A door opening alarm circuit is a security alert device which activates while opening the device attached to the door. And the alarm remains ON as long as the door is kept open.
Reed Switch:-A reed switch is an electrical switch activated by a magnetic field. It looks like a small tube like device contains 2 ferrous materials leads.
Reed switch comes in 2 forms
1. Open switch
2. Closed switch
1. Open Switch:- When any magnetic field comes near to the reed switch, it closes the contacts and creates a conducting path.
2. Closed Switch:-When any magnetic field comes near to the reed switch, it opens the contacts and breaks the conducting path.
Alarm circuit using transistor diagram:-
The above circuit is a simple switching circuit using transistor. Here we are using open reed switch. When the door is closed, the reed switch shorts the base-emitter of the transistor which switches of the buzzer. So the alarm will remain OFF. When a magnet is placed near the reed switch, short-circuit across base-emitter gets removed and the transistor switches to ON state and the buzzer will get ON.