The circuit shown here is used as an anti-theft alarm. LDR stays in such a place that when the thief enters our house then a shadow will fall on the LDR. A small beam of light source is also required to provide a continuous signal to LDR. For the best light source we can use a laser diode that will run for a few kilometers. For home use the infrared LEDs will be good and will be difficult to thief and work with the same efficiency at night.
This circuit uses a popular timing IC which is 555. The IC 555 is connected as a comparator with the pin 6 connected with positive supply, the output goes high-1 when the trigger pin 2 is at a level lower than 1/3 of the supply voltage the output low of 0 when it is above 1/3. Such a small change in the voltage of the pin 2 is sufficient to change the output state of the pin 3 from 1 to 0 and 0 to 1. The output has only two high and low states and can not remain in any intermediate stage. It is power by 9V battery for portable use. The circuit is economical in energy consumption. Pin 4, 6 and 8 are connected to the positive supply and pin 1 is connected to ground.
To detect the current thief we have used LDR and a light source.
LDR is a special type of resistance whose value depends on the brightness of the light, which falls on it. It has a resistance of about 1 megohms when it is in total darkness, but a resistance of only about 2-5k ohms when it is brightly lit. It responds to a large part of the spectrum of light.
The light source and LDR is adjusted in such a way with a reflector that the light will fall directly on the LDR, but when the thief enters the interior, it will block the beam of light and the LDR will be under darkness.
We have made a potential divider circuit with LDR and variable resistance of 100 K connected in series. The voltage is directly proportional to the conductance, so more voltage will be obtained by this splitter when LDR is getting light and low voltage in the dark. Sensitivity can be adjusted by variable resistance. The split voltage is given to the 2nd pin of 555. As soon as LDR oscillates the voltage of pin 2 drops 1/3 of the supply voltage and pin 3 gets high and Buzzer Beeps.
In this demonstration we have used ordinary LED.
LED = light emitting diode
LDR = Light-dependent resistance
IC = Integrated Circuit
Components: -
1) 9V battery with snap-on
2) LDR
3) Variable resistance 100K ohms
4) Resistance 470 ohms
5) LED
6) IC 555
7) Switch