Normally closed contact anti-theft alarm

 The circuit principle is shown in Figure 1. SCR , R1 and AN form a thyristor trigger switch circuit; IC1, R2, VT1, VT2 and BL form an analog siren circuit. Normally, AN is oppressed by household appliances, causing its two normally closed contacts to disconnect. The SCR is blocked without a trigger signal, and the alarm does not work. When the household appliance is lifted, the AN contacts are automatically closed, and the triggering end of the SCR obtains the triggering signal from the positive pole of the power supply via R1. The SCR is turned on, and IC1 is energized to work. The siren acoustic and electrical signal output by its output end is powered by VT1 and VT2. Amplify and push the speaker to make a loud alarm sound. At this time, even if the home appliance is put back to its original place or the AN is destroyed, the alarm sound cannot be stopped. Only when the owner opens the switch S, which is installed together with the alarm and is difficult for others to find, can the alarm be released.