I have put together:
- an Raspberry Pi Pico that manages a keypad and
- a simple audio amp based on LM386N.
The audio amp is connected via GP22 which provides PWM and plays the role of sound-audio feedback on a successful key-in pressed. The system works correctly and audio beeps are played from the speaker attached. However there is unwanted noise that is coming out of the speaker, sporadically.
Specifically the parasite or noise is very precise and is repeated every time the wifi/mqtt publishes data to the network plus some other times, but definitely it doesn't seem to be random noise.
How do I tackle this? Of course I would prefer this to be as silent as possible.
Here is the schematic:
This is the pcb layout: