Help with monostable multivibrator

[vk6zgo]

Thanks! The thing is at the moment there is no hold time at all, the buzzer just stays on. I will make a video to show you the behaviour, give me an hour or two.

Here is the video demonstrating the problem:

If Q5 turns on, the base of Q1 goes low, turning Q1 off, so the description is incorrect.

You seem to have multimeters coming out of your ears, so initially look at the collectors of Q5 & Q1.
By my reading of the schematic, Q5 collector voltage will be high when the LDR is illuminated, because its resistance will be low, taking the voltage at the point where Q5 base is connected below 0.6v, cutting off the transistor.
Q5 collector is connected to Q1 base, biasing it on, so its collector voltage will be low.

Break the light & Q5 will turn on, its collector voltage will fall, cutting off Q1, whose collector voltage with go high & so on.
As far as I can see, the astable M/V is always operating, & all the monostable does is connect it to the buzzer by biasing Q6 on. The base of that transistor is already fed with the  astable output.

The best way to troubleshoot that circuit is with an Oscilloscope, but I've guessing you don't have one.

[jj5]

The best way to troubleshoot that circuit is with an Oscilloscope, but I've guessing you don't have one.

I have a scope on my bench! An Rigol MSO5074. It has four channels and all my probes are set at 10x attenuation. What points of interest should I look at?

[vk6zgo]

The best way to troubleshoot that circuit is with an Oscilloscope, but I've guessing you don't have one.

I have a scope on my bench! An Rigol MSO5074. It has four channels and all my probes are set at 10x attenuation. What points of interest should I look at?

Using only one probe, connect the ground clip to the negative terminal of the 9v battery (marked "2" on your schematic).
With the LDR illuminated, look at Q5 collector with your 'scope set to DC coupling---note if the voltage is around +9v(high) or very much lower.(low).
If Q5 is an NPN transistor as drawn, with the LDR in its low resistance condition, Q5 should be turned off, with its collector "high".
Following the schematic, if Q5 collector is "high", Q1 base should also be "high", turning that transistor on.

Now remove the illumination of the LDR----rechecking Q5 collector, it should be "low", as should be Q1 base.

This will prove out the operation of the LDR & Q5 circuitry.

You can now follow the schematic through the monostable, but I would suggest now going to the emitter of Q6.
If your mono is working, Q6 emitter should go "low" when you remove the light from the LDR, turning that transistor on & making the alarm sound.
After some time, the mono should time out, taking Q6 emitter high & turning the transistor & alarm off.

From my reading of the schematic, the astable should be continuously running, so you should see its output appearing at Q6 base.

It is incredibly tiresome writing (& no doubt reading) these troubleshooting steps, whereas if I were standing alongside you in your lab, they would be but a matter of seconds.
I also know that there are always "vultures" waiting to pick such dissertations to pieces!

[Zero999]

And one thing I didn't understand is that the multivibrator won't start transitioning back to its stable state until *after* the stimulus is reapplied.

Swapping the diode for a small capacitor, which makes it AC coupled will change that.

Now the monostable is edge triggered. Note that C2 blocks DC and slowly changing signals, so it doesn't hold C1 at 0V, when Q3 is continuously on.

[jj5]

And one thing I didn't understand is that the multivibrator won't start transitioning back to its stable state until *after* the stimulus is reapplied.

Swapping the diode for a small capacitor, which makes it AC coupled will change that.

Now the monostable is edge triggered. Note that C2 blocks DC and slowly changing signals, so it doesn't hold C1 at 0V, when Q3 is continuously on.

Excellent! Thanks very much for explaining this to me. If it's called edge triggered when it is AC coupled with a capacitor, what is the triggering called when it uses the DC pulse and a diode, as with our earlier design?

[Zero999]

And one thing I didn't understand is that the multivibrator won't start transitioning back to its stable state until *after* the stimulus is reapplied.

Swapping the diode for a small capacitor, which makes it AC coupled will change that.

Now the monostable is edge triggered. Note that C2 blocks DC and slowly changing signals, so it doesn't hold C1 at 0V, when Q3 is continuously on.

Excellent! Thanks very much for explaining this to me. If it's called edge triggered when it is AC coupled with a capacitor, what is the triggering called when it uses the DC pulse and a diode, as with our earlier design?

They're both edge triggered I suppose. It's just the capacitor ensures it doesn't remain triggered, if the input is still high, once the time cycle completes.