The circuit diagram of window foil burglar alarm has been published here. The main parts of this circuit are SCR H1 (TYN 612) and transistor Q1 (BC 107). This burglar alarm can be fitted in windows to check breaking attempts.
Here the sensing element of break attempt is a network of fine wire element fitted through the window glass. At normal condition, the transistor Q1 will be in off state since its base is connected to the ground. Then SCR will be in non-conducting state since it has not been triggered. When a break attempt of window comes, then the wire elements will break and the base of transistor will connect to positive potential. This turns on Q1 and the output of it triggers SCR. This turn on of SCR turn horn to blow and the circuit remains on unless power supply is switched off.
- To setup the circuit, make switch S1 open and adjust R2 to get a 1V between base of Q1 and ground. Now transistor will be conducting and horn will be blowing.
- Now, close S1 and reset the power supply .The transistor Q1 will be off and horn will not be blowing. Then circuit is ready.
- Make a loop of very fine wire through the window glass to make the window foil (two or three lines running in both direction is enough).