Transcript Document

Laser Door Alarm
By
BySagar Adhikari
Ved Prakash
Indian Institute of Technology
Hyderabad
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
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Resistors
Buzzer
Inductor
Diodes
Relays
LEDs (Light Emitting Diode)
Capacitors
PCB(Printed Circuit Board)
References
www.google.com
www.texas.com
www.efymeg.com
www.efy.com
www.micro.edu/echips.com
www.answers.com
www.google.com
www.national.com
www.ascom.com
www.electronicsconsulting.co.uk/
www.radarsystem.com
www.electronicsproject.com
www.scienceproject.com