# Monostable Multivibrator with Trigger at the Collector

#### zuhriri

Joined Jan 1, 2021
4
Hi!

I need help in understanding this one particular monostable multivibrator circuit.

I have gone through a lot of readings about monostable multivibrator and I understood how it works.
The only difference between all the readings that I had done with this particular monostable multivibrator circuit is the trigger.

The monostable multivibrator circuit, that I have no problem understanding it, has its trigger at the base of Q1.
But the monostable multivibrator that I had shown above has its trigger at the collector of Q1.

The problem is I can't visualize how the circuit work when the trigger is at the collector of Q1.

#### hobbyist

Joined Aug 10, 2008
889
First of all when your working with transistor switching circuits, temporaarily remove any AC components out of the circuit and analyse the DC bias of the transistors.

By looking at the bias arangements of both transistors, do you see which transistors are conducting (assuming saturation , as that would be switching charactersitics) in the circuit.

You may say, well they both are to some extent, and you may be right depending on current and voltage drops, without actually doing alot of loop analysis and such.

But just using some logic, do you see any cross coupling, a multivibrator has some sort of cross coupling feedback.
Look at where the feedback is happening, then try to visualize current flow starting from the positive side of the battery through the base emitter junction to ground.

Is there current division at the collector of Q1 or Q2, remember DC current only, keep the caps out of the loop for now.
Do you see how the bias currents are flowing, which base emitter junction assumes to be getting the most amount of current to saturate its transistor.

Now once you get that established, do you see which transistor is actually saturated as a "on" switch, and is it able to keep the other transistor off.

If so you now have determined the (non signal condition of the circuit).

Now put the caps back in, are they filter caps, or are they AC signal transfer caps, or are they timing caps.
Once you put those caps in the circuit, they react to the voltages on each side of them, analyze the voltages across the caps after they have become charged up.

What is the polariztion voltage on each side of each cap.

Now finally visualize the input diode becoming forward biased through the appropriate signal, how do the caps respond, and how does the original current branches react.

From there you should be able to trace the input signal through the circuit to it's output, and understand the sequence of events that take place in the circuit.

#### MrChips

Joined Oct 2, 2009
23,531
The trigger may be at the collector of Q1.
More importantly, the trigger is applied to the base of Q2.
Once the trigger signal can overcome the impedance of Q1 and R2, it propagates through C1/R3 to the base of Q2 which gets the ball rolling.

In the initial state, Q1 off and Q2 is on.
When the input trigger is pulled low, the base of Q2 goes low which turns off Q2.
Collector of Q2 goes high and the base of Q1 rises slowly as C2 charges. Eventually Q1 turns on. In the meantime, C1 charges and brings up the voltage on the base of Q2. Q2 is on again and the output signal goes low.
The base on Q1 goes low and the circuit is back to its original idle state.

#### michael8

Joined Jan 11, 2015
137
A confusing circuit since the positive going pulse is going in reverse through the 1N4007. It would make more sense if
the input pulse was normally high and went low to trigger.

#### MrChips

Joined Oct 2, 2009
23,531
This is just for illustration.
I bet the circuit was drawn with some kind of CAD software and there was no provision to show a negative going pulse.