Indian Institute of Technology Hyderabad Laser Door Alarm By By- Sagar Adhikari Ved Prakash

Laser Door Alarm This laser door alarm is based on the interruption of Laser beam. A low cost Laser pointer is used as the source of light beam. When somebody breaks the laser path, the alarm will be generated for few seconds.

Working The laser door alarm circuit has two sections. The laser transmitter is a laser pointer readily available. It should be powered with 5 volt DC supply and fixed on one side of the door frame. The receiver has a Phototransistor at the front end. Bc547NPN Darlington phototransistor is used as the laser sensor. IC1 is used as a voltage comparator with its inverting input tied to a potential divider R2-R3. So that the inverting input is kept at half supply voltage. The non inverting input receives a variable voltage based on the conduction of T1. The receiver should be fixed on the opposite door frame and should be properly aligned to the laser beam. Normally the laser beam illuminates the face of phototransistor and it conducts. This keeps the voltage at pin 3 lower than pin 2 of IC555.

Working As a result, output of comparator remains low. LED and Buzzer remain off in this state. When a person crosses the door, laser beam breaks and T1 cease to conduct. Collector voltage of T1 rises and voltage at pin 3 of comparator increases and its output becomes high. This activates LED and buzzer. Capacitor C1 keeps the base of T2 high for few seconds even after the output of IC555 becomes low again. C2 gives current to the buzzer for few seconds even after T2 turns off.

Buzzer and Bleeper These devices are output transducers converting electrical energy to sound. They contain an internal oscillator to produce the sound which is set at about 400Hz for buzzers and about 3 kHz for bleeper’s. Buzzers have a voltage rating but it is only approximate, for example 6V and 12V buzzers can be used with a 9V supply. Their typical current is about 25mA. Bleepers have wide voltage ranges, such as 3-30V, and they pass a low current of about 10mA. Buzzers and beepers must be connected the right way round, their red lead is positive (+). An inductor may be connected either way round and no special precautions are required when soldering.

Relays A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and they are double throw (changeover) switches. Relays allow one circuit to switch a second circuit which can be completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit. There is no electrical connection inside the relay between the two circuits, the link is magnetic and mechanical.The coil of a relay passes a relatively large current, typically 30mA for a 12V relay, but it can be as much as 100mA for relays designed to operate from lower voltages. Most ICs (chips) cannot provide this current and a transistor is usually used to amplify the small IC current to the larger value required for the relay coil. The maximum output current for the popular 555 timer IC is 200mA so these devices can supply relay coils directly without amplification.

Components Used Resistors Buzzer Inductor Diodes Relays LEDs (Light Emitting Diode) Capacitors PCB(Printed Circuit Board)

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