never built one, but used one. They are nothing but a coil, insulated wire, and a DCV source. The one I used had a bridge circuit, used 120vac, and it also had a 10 amp circuit breaker. The more turns of wire, the stronger the MF. The more current the stronger the MF.
The difference between a magnetizer and a demagnitizer is you use ACV instead of DCV. Pull the object through the varying MF to demagnetize.
Should be very simple to build.