Hi, I've used this very simple audio amplifier to be able to hear serial mp3 on a small 8ohm speaker.
It works perfectly if I connect it to a separate 5V input (it's very robust in fact, works very well even when changing the input voltage or resistor).
However when I'm trying to use the exact same circuit with one of the Arduino Uno's output pins the speaker just makes a nonsense loud sound. It happens also when using the 5V output directly.
Any idea what I'm doing wrong?
Thanks!
I'm guessing you are supplying 5V (not 9V as show) to Vcc from your seperate 5V supply.
and "8 ohm" speaker is NOT a resisitor; it could easily draw over 1A from the supply, which will upset the arduino, whether you try to get the 5V from the arduino "5V" OR an output pin.
Are you providing the audio from the arduino Uno?
Two things to be careful of…
Is the 9V negative connected to the circuit ground
You really should use a diode to protect the FET from back EMF, or a small cap to pass only the ‘ac’ edges of the ‘audio’ waveform.
the audio is coming from a small mp3 serial module, which I am powering and controlling from the Uno. I see your point about current, I guess there's no way around it then... thanks
Thanks, where and which diode would you use for this circuit? The ground is separate, only the mp3 module outputs (a 3.5 audio jack wires) are connected to this circuit.
The diode should go across the speaker ‘pointing up’ to dump any negative spikes generated by the coil in the speaker.
The negative rail of BOTH power sources should be connected, so the FET has something to reference it’s Gate to when switching the output on and off.
Hi, @stevejolt
Welcome to the forum.
Can we please have a circuit diagram?
An image of a hand drawn schematic will be fine, include ALL power supplies, component names and pin labels.
Please include the UNO and the MP3 module.
Thanks.. Tom.. 
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