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I have an arduino mega that will be communicating through serial to a pc. One of the things I want it to tell the pc is whether or not it is connected to a wall power to 12V converter (where there is a switch between the arduino and the wall power). I was just wondering if there was a way to do that in software without having any additional hardware.

Thanks!

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No, you can't do it without extra hardware.

You need to pass VIN through a voltage divider so it sits within 0-5V when powered by 12V, then feed that into an ADC input. Protect the on-board voltage regulator with a diode between 5V (anode) and VIN (cathode). Then you can measure the voltage on VIN and make a decision on if it is around 4.5V or 11.5V

A 22K/10K voltage divider will reduce 12V down to 3.75V, which gives plenty of "headroom" for over-voltage situations (where your 12V unregulated power supply actually provides, say, 15V):

enter image description here

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  • I don't think the diode is needed. You say it is needed for protection. Are you sure the diode is needed? – jantje May 24 '16 at 11:52
  • Yes. Without it you will be drawing current backwards through the 5V voltage regulator when powered by USB. The diode bypasses the regulator when VIN is at a lower voltage (i.,e., not connected) than 5V. – Majenko♦ May 24 '16 at 11:54
  • Ah, oh well. Thanks for the hardware implementation too, at least its super simple! – NotCras May 24 '16 at 17:29

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