Laser Alarm Circuits >>

This circuit may be a optical device alarm just like the one we tend to see in varied movies. It uses a optical device pointer beam to secure your valuables and property. primarily, once the beam gets interrupted by someone, animal or object, the resistance of a photodiode can increase associated an alarm are going to be activated. The optical device and therefore the receiver will be fitted in same box, sharing a standard power offer. because the receiver attracts but ten mA on the average, you’ll before long notice that the optical device is that the most current hungry device! Mirrors area unit accustomed direct the beam in no matter setup you need. samples of a passage and {an area unita|a neighborhood|a district|a region|a locality|a vicinity|a part|a section} protected by the alarm are shown within the diagram.

In the circuit diagram we discover a TL072 op-amp (IC1.A) designed as voltage comparator between the voltage reference provided by the adjustable resistance P1/R4 and therefore the light-dependent voltage provided by the resistance consisting of photodiode D1 and glued resistance R3. once the shaft is interrupted, the voltage on comparator pin a pair of drops below that at pin three, inflicting the output to swing to (almost) the positive offer voltage associated indicating an alarm condition. This signal will drive a siren, a laptop or a light-weight that hopefully can deter the interloper.

Alternatively it can be used to ‘silently’ trigger a more sophisticated alarm. Resistor R2 provides some hysteresis to prevent oscillation when the two comparator input voltages are almost equal. Capacitor C1 makes the circuit immune to short, accidental interruptions of the beam, e.g., by flying insects. If you want your circuit to have faster responses you can reduce its value to 1 µF. The operation of the circuit is illustrated by the waveform diagram, which also proves the hysteresis action that sets an upper and a lower threshold on the input voltage. You can also see the delay introduced by capacitor C1.

The circuit is simple and could be assembled on a piece of breadboard. After assembling the circuit and testing it, you should mount it in a black box that has just a small hole. You may decide to put the laser in the same box but only if you are sure there is no way the photodiode can ‘see’ the laser beam directly. The small hole should be filled with a black drinking straw so that only light from the direction of the laser beam can enter. With the appropriate setup of the box and the mirrors, the laser beam is so intense that even direct sunlight cannot affect the operation of the photodiode.Electronics Circuit Application

Author: Dimitris Kouzis-Loukas - Copyright: Elektor Electronics Magazine