LM311 ZCD out to LM555 reset

Question

I have no background in electrical engineering other than curiosity; I'm more of a copycat. I have been trying to reproduce this schematic:

I have searched the forum but don't see anything remotely close to what I am doing.

I have built it as per the schematic. I did add an LED to pin 7 of the LM311 as an indicator the signal is working (one thing of note: I can only get a signal on output if I press the "normally open" momentary button).

I am feeding the audio through a BT module. When I scope the output I do get a reading:

I have no idea if it's correct. I never used a scope before.

When pin 7 of the comparator is connected to the reset pin of the 555 I should get a polyphonic square wave (I believe) from pin 3, which drives a BC327 transistor which will in turn operate the gate of an IRFP260 MOSFET.

The two red LEDs tell me the 555 and the BC327 are both outputting power constantly. They should both be flashing if the reset pin is being triggered properly. I don't know if I need a pulldown resistor on pin 4 or if my output from pin 7 is not switching to low as I understand it should. Any help is appreciated.

Here's a link to the finished project: https://youtu.be/DBukpYea4mQ

Answer 1

Perhaps you have a grounding problem.

If the voltage source powering the bluetooth audio device shares a common ground with this circuit, then it could easily upset the conditions that your circuit so carefully sets up.

R2, R3 and C2 establish a more-or-less constant 3.8V at the top of C2. This is the average "centre potential" about which the input signal potential must oscillate.

Usually, if a completely independent, electrically isolated audio input signal is presented at the input jack, all will work well, because your circuit holds one side of the input at its own +3.8V, while the other side wiggles up and down around that.

However, if the input source is not isolated, that audio signal will oscillate up and down around its own average D.C. offset. Connecting it to your input jack here will result in a war between two parties over which gets to control the potential at the top of C2. The winner will be the source (+3.8V vs. audio D.C. offset) has the lowest impedance. In layman's terms, the winner is the one who insists the hardest.

To alleviate this problem, you have two solutions:

1. Completely isolate the audio source from this circuit, by using completely independent supplies (like two separate batteries), and remove any common ground connection between the two parties. This allows the ZCD to adopt its own ground potential relative to the signal source:

^{simulate this circuit – Schematic created using CircuitLab}

2. "A.C. couple" the audio source to your circuit's input with a capacitor. This allows the two circuits to share the same ground, because the capacitor will charge to some average D.C. "offset" to "span" the potential gap between +3.8V and whatever average value the audio source has.

^{simulate this circuit}

Answer 2

Rebuilt it and took my time. Used all the same components and didn't crowd them together. Now it works as designed. Don't know how to mark this solved.