THIS 2-CHANNEL vibration sensor costs almost nothing to make but is sensitive enough to detect a cat walking past on a wooden floor!

To make it, you’ll need a discarded (but still working) cassette deck that has VU meters (these can be either analog or digital) plus a couple of loudspeakers, which can be easily salvaged from an old stereo TV. If you can’t score that lot for under $5.00, you’re not really trying.

High-gain preamplifiers

The unit takes advantage of the fact that a cassette deck uses two high-gain preamplifier stages that work with very small signals. Normally, these signals are read off the tape by the heads but what we do here is feed in new signals which are derived from coils of wire moving in a magnetic field. And since loudspeakers have very strong magnets, coils with lots of windings and very small internal clearances, they make ideal sensors for our vibration detector.

It looks like a $1000 instrument but costs less than $5 to make. This 2-channel vibration detector is actually based on a slightly modified cassette deck and uses conventional loudspeakers as vibration sensors. It's sensitive enough to detect a cat walking past on a wooden floor.

If the speaker basket (or frame) is firmly attached to the ground and a vibration occurs, the basket and the cone will tend to move at different rates. For example, if there is a sudden movement upwards, the inertia of the cone means that it gets left behind for a moment.

As a result, the magnet will move in relation to the coil (which is attached to the cone) and a small voltage will be generated.

This voltage is amplified and dis-played on the cassette deck’s VU meters. The greater the needle deflection, the greater the amount of vertical vibration that has occurred.