Based on logic gates and NE555 timer this burglar alarm features two door sensors and an activation switch.
The door sensors can be connected in NO or NC
configuration because the circuit detects the change in their condition.
When the activation switch is open the input pins of the IC2 timer are shorted to the negative pole of the power supply.
In that condition the burglar alarm does not work because the collector of the Q12 transistor keeps off the Q3-Q4
By closing the activation switch the output of the IC2 timer takes the low level with a time delay depending on the C5
capacitor. The LED indicates that the burglar alarm is now working. If the circuit detects a change in
the condition of a
door sensor the output 3 or 11 of the EXOR gate turns on for a bit. The output pin of the IC1 timer turns on due to
the Q3-Q4 transistors. Since the Q7 transistor turns off the input pins of the IC3 timer take the low level with a
time delay depending on the C3 capacitor. The output pin of the IC3 timer lets the alarm sound and starts the count on
the IC4 timer. This one stops the alarm sound and gets the circuit to the start condition.
By using the components indicated in the circuit schematic the burglar alarm should work with a time delay of
about 10-15 seconds (the circuit in the
features smaller C3 and C4 capacitors).