Burglar Alarm Circuit

Just for fun I created a burglar alarm circuit using NE555 and

logic gates. Two door sensors can be connected to the circuit in NO or NC configuration.

When the ON/OFF switch is open, the device does not work.

When you close the ON/Off switch the burglar alarm starts working with a time delay given by the C5 capacitor.

If the circuit detects a change in the condition of a sensor, the alarm sounds with a time delay related to the C3 capacitor.

The C4 capacitor turns off the alarm, then the circuit backs to the start condition.

The bell works even if the sensor backs to the start condition in a very small instant, so the circuit can be used to detect vibrations.

Since this burglar alarm does not feature a remote control, you may use it in combination with a resistor key.

Step 1:

If a door sensor changes condition, even for a bit, an output of the CD4030 activates the IC1 timer; this one turns off the Q7 transistor.

The output pin of the IC3 timer triggers the burglar alarm siren through the Q8 transistor; the time delay depends on the C3 capacitor.

The IC4 timer turns on with a delay given by the C4 and the circuit gets to the start condition.

As you can see this burglar alarm is not wireless, so it may be used to protect small apartments.