# Monostable multivibrator BJT

#### janraid_

Joined May 27, 2023
14
Hello, I'm a computer science student and I was assigned by my professor to make a monostable multivibrator circuit on a bread board with an unstable duration of three seconds. Keep in mind I have very little knowledge about circuitry, we were only given the schematic and were told to find the values for the components ourselves. After watching a YouTube video I got some of the values myself and ran the circuit on a simulator and the stable state worked fine, however when I applied a trigger pulse through the differentiator circuit the output (in this case tracked by a LED) turned on for less than a second then turned off. I can't determine the cause for this and as to why the light won't stay on for three seconds, I did the math for the time constant which is T = 0.693RC, at this point I'm thinking the differentiator circuit values are to blame.
view attached (source supply is 9V)

#### sghioto

Joined Dec 31, 2017
4,918
Multivibrator circuit again?
I just did this in Homework! You guys must know each other.
Anyway the formula for the OFF time of Q2 is: T = RC
Your timing components are a 5uf cap and a 1K resistor, T = .005 seconds
Change the values shown in the edited schematic and run that through the simulator.

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#### sghioto

Joined Dec 31, 2017
4,918
this point I'm thinking the differentiator circuit values are to blame.
What is a 15mf capacitor?

#### janraid_

Joined May 27, 2023
14

#### sghioto

Joined Dec 31, 2017
4,918
Good lord! Why so big? You only need a 100nf or less.
Have you tried my circuit modifications?

#### janraid_

Joined May 27, 2023
14
Alright so the light did turn off but the time interval is unknown since this simulator doesn't really show the time well, I'll try it on the breadboard today. Everything looks good thanks, however could you tell me why you chose 100nF and 1k for the differentiator circuit.

#### janraid_

Joined May 27, 2023
14
ps: what's the reason behind the light only turning on once and won't turn on again after that time until the circuit is reset.

#### sghioto

Joined Dec 31, 2017
4,918
Only need a brief pulse to turn ON Q1.
No need to load down the trigger input with a small 100 ohm resistor in fact a 10K works fine as well.
That's why it's called a monostable. It only has one stable state. When triggered it resumes that state after the delay time .has expired.

#### janraid_

Joined May 27, 2023
14
That makes sense. on the other hand, here's what I found out: 100k resistance is too much for the 2n2222 transistor I was using, it won't saturate (or so my professor said). I instead used a 10k resistor with a 300uF capacitor.

#### sghioto

Joined Dec 31, 2017
4,918
I instead used a 10k resistor with a 300uF capacitor.
Did that work?

#### janraid_

Joined May 27, 2023
14
Did that work?
yes in the simulator, however I still have yet to apply it on the breadboard.

#### sghioto

Joined Dec 31, 2017
4,918
OK. You can also try other values for R and C to figure out the smallest value cap required.
I would start at setting R = 20K and C = 150uf, keeping the time constant the same until the value of R becomes too high to saturate Q2.
On my breadboard the circuit worked fine with a 100K and 33uf cap using 2N2222A transistors.

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#### janraid_

Joined May 27, 2023
14
I did the circuit on the breadboard here's what I'm facing: the second I plug in the source battery, the light (output) turns on for 3 seconds (time interval) then turns off, it is not activated by the trigger pulse from the button. I'm using BC107 transistor could that be the problem? I'll try finding 2n2222 transistors

#### crutschow

Joined Mar 14, 2008
32,926
, it is not activated by the trigger pulse from the button.
Show the schematic for how the button is connected.
Do you have a resistor to ground at the button output to discharge the capacitor after it is pulsed?

#### sghioto

Joined Dec 31, 2017
4,918
I'm using BC107 transistor could that be the problem?
It's not the transistors or the LED would not have turn ON when first powered up.
Suspect the diode is in backwards or open. But as Cruts mentioned above you need to add a resistor.
Shown below is the 10K resistor needed.

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#### janraid_

Joined May 27, 2023
14
After retouching it a bit I got it to work and here's what I got, to be honest I'm quite happy with the result. I assume the initial light is the capacitor discharging and the second one is in response to my trigger. I did add that 10k resistor to the switch and I didn't notice much difference with or without.
https://streamable.com/ms21g8

#### janraid_

Joined May 27, 2023
14
PS don't mind the messy wiring this was merely a prototype, hopefully now I can work on making it presentable.

#### sghioto

Joined Dec 31, 2017
4,918
I don't see the 100nf cap on the board.
EDIT: I see the cap now but if it's installed correctly it will only trigger the circuit one time.

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#### janraid_

Joined May 27, 2023
14
Its there I just extended it's prongs a bit too much haha, but yeah here it is, it was out of shot in the video.

#### sghioto

Joined Dec 31, 2017
4,918
What is that connection on the side of the switch I circled in red?