Circuit Diagram
Description
The door latch alarm circuits are simple and effective and easy to
construct. Normally the alarm is in de-activated mode by keeping the
switch s1 in off state which can be done with a lock and key kind of
switch. Number of doors can be guarded against the intrusion by keeping
or connecting the reed switch in series fitted with the help of a
magnet.
The reed switch can be fitted in such an way that the magnet is
fixed on the door, in normal conditions when the door is closed reed
switch will be held close to the magnet and hence the reed switch is
kept closed , when the door is opened the reed switch contact is opened ,
this triggers an SCR, the gate of an SCR is connected to its cathode
through the reed switch as long as the switch is closed the SCR dose not
fire but when the door is opened the SCR is fired supplying an voltage
to the alarm circuit.
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