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I am building a tiny project using an android phone -- it simply has to drive a transistor switch on and off. Instead of going down the route of trying to interface to a micro controller via USB/Bluetooth, etc., I thought it would be easy to drive the switch through the headphone jack: play a sound through the jack and have the sound translated into a DC voltage that can drive the transistor.

I found this post, but it didn't seem to lead anywhere.

EDIT: A bit more searching led me to an LM2907, which seems like it might do the trick.

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The signal from the headphone socket on the android may be enough to activate a transistor or, it may not be enough without resorting to some form of voltage amplification. Given that you probably won't want to use a small battery to power the amplifier it may be sensible to consider a small voltage step-up transformer. It may not work effectively at bass frequencies but it should be able to be made small enough for operating at mid-range audio frequencies. The output would need to be rectified and smoothed and then fed to the base of a BJT via a base resistor. It may be suitable for a MOSFET too.

Another option is an energy harvesting chip such as: -

enter image description here

This will take the rectified and smoothed signal from a low voltage audio device and convert it to a voltage suitable for turning on a mosfet.

An LM2907 is unsuitable for turning a small dc voltage (rectified audio) into a voltage suitable for driving a BJT or mosfet - it needs a power supply to operate and it's applications are not in this field.

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