The cheaper alarms are just two magnetic coils working in conjunction. There's a nice write by Hackaday on them - http://hackaday.com/2015/12/03/how-store-anti-

The cheaper alarms are just two magnetic coils working in conjunction. There's a nice write by Hackaday on them - http://hackaday.com/2015/12/03/how-store-anti-theft-alarms-work-magnetostriction/ http://hackaday.com/2015/12/03/how-store-anti-theft-alarms-w...