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Single Transistor LED Flasher

Last Updated on by 65 Comments

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

How it Works

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

Circuit Diagram

 



LED flasher circuit using single transistor

Video Clip:

 

PCB Design

single transistor flasher PCB design

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  1. Ha! If I did not see the video i would not have believed it.
    Does not work in simulator.
    www.falstad.com/circuit/circuitjs.html?cct=$+1+0.000005+12.050203812241895+53+5+50%0AR+656+160+656+80+0+0+40+12+0+0+0.5%0Ag+656+464+656+528+0%0Ar+656+160+656+256+0+2700%0Ar+656+464+576+464+0+100%0A162+576+352+576+464+1+2.1024259+1+0+0+0.01%0At+384+336+576+336+1+1+0.00012985325697745242+-0.30249585765328524+500%0Aw+576+320+576+272+0%0Aw+576+272+656+256+0%0A209+656+256+656+464+0+0.00033299999999999996+0.8571050617707789+1%0Ao+4+64+0+4099+0.625+0.00009765625+0+2+4+3%0A

    Reply

      • Considering simulators like LTspice can easily simulate op-amp and power supply circuits, it should easily be able to simulate your circuit, but try as I might, I too cannot get it to simulate.

        BC547 A, B or C ?
        LED: which one did you end up using (what breakdown voltage, forward current, partnumber…)

        Reply

        • When only BC547 is written it means any BC547 can be used since the supply voltage is only 12V, in the video you can see that the LED is a 5mm 20mA type.

          A simulator may have difficulty in simulating because the circuit works with unconventional principles.

          Reply

  2. Hi Swagatam,

    I have seen it’s possible to make this LED stop blinking by disconnecting the capacitor. Is it possible to disconnect / reconnect the capacitor by means of an additional transistor on the cap side of the circuit? I would like the LED to be on then when the second transistor gets signal the cap is connected and flashing starts. I’ve achieved this with relays but as I will have many of these in my circuit the cost of relays isn’t a good choice.
    Thanks for an excellent website!

    Reply

    • Thank you Theuns, yes that may be possible, but instead of modifying the C1 section, I would recommended you to configure the ground supply line of the above circuit with an NPN such that when the NPN is triggered the circuit gets activated. For this you may connect the collector of the NPN with the ground line of the circuit,and the emitter with the supply negative.

      Reply

  3. Thank you for your response sir.. How can i accomplished that? Can you please show me a circuit diagram with BJT circuit that i can follow sir..
    Thanks again sir..

    Reply

    • Paul, I’ll explain you verbally:

      In the above circuit, remove the LED and join R2 directly with collector of T1, but make R2 = 1K

      now take a TIP127 BJT, connect its base directly with the above junction of R2 and T1 collector, join its emitter with the +12V line…and connect the collector with the bulb…bulb other end will go to the ground line…I hope you understood.

      Reply

  4. Sir Swagatam,

    Can I use this circuit as replacement for my turn signal flasher on my motorcycle? or is there another circuit you can refer to me..
    As a 12v signal flasher for 12v led light.

    Thank you very much..
    Paul

    Reply

  6. Hi Mr swagatam,
    Can you guide me to make an automatic switching for my led emergency light using Transistors? I have connected 50 no's of led and planned for 6V/4.5 Ah battery.pl explain with circuit diagram.

    Reply

    • Hi Mohan, you can refer to the following circuit and do it accordingly for getting the required results:

      https://homemade-circuits.com/2011/12/how-to-make-efficient-led-emergency.html

      Reply

    • 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.

      Reply

  10. 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..

    Reply

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

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

      Reply

    • 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

      Reply

  12. 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

    Reply

    • 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.

      Reply

  14. 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

    Reply

  16. 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

    Reply

  17. 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?

    Reply

    • 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.

      Reply

  18. 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…www.youtube.com/watch?v=Rj5tqhGypAE&feature=player_embedded. I found 3mm flashing LEDs …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?

    Reply

  21. 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

    Reply

    • 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.

      Reply

  24. 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 🙂

    Reply

  25. 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?

    Reply

  26. 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

    Reply

  29. 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 . .??

    Reply

  30. 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

    Reply

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