Simple 2 LEDs flasher

I remember that this was my first circuit several years ago when I used to go to an Electronic Club in my home town.
Few days ago I wanted to do something simple (I do this kind of stuff as relaxation, in the day-by-day life I work as a programmer) with LEDs so this was the right choice :).
So here is the circuit :


and the components for it :

  • T1, T2 : 2N3904 or almost any NPN transistors (BC171 , BC172 , 2N2222 etc)
  • C1, C2 : 10uF / 16V
  • D1, D2 : LEDs
  • R1, R4 : 470 Ohms @ 0.25W
  • R2, R3 : 39 KOhms @ 0.25W

The circuit is extremly simple to build :D, the LEDs flashes around once/second. If you want to increase/decrease the flashing rate modify the values of the R2, R3 resistors. The formula for it is about 3*R(in KOhms)*C(in uF)/1000 seconds so, as an example, for this specific circuit :


Here are some pics I made during the building :



Here is a sample video of how it works:

Happy building 😉

11 comments on “Simple 2 LEDs flasher

  1. dear sir..
    what is the function theory of the design…?
    how each capacitor effects the opposite transistor ? and why have you used polarized capacitors??

  2. thanks.but i dont understan how the capacitors work in this circuit…?
    which side of the polarized capacitors charge and discharge (positive or negative)??

  3. Thanks…tried many 2LEDs flasher circuits for my train crossing -signal light. Only this one work ( I’m new to elec. 🙂 ) and your formula for flash timing & link to study more -very useful to me thanks again.

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