What's the purpose of the arduino? Couldn't you setup a push button that needs to be pressed every 5 minutes or so to keep the circuit active, like a watchdog?

What's the purpose of the arduino? Couldn't you setup a push button that needs to be pressed every 5 minutes or so to keep the circuit active, like a watchdog? You could also add a small piezo speaker or blinking LED to warn you to press it 30 seconds before the timeout. Or even more advanced, connect it to a computer/tablet through USB or bluetooth, mimic a keyboard/mouse and use your real keyboard key presses/mouse movement to reset the countdown. I wouldn't use this directly though, but instead through opto-couplers to avoid electric shocks or other safety issues! Related to watchdog, you should probably also activate the hardware watchdog of the microcontroller to make sure it doesn't crash. 27V at 100mA is 2.7Watt dissipation, quite a lot when in contact with skin!