I have a PCB with 3 buttons on it, each button connected in series with a different value resistor forming a voltage ladder (schematic below). On the same PCB, I also have connected a speaker. The negative terminal of the speaker is connected to the ground plane of the PCB and then the positive terminal is connected to a pin on the PCB.
This entire PCB is actually the handheld microphone of a CB radio and is connected to the main unit through a cable. The sensing of the analog voltage from the buttons is done by an IC on the PCB of the main unit, on the other end of the cable. The problem is that whenever audio is played back through this speaker (be it even static noise) it somehow messes up the buttons. If there is no audio, the buttons work correctly. If there is audio, buttons no longer respond.
Can anyone shed some light as to why / how the speaker interferes with the voltage ladder button array and how I can prevent this interference from happening and have the speaker play audio and have functional buttons at the same time?
I have a small CB radio transmitter / receiver that has a handheld microphone. Originally, the microphone had 3 buttons (PTT, channel up and channel down) and, obviously, the actual mic. The speaker is placed inside the actual CB radio unit.
My issue is that the original speaker isn't loud enough to be heard, especially with the AC on. I noticed that the handheld microphone also had what seemed to look like a slot for a speaker, so I took it apart and, indeed, it had a slot to comfortably fit a 35mm diameter speaker. Maybe a different CB from the same line up came with a speaker in the handheld mic. So I thought why not take it apart and add a speaker so that I can hold the mic close to my ear so I can hear better.
I then began studying the circuitry inside the mic and this is what it looks like:
Notice the Extra pin. That pin was not connected to anything on the PCB. The traces for that pin seem to lead to an extra slot for another resistor but the trace ends there and is not connected to anything.
I bought a 35mm speaker, glued it in place, soldered the positive terminal to the Extra pin on the PCB and the negative terminal to a free ground plane via on the PCB.
Finally, I opened up the main unit, traced the cable from the mic and I noticed that the extra pin on the mic was actually connected to the ground plane on the main board (when I was testing for continuity on the microphone PCB, the cable was unplugged from the board so I did not notice that the extra pin and the GND pin were connected). I desoldered the old speaker, then the cable connected to the extra pin and then I soldered the cable to the pin on the PCB that was connected to the positive terminal on the old speaker.
So now I have a new speaker whose negative terminal is connected to the ground plane on the mic PCB, positive terminal is connected to the extra pin, and finally the extra pin connected to the speaker+ pin on the main board.