Hi,
This setup (schematic below) was once working perfectly for 2 months. But suddenly it burnt out. It's a mystery to me why, as when I remake it, the replacement UNOs burn out almost immediately before any script is actioned or buttons pressed.
It uses a 12 volt adapter, and now I've even replaced that as well. The chip that seems to heat-up to the point it hurts to touch it, is circled in the pic below. Help me, it's driving me crazy! 
Is the Uno plugged into USB? When you turn off the Standby switch then everything will be drawing power via USB, which could burn out the fuse (which is the component you have circled).
BJHenry:
Is the Uno plugged into USB? When you turn off the Standby switch then everything will be drawing power via USB, which could burn out the fuse (which is the component you have circled).
Hello. When I plug in only the USB into the UNO to upload, it is fine. But as soon as I make the schematic you see, the chip burns up when only the USB plugged in. When I unplug the USB and use the 12 volt supply instead, this chip below it burns up instantly (circled).
Could this have something to do with the speaker in that it's not drawing much power and so it's overloading everything?
4th NANO burnt up This is costing me a fortune.
Ok. I've got some good news for you- the problem isn't hard to overcome, and you can still use your damaged Unos.
The problem is that your circuit is drawing more current than the Uno can supply. When you have the Uno plugged in to USB this overdraw flows the fuse, which is the part you've circled in the last photo. When you have the Uno powered by external power then the overdraw damages the 5V regulator, which is the part that you've circled in the second photo.
I'm having trouble seeing exactly what else is in your circuit since I don't want to deal with Photobucket nonsense (see here for a better way to post photos on these forums). From what I can see you're running an SD card and some sort of speaker all from the Uno's 5V pin. I reckon the speaker is going to what's causing your problems. If you can run the speaker straight from the input voltage (ie the power coming through the Standby switch) then you should do that. If not, you should change the input voltage to 5V and connect it to the 5V pin of your Uno, rather than the VIN pin.
If you use the second method it will allow you to keep using the damaged Unos too, since you won't be using USB power or the onboard regulator.
BJHenry:
Ok. I've got some good news for you- the problem isn't hard to overcome, and you can still use your damaged Unos.
The problem is that your circuit is drawing more current than the Uno can supply. When you have the Uno plugged in to USB this overdraw flows the fuse, which is the part you've circled in the last photo. When you have the Uno powered by external power then the overdraw damages the 5V regulator, which is the part that you've circled in the second photo.
I'm having trouble seeing exactly what else is in your circuit since I don't want to deal with Photobucket nonsense (see here for a better way to post photos on these forums). From what I can see you're running an SD card and some sort of speaker all from the Uno's 5V pin. I reckon the speaker is going to what's causing your problems. If you can run the speaker straight from the input voltage (ie the power coming through the Standby switch) then you should do that. If not, you should change the input voltage to 5V and connect it to the 5V pin of your Uno, rather than the VIN pin.
If you use the second method it will allow you to keep using the damaged Unos too, since you won't be using USB power or the onboard regulator.
Thanks so much for the feedback 
I thought I was going mad thinking I had some sort of static charge on me lol... I shall give it a go as well as using something better than Photobucket 
I blew 6 Unos voltage regulators before I wised up and stopped using the 5V pin! Get a buck volt reg for about a buck each on amazon, aliexpress, etc.
sillyface:
Thanks so much for the feedback
No worries, I'm glad I could help
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.
SupArdu:
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?
Using an external amplifier is the best way to do it, and power the amp from the external power not 5V.
