I have built Tony Messina's bat detector (http://pw1.netcom.com/~t-rex/BatDetector.html) and after putting it in an enclosure (small Pactel type), it dawned on me that I should add a power switch. Now it wouldn't be hard just to add a power switch, but then I thought it would be nice if the device just turned on when you plugged the ear phone in. The phone jack I'm using is a mono jack with a shunt lead which gets disconnected from ground when an ear phone is plugged in.

So, the question is does anyone have any ideas for a way to turn on the device when the ear phone is plugged in but draws almost no current when off? (A different ear phone jack would work, of course.)

Some more specs: the circuit draws about 15ma and uses a 9v bttery. I think I could tolerate 1 uA in power down mode, but less would be better. Currently I'm investigating using a MOSFET (2n7000 because that's what I have) with a 10 M ohm pull-up on the gate.

 

You have answered your own question:
The circuit as drawn shows one way to do it.

JimB

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