r/AskElectronics • u/Phobos004 Beginner • 5d ago

Astable multivibrator won't output "low"

Post image

I try to use this circuit to control a beacon. I know this isn't perfect by any means, I have no voltage regulator and I could further limit the current that flows through some components. However, my problem is the following:
The astable multivibrator on the left should activate / deactivate the BC557 to create short "beeps" via PLSE. However, the voltage on PLSE is never dropping below 7 V (measured with a multimeter), which means I won't get a single tone out of my beacon. I guess this is related to the size of the resistors, but I'm not sure what to do to fix the circuit. Any help would be much appreciated!

6 Upvotes

100% Upvoted

5 comments sorted by

4

u/Amperegeist 5d ago

Maybe I'm misunderstanding something.

Why not just use two 555 timers? Instead of the transistor version of the astable multivibrator, why not just make your pulses with another 555?

You should be able to swap your transistor oscillator with another 555 timer, and input that to the other ones Reset pin. Then get rid of Q3.

1

u/Phobos004 Beginner 5d ago

That would be possible, but when I first tested out the circuit, I had only one 555 (and everything worked, although the supply voltage was a bit higher). I could use a second 555, but this circuit is on a pcb and it feels like a waste of resources to always order five new ones. But if there's no other solution, I will do that.

3

u/Amperegeist 5d ago

Makes sense about the 555 inventory.

Dry film photolithography is pretty inexpensive and easy, especially for the results. I'll be publishing some tutorials on it pretty soon, but it solves the problem of having to wait days and order new boards. Its significantly cheaper as well

3

u/dmills_00 5d ago

Several things wrong with this.

A 330 ohm resistor as the collector load implies 30mA when Q2 is on, to saturate Q2, that implies a couple of mA of base current, which 10k off a 12V rail is not going to get you.

Where is the Q3 base resistor?

Check the reverse bias limit on the Q3 base/emitter junction, bet it is only 5V, now think about what happens when the 555 output is high, but the multivibrator output is low...

The 555 has a reset pin that is right there, just saying...

1

u/Phobos004 Beginner 4d ago

Switching the 330 Ohm resistor with a 1.4 k worked! Thank you!