Here is a simple, low-cost intruder detector that uses an invisible laser beam to detect the intruder. The laser beam is produced using a 3V DC or 4.5V DC laser pointer or torch that is available in the market. The 3V DC or 4.5V DC power supply for the laser transmitter can also be given using a bridger ectifier or full-wave rectifier.
Fig. 1 shows the block diagram of the complete unit comprising the transmitter and receiver sections. The laser beam from the transmitter after reflection from various mirrors (M1 through M6, as shown in Fig. 1) is made to fall on the photodetector in the receiver circuit.
Once the laser beam is positioned, the receiver circuit is powered by closing switch S. An alarm unit operating on 230V AC is connected to the relay RL in the receiver circuit.
When an intruder interrupts the path of the beam or switches off the laser torch, the alarm unit becomes activated. The alarm unit remains activated until reset switch S is opened. To activate the alarm circuit again, reset switch S should be closed. The total distance travelled by the laser beam should be less than 800 m for 4.5V laser torch and 500 m for 3V laser torch.
The circuit of the receiver is shown in Fig. 2. When reset switch S is closed, the circuit is powered on. As the laser beam falls on the photodetector, transistor T (BC547) conducts, resulting in the collector being pulled down to ground potential. Thus no current flows to the gate of the SCR and it remains off.
Once the path of the laser beam is interrupted, the base current of the transistor becomes very low and the transistor is driven to cut-off. Now the current starts to flow through resistor R1 and to the gate of SCR. Hence the SCR is fired and it begins to conduct. Thus relay RL connected to the anode of SCR is switched on and the alarm is activated. The alarm sounds until reset switch S is opened to turn off power to the circuit.
EFY Lab note. We tested the circuit using only one mirror and found its range to be 25-30 metres. The range depends on the intensity of laser beam falling on the photodetector.
The circuit (excluding laser torch and mirrors) costs around Rs 100.