Bistable multivibrators in circuit using BJTs are triggering each other. Need a fix.

Thread Starter

Quincy96

Joined Jan 12, 2021
3
I'm trying to use 2N2222A BJTs as a toggle switch using the bistable multivibrator circuit connected to JFETs. I based it off of what BOSS uses in their guitar pedals, which is highlighted in the photo. I have 6 of these circuits set up and they are only connected to each other through Vcc and ground. Vcc is around 16V instead of 9V like BOSS does. I have 2 momentary switches hooked up to a rotary switch. The rotary switch is being used as a selector to control which toggle circuits will receive the triggering pulse from each momentary switch. I can only control 2 of the 6 BJT toggle circuits at a time (each connected to one of the momentary switches). However, I'm finding that triggering one of the BJT toggle circuits triggers some of the other ones sometimes even though they're not connected to the momentary switch that was pressed.

So my question is... Why would multiple BJT bistable multivibrators affect each other when hooked up to the same power source, and what can I do to keep them from affecting each other? Most of the collectors of the BJTs go to JFETs, and each multivibrator has an LED attached similar to the BOSS schematic. Most of the values are the same as the BOSS schematic, but the capacitors connected to the collectors are not equal so one side is favored on power up. Also one of the toggle circuits is using 62K resistors because I ran out of 56K. The zener diodes for the LEDs are 9.1V since Vcc is 16V instead of 9V. I thought the circuit would be pretty forgiving with values, so not much calculation went into this. An explanation would be greatly appreciated. BOSS Schematic.JPG
 

AlbertHall

Joined Jun 4, 2014
11,704
My first guess is that you need some decoupling capacitors across the supply to ground, I would suggest 100nF with the shortest wires possible, one for each bistable.
How is this constructed?
 

Marley

Joined Apr 4, 2016
450
Why is Vcc 16V instead of the 9V the circuit is designed for?
All 6 of the circuits need to be well de-coupled from each other.
If you have a 16V supply, one way to do the decoupling would be to use a 9V linear regulator for each circuit. Power all of them from the 16V supply.
 

AnalogKid

Joined Aug 1, 2013
9,639
The Boss schematic is good as a reference, but we need to see your schematic for your circuit, including power sources, switches, connectors, etc., to really evaluate the circuit's performance.

Photos of the construction will help.

ak
 
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Thread Starter

Quincy96

Joined Jan 12, 2021
3
My first guess is that you need some decoupling capacitors across the supply to ground, I would suggest 100nF with the shortest wires possible, one for each bistable.
How is this constructed?
Why is Vcc 16V instead of the 9V the circuit is designed for?
All 6 of the circuits need to be well de-coupled from each other.
If you have a 16V supply, one way to do the decoupling would be to use a 9V linear regulator for each circuit. Power all of them from the 16V supply.
Vcc is 16V because the main circuit that these multivibrators are going into is running on 16V. I did try using a zener diode to create 9V for the multivibrators, but since that didn't change anything I went back to 16V. Everything is being tested on three breadboards with two of the multivibrator circuits on each breadboard. Regarding the problem, I was able to fix it using another version of the BJT bistable multivibrator circuit. The switching isn't as consistent though. I heard that using the 470pF capacitors from the collector of each BJT to the base of the other BJT like shown in the BOSS schematic helps make the transistor switching "faster and surer", but it's making the switching worse when added to this version and the LEDs don't fully turn off with them added. I will try adding decoupling and see if that will fix it too so I can keep using the version in the BOSS circuit which has more reliability. Thank you all for your help so far!
Electronics Tutorials Bistable Multivibrator.JPG
 

Thread Starter

Quincy96

Joined Jan 12, 2021
3
I was hoping to see your schematic, not something lifted from a website.

ak
I understand. That was just so you'd have a better depiction of the circuit so you could help right? I feel like I'm at a point where I can move forward since I have at least one way to fix the problem now, so I didn't think it would be needed anymore. The comments on decoupling caused me to look at some YouTube videos and I came across one that I'll link here. I think my issue is related. I don't even have a full schematic created for this part of my project, but if I find that I could use further assistance I'll put something together. Thanks for responding!
Decoupling circuits from supply rail. " Is it really necessary"?
 

AlbertHall

Joined Jun 4, 2014
11,704
The comments on decoupling caused me to look at some YouTube videos and I came across one that I'll link here.
That's a good demondtrstion of the problem and solution.
Exactly what and how much decoupling is needed will depend on the circuit and the layout. It is difficult to predict so it is usually a good idea to incude it.
 
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