astable multivibrator Problems

Thread Starter


Joined Jul 4, 2019
Hello everyone.
im sure this has been answered many times but I can’t seem to find the answer.
Im trying to build astable multivibrator to use in a 12v Scalextric police car. I have it switching at the speed I desire but the leds don’t turn off fully when they are switching. Can’t post a video of it but I’m sure you can work out what 2 leds alternating between bright and slightly dimmer look like.
Any help would be most appreciated



Joined Nov 6, 2012
It looks like you may be exceeding ~20mA with your 330-Ohm Resistors.
Make sure that your LEDs can tolerate that much Current.

Here's how I would do it .............
Square-Wave Generator 2 .png



Joined Mar 14, 2008
Your LEDs don't go out because the current to charge the capacitors goes through the LEDs when its transistor goes off.
To prevent that you need to isolate the LED from this current.

See LTspice simulation below where D1 and D2 provide the needed isolation:

D3 and D4 provide a path for discharging the capacitors and also keeps the reverse base bias going to -9V when the transistor is off, which would exceed the typical max Vbe rating of -5V.



Joined Feb 16, 2022
Take in mind, if supply voltage is 5V or higher, a specific so called chopper transistors should be used- MPS404, MPS404A, etc. They can tolerate higher base-emitter voltages. Or, diodes to be added, as shown in the post #4


Joined Dec 31, 2017
the leds don’t turn off fully when they are switching.
It might be because of the sensitivity of the LEDs you are using. If that's the case installing a 10K resistor in parallel with the LED will usually eliminate the problem.
I breadboarded this circuit with a 1 second switching speed using sensitive LEDs and verified the issue. The 10K resistors fixed the problem.


Joined Mar 14, 2008
I breadboarded this circuit with a 1 second switching speed
That circuit applies a pulse of near -12V from the capacitors to the transistor bases, which is well beyond their Veb 6V rating.
A diode added in series with each transistor base will eliminate that problem.



Joined Mar 14, 2008
What do you mean, if diode is added in parallel to B-E, as proposed in post #4 ? What is the difference ?
The diode in parallel reduces the oscillation period since it clamps the reverse voltage, which determines the capacitor discharge time.
It also has a similar effect in the circuit I posted, but that circuit requires the diodes (or another resistor) to provide a path for the capacitor discharge.

See difference below:

1683480417187.png 1683480516582.png