2014 at 12:40 PM delete

t = -log((V-Vc)/V)R*C

To calculate Vc at a specific time, the formula can be modified to:

Vc = V-(V*exp(-t/(R*C)))

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March 14, 2014 at 10:57 PM delete

I am trying to imitate cloud to cloud lightning using the smallest possible circuit. I found this project that uses only the flashing LEDs and a battery...http://www.youtube.com/watch?v=Rj5tqhGypAE&feature=player_embedded. I found 3mm flashing LEDs ...http://lighthouseleds.com/led-component-lighting/animated-leds-flashing-blinking/3mm-led-flashing-round-top.html
Can I power 2-3 of these (amber and/or white) using one CR2032 battery? Can I get them to flash at different rates? ...I think that may drain the battery too quickly since there are no resisitors used, is that correct?

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March 14, 2014 at 11:09 PM delete

I am trying to imitate cloud to cloud lightning using the smallest possible circuit. I found this project that uses only the flashing LEDs and a battery (Candle Flicker Hair Bow). I found 3mm flashing LEDs (3-3.4V, 20mA, 1 Hz flash frequency)
Can I power 2-3 of these (amber and/or white) using one CR2032 battery? Can I get them to flash at different rates? ...I think that may drain the battery too quickly since there are no resisitors used, is that correct?

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March 15, 2014 at 2:32 PM delete

you can include a 22 ohm resistor in series with the LEd in order to make the battery drain slower.
alternatively you can employ a joule thief circuit for making the batt last for ages.

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March 15, 2014 at 2:34 PM delete

a flashing LEd could have an automatic flash rate changer built-in, so it could go on changing the flash rate by itself.

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March 29, 2014 at 1:41 PM delete

hi my name is madhu i want electronic variable wattage choke circuit diagram for flurocent tube. pls send to the mail below
email: madhubabu.sv@gmail.com

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March 30, 2014 at 1:05 PM delete

sorry, presently I do not have this circuit.

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April 3, 2014 at 6:25 PM delete

HI
will this circuit work for 16v?

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April 3, 2014 at 6:29 PM delete

I want to design a LED Flasher Cirrcuit with Input : 9V-16V, On time : 0.5s +/- 10%, Period : 1s +/- 10%, Reverse polarity protection : required can you help me with the above circuit? Will it work for this conditions?
need your response
thank you

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April 4, 2014 at 1:14 PM delete

the above circuit won't be appropriate, search for:

transistor astable multivibrator circuit .....or 555 astable multivibrator circuit

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April 4, 2014 at 3:43 PM delete

Can you tell me in technical prospectivem, why cant we use this circuit??

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April 5, 2014 at 6:30 PM delete

the above circuit is not configured in a conventional manner so could have limitations in terms of interval setting accuracy...you can try it out, though.

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April 7, 2014 at 5:57 PM delete

Hi,

thanks for your prompt response.
Could you Just modify the circuit for this specifications LED Flasher Circuit with Input : 9V-16V, On time : 0.5s +/- 10%, Period : 1s +/- 10%, Reverse polarity protection required, by using any other stage of RC or any other way. Using astable multivibrators is cost effective for my project, need a simple circuit. Hope for your response.

Thanks in advance

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April 8, 2014 at 12:33 PM delete

It will need to be tweaked and verified practically for those results....there are no formulas for it.

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August 10, 2014 at 5:00 PM delete

Hi, can you please tell me how the transistor is activated without the base connected?

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August 11, 2014 at 5:55 PM delete

In the circuit, the capacitor is charged by R1 until the voltage becomes large enough to get the emitter-base junction to avalanche which lights up the LED and also discharges the capacitor to trigger a fresh cycle....it basically takes place due to the negative resistance characteristic of the BJT

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March 31, 2015 at 9:03 PM delete

Hey buddy,i have installed 12v3led strip in my bike and i need them to blink.i knw i can use this circuit but what all i need to alter?to adjust speed and to power led 12v..

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March 31, 2015 at 9:05 PM delete

Please reply soon thnkx:-)

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April 1, 2015 at 10:13 AM delete

Mukesh you cannot use the above circuit, instead you can try any of the following designs:

http://www.homemade-circuits.com/2012/01/how-to-make-any-light-strobe-light.html

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December 18, 2015 at 6:07 PM delete

Hi Swagatam, were you able to write the comprehensive article explaining how this works?

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December 19, 2015 at 10:05 AM delete

Hi Siddarth, sorry no I could not complete it due to lack of time and other more complex assignments...you can Google "Esaki single transistor flasher circuit" to learn more about it.

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