Build a Magno-Sniffer using a hall effect sensor, 555 oscillator circuit, piezo speaker, tongue depressor, hot glue, and a few other doo-dads.
Step 1: Whatsit Do?
Step 2: Disclaimer
Some knowledge of electronics and soldering.
Safe handling practices for ESD (Electrostatic Discharge) sensitive devices like the CMOS 555 or flagrant disregard thereof.
Step 3: Circuit Chunks
VCO - Voltage Controlled Oscillator. A circuit that oscillates (makes a tone) at some pitch. The pitch changes when you feed it a changing voltage from the sensor.
Piezo Speaker - Changes the electric signal from the VCO to an annoying audible tone.
Battery, Regulator, and Switch - 9V battery power is reduced to 5V by the regulator because this hall sensor doesn't want to see more than 6V power. Pushbutton switch switches power like switches do.