I guess the problem is the speaker. I cannot read the details in your circuit diagram. But if the speaker is connected to 5V and the MOSFET is on continuously for some time the speaker is drawing a lot of current. Therefore insert a capacitor, 10 or 20 uF, in series to the speaker.
Thanks for the advice.
I’m currently getting different opinions and will be going through them all until it’s fixed. I tried similar advice regarding controlling the power of the speaker below, but burnt out my 5th UNO…
“The reason that the power regulator and the fuse get damaged is that the speaker is just 8 ohm. The current draw of that branch of the circuit is I = V/R = 5/8 = 625 mA, which is too much for the Arduino regulator to supply. You can use a speaker with higher impedance, or add a series resistor to the speaker (+) connection. Just around 100 ohm should be enough to get the current draw to safe levels (I = 5/108 = 46 mA). I don’t recommend powering the speaker directly from the +12 V line, since the current draw then would be 12/8 = 1.5A. Even though the power source might be able to source that current, I doubt the speaker would survive it.”
The next piece advice is this, which I’ve placed an order for the part…
“Replace the single MOSFET with actual amplifier, something like this https://www.instructables.com/id/PAM8403-6W-STEREO-AMPLIFIER-TUTORIAL/
That way when something gets destroyed due to a faulty speaker it’s the cheap amplifier board, not the Arduino”
Before I burn out my 6th UNO, does anyone feel this might be worth a shot?