This device is good for listening to movement and vibrations through doors and floors. Picks up on the smallest noise and is useful for using before entering rooms. I push the mic to the surface (door/floor) and stay completely still so as not to move it/provide extra noise myself.
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Made with a few bits of wire, dissasembled bits of a £1 kiddies "spy ear" and a battery pack. Then tinkered a bit. Runs off of a single AA battery, has a variable volume control and On/Off at the low end of the volume control.
Outputs to a normal size headphone jack which I'm currently making a one ear headphone for it, DJ style-ish!