This flashlight triggered burglar alarm circuit will instantly switch ON an alarm as soon as it detects light from the burglar's torch or flashlight.
Burglars and thieves normally set out to steal or rob during nights, when it is completely dark, and depend on a flashlight to investigate the area.
Our circuit is setup and activated in the area which needs to be protected.
As long as it is dark, the circuit remains switched off. If in case a burglar tries to investigate the area with his flashlight ON, our circuit detects the light from his flashlight and sounds the alarm.
Once the alarm is triggered, it gets latched and can be deactivated only by powering it off.
As can be seen in the following diagram, the trigger circuit consists of a couple of transistors, configured as a latch.
The feedback loop through R3 is very crucial as it becomes responsible for implementing the latching operation as soon as an input trigger signal is detected.
Initially, while is completely dark and power switched ON, the circuit stays in its standby mode. In this position The LDR resistance is very high, at around 5 to 10 meg Ohms.
Therefore, T1 is unable to get any base bias, and remains switched OFF. With T1 in switched OFF condition, T2 also remains switched OFF, and keeps the siren shut off.
Now suppose a burglar or an unwanted guest arrives at the location with a flashlight and tries to investigate the area with its flashlight ON.
In this situation, as soon as the flashlight light happens to fall on the LDR, causes the LDR to resistance to drop drastically, which is enough to turn ON T1 momentarily.
Even a millisecond switch ON for T1 is enough to turn ON T2 and latch the circuit in that position.
The above latching of the circuit also turns ON the siren which starts blaring loud, alerting the owner regarding a security breach.
The siren can be turned OFF only by switching OFF power to the circuit.
- All Resistors are 1/4 watt CFR
- 10k, 47k, 33k, 1k, 4.7k = 1 each
- LDR (any standard) = 1
- Capacitor 1uF / 25V Electrolytic = 1
- Transistors, BC547, TIP32 = 1 each
- 12V Siren = 1
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