Photo from ?? Thanks, Mike Guerrucci.
This is a simple device. That means it’s just a buzzer and a battery. And it has a super sensitive magnetic switch. This detector is always open until it is disturbed by a magnetic field. They call it a magnetometer.
One way to make this is to use a compass. The compass needle is set to point so it’s between but doesn’t touch two wires. If the magnetic field is disturbed it will touch one or the other wire.
BZZZT!
Easy peasy!
These are common in California. Why?
The seismologists use them along with tiltmeters and seismometers to detect changes in the earth to tell if an earthquake is eminent.
The circuit may use a relay that latches it on once it has triggered. The button on the front may be to reset it to stop buzzing.
If I would build this I would use a capacitive detector for detecting the needle movement. It could improve the sensitivity many times. When the needle moves the distance between plates changes and the capacitance change will cause the circuit to change. It could change frequency, or timing of an oscillator. Or it could change the balance of a bridge circuit, similar to a wheatstone bridge.
