LED Flasher Circuit Not Working

Thread Starter


Joined Sep 23, 2021
Hello Everyone ,
I am trying to simulate a circuit of LED Blinking through Transistors BC547 and BC557 . The Schematic is being attached . While Stimulating BC557 is not conducting. Ideally it should conduct as the circuit . The link of the refrence i have taken is this -> https://www.petervis.com/GCSE_Desig...er-using-bc547-bc557/led-flasher-blinker.html .
help me out with this . I have tried all the voltage source like 12v , 9v , 5v but it does'nt work



Joined Jun 4, 2014
This circuit won't work. When the capacitor has charged up to the supply voltage then current needs to be able to flow through the base-emitter junction of Q1 but there is no path for this current to flow as Q2 will be off until that current flows.

schmitt trigger

Joined Jul 12, 2010
This is a circuit which relies in component parasitics to work.
Namely leakage on the lower transistor to create a regenerative feedback which eventually fires it. If the leakage is low, it won’t fire.

However, once it fires, there is nothing to limit the current except the battery’s impedance and the lower transistor’s gain.
All in all a terrible design.

If you really, really want to build a discrete transistor Led flasher, then build a classic Eccles-Jordan astable circuit.


Joined Mar 14, 2008
This circuit won't work.
But it will (although how reliable is a valid question). ;)
Below is the LTspice sim:
I did add R3 so it doesn't depend upon stray parasitics to operate, and R1 to limit the peak LED current.
If you want to lengthen the LED flash length, you can add a resistor of the appropriate value in series with C1.