Help with monostable multivibrator

[jj5]

Hi there. I'm trying to assemble and test the circuit shown below. I'm fairly sure I have followed the instructions properly and the circuit is wired as shown. There is one mistake I found in the schematic and that is that the 'E' and 'C' labels for the emitter and collector of Q1 are on the wrong side. The thing is, this circuit doesn't seem to behave like a monostable multivibrator. The astable multivibrator on the right formed with Q3 and Q4 works properly to produce the tone, but the monostable multivibrator on the left made from Q1 and Q2 doesn't seem to be working. I was expecting the buzzer to sound for a short period and then turn itself off, but what happens instead is that the buzzer goes on and off immediately when the light source is varied over the CdS cell. Is the circuit diagram for the monostable multivibrator correct? My hunch is that maybe the 10uF and 33K components in the monostable multivibrator need to be flipped? Thanks very much if you can help me with this. I can provide more details if needed.

[madires]

The holdtime (of the unstable state) of the monostable multivibrator is about ln(2)RC. With R = 33 kOhms and C = 10 µF t is about 0.23 s which is quite short. For a longer beep you could increase the value of the capacitor. 22 µF would increase the holdtime to 0.5s, 33 µF to 0.75 s, and 47 µF to 1 s.

[jj5]

The holdtime (of the unstable state) of the monostable multivibrator is about ln(2)RC. With R = 33 kOhms and C = 10 µF t is about 0.23 s which is quite short. For a longer beep you could increase the value of the capacitor. 22 µF would increase the holdtime to 0.5s, 33 µF to 0.75 s, and 47 µF to 1 s.

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.

[madires]

Which transistor are you using for Q1/Q2? Could be an hFE issue.

[jj5]

Which transistor are you using for Q1/Q2? Could be an hFE issue.

The transistors are NPN transistors marked STC 945 G N2.

[jj5]

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:

[wasedadoc]

Typical silicon transistors have a base to emitter reverse breakdown voltage of only 5 to 6 Volts. Running those circuits from 9 Volts exceeds that. The best case result is that the actual periods when a transistor is off, is significantly shorter than the calculated time.

[jj5]

Typical silicon transistors have a base to emitter reverse breakdown voltage of only 5 to 6 Volts. Running those circuits from 9 Volts exceeds that. The best case result is that the actual periods when a transistor is off, is significantly shorter than the calculated time.

I tried running the circuit on 5V instead of 9V and it behaved the same way. That is, the LED lights when light is removed, and the LED goes off then light is applied. There is no unstable state that resets after a period.

[madires]

The STC945's datasheet states an hFE between 70 and 700, a large range. You could increase the base current by changing the 33 kOhms resistors to 10 kOhms, and check if this helps.

[madires]

Typical silicon transistors have a base to emitter reverse breakdown voltage of only 5 to 6 Volts. Running those circuits from 9 Volts exceeds that. The best case result is that the actual periods when a transistor is off, is significantly shorter than the calculated time.

To solve that problem one can insert a diode between the cap and the transistor's base (to prevent the negative base voltage).

[jj5]

The STC945's datasheet states an hFE between 70 and 700, a large range. You could increase the base current by changing the 33 kOhms resistors to 10 kOhms, and check if this helps.

I put 10 kOhm resistors in parallel with the two 33 kOhm resistors but the behaviour of the circuit didn't change. I tried testing at both 9V and 5V but the behaviour was the same.

[madires]

Then back to the original circuit and change the 33 kOhms resistor at the cap and the base of Q1 to something like 47 kOhms.

[jj5]

To solve that problem one can insert a diode between the cap and the transistor's base (to prevent the negative base voltage).

I put an 1N4007 between the base of Q1 and the 10uF cap. I will leave this in the circuit unless instructed to remove it. The addition of this diode didn't seem to change the behaviour of the circuit at all.

[jj5]

Then back to the original circuit and change the 33 kOhms resistor at the cap and the base of Q1 to something like 47 kOhms.

I put a 47 kOhms resistor in place of the 33 kOhms resistor (and also the diode you suggested) but the behaviour of the circuit remains unchanged.

[Zero999]

The time delay is only 230ms,

I expect the monostable is working.

The monostable is triggered by a brief positive pulse on the base of Q5, causing it to activate the buzzer.

One quirk of this circuit is, if the pulse lasts longer than the time delay, then it will remain on until the pulse terminates.

To confirm this, increase the capacitor value to something like 47µF or 100µF, to make the delay longer, so it's easily perceivable.

Now, briefly flash the light at the sensor and the LED should turn on for about 1 to 2 seconds, before going off.

[jj5]

The time delay is only 230ms,

I expect the monostable is working.

The monostable is triggered by a brief positive pulse on the base of Q5, causing it to activate the buzzer.

One quirk of this circuit is, if the pulse lasts longer than the time delay, then it will remain on until the pulse terminates.

To confirm this, increase the capacitor value to something like 47µF or 100µF, to make the delay longer, so it's easily perceivable.

Now, briefly flash the light at the sensor and the LED should turn on for about 1 to 2 seconds, before going off.

I put 100µF in parallel with the 10µF capacitor and the behaviour of the circuit is unchanged. I'm going to rewire the whole thing to ensure I haven't made a mistake.

[madires]

Bummer! Have you tried with turning the potentiometer up/down?

BTW, as long as the light intensity is too low Q5 stays on and shorts the base of Q1 (Q1 is off). This drives Q2, enabling the buzzer/LED. The monostable multivibrator's holdtime is triggered by the light intensity going up again (Q5 off).

[jj5]

Bummer! Have you tried with turning the potentiometer up/down?

Yes. When the pot is low the LED flashes on and off slowly (around 2Hz). When the pot is high light on the CdS cell either turns the LED constantly on (when there is no light) or constantly off (when there is light).

BTW, as long as the light intensity is too low Q5 stays on and shorts the base of Q1 (Q1 is off). This drives Q2, enabling the buzzer/LED. The monostable multivibrator's holdtime is triggered by the light intensity going up again (Q5 off).

Oh I thought the initial pulse was supposed to drive the whole thing. If the multivibrator's holdtime is triggered by the light going back up, then perhaps what we're seeing is to be expected? I have to admit I am quite confused.

[Zero999]

The time delay is only 230ms,

I expect the monostable is working.

The monostable is triggered by a brief positive pulse on the base of Q5, causing it to activate the buzzer.

One quirk of this circuit is, if the pulse lasts longer than the time delay, then it will remain on until the pulse terminates.

To confirm this, increase the capacitor value to something like 47µF or 100µF, to make the delay longer, so it's easily perceivable.

Now, briefly flash the light at the sensor and the LED should turn on for about 1 to 2 seconds, before going off.

I put 100µF in parallel with the 10µF capacitor and the behaviour of the circuit is unchanged. I'm going to rewire the whole thing to ensure I haven't made a mistake.

Q5 is discharging the capacitor, when the other side is shorted out by Q2. Q1's base needs a diode connected in series to prevent this. It's not just there to protect the base-emitter junction, but is part of the circuit.

[jj5]

Q5 is discharging the capacitor, when the other side is shorted out by Q2. Q1's base needs a diode connected in series to prevent this. It's not just there to protect the base-emitter junction, but is part of the circuit.

Thanks! I finally got this circuit to work! Things I needed to do:

• put a diode on the base of Q1
• change the resistor on the base of Q1 from 33kΩ to 47kΩ
• put a 220μF cap in parallel with the 10μF cap

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.

Thanks to everyone for helping me with this one! <3

[Zero999]

Good, I'm glad you got it working.

Here's a SPICE simulation on the circuit.

Note how the voltage on the RC circuit (yellow trace) goes negative, when Q2 turns on. Without D2, Q3 simply shorted it straight to 0V.

It's really frustrating when there are errors in circuits, but fixing them is good learning experience.

[jj5]

Here's a SPICE simulation on the circuit.

Excellent. Thanks! What software do I need so I can do my own SPICE simulation?

Note how the voltage on the RC circuit (yellow trace) goes negative, when Q2 turns on. Without D2, Q3 simply shorted it straight to 0V.

I see. Thank you.

It's really frustrating when there are errors in circuits, but fixing them is good learning experience.

Yes, it's all good learning experience!

Speaking of learning, why did you set R4 at 1kΩ for your simulation? I had to pick a current limiting resistor myself and I picked 220Ω, much less than your 1kΩ. Is 1kΩ a good value to use in general for such purposes?

[Zero999]

Most people here use LTSpice. Unfortunately many people find the GUI frustrating and there are some unintuitive aspects to it, but it's free, is considered one of the most accurate simulators and lots of people here can provide support.

[jj5]

Most people here use LTSpice.

I will check it out, thanks!

I also had the question about why you set R4 at 1kΩ for your simulation? I had to pick a current limiting resistor myself and I picked 220Ω, much less than your 1kΩ. Is 1kΩ a good value to use in general for such purposes?

[Zero999]

Most people here use LTSpice.

I will check it out, thanks!

I also had the question about why you set R4 at 1kΩ for your simulation? I had to pick a current limiting resistor myself and I picked 220Ω, much less than your 1kΩ. Is 1kΩ a good value to use in general for such purposes?

No particular reason. Modern LEDs tend to be bright enough at 3mA, so 1k tends to be my go to value for 5V. If it's not bright enough, then reduce it.