A secret listening device can be used to eavesdrop in on conversations that you wouldn't otherwise be privy to. You can make an inexpensive one using a digital recorder and a stethoscope. This will allow you to listen to people talking through walls. However you should only use such a device in accordance with the law. So make sure that you do not use it at a property that you do not legally rent or own.
Buy a stethoscope and two multimedia microphones. The stethoscope is available at local chemists for just £6.50 as of 2010. You can use an old stethoscope if you have one. The multimedia microphone can be bought at a local electronics shop for around £9.70 as of 2010.
Disassemble the microphone carefully. You will notice two mini-condenser microphones that are installed inside the casings. Do not disconnect the mini-stereo plugs from the condenser microphones. Just remove the casings that enclose the microphone.
Remove the earpieces on the stethoscope and enlarge the holes using a drill so that they match the earpieces on the mini-condenser microphones
Put the mini-condenser microphones into the holes of the earpieces. Reassemble the earpieces on the stethoscope. If necessary use glue to stick the earpieces back into their position.
Connect the mini-stereo plugs into the digital mini-connector using the Y-connector. Place the chest piece of the stethoscope to a wall or door where you wish to listen to a conversation and press the record button on the recorder. You can also hide the stethoscope inside of a room with just the part that goes against the chest sticking out.