Burglar Alarm 4 in 1 Schema

In this circuit ( Burglar Alarm 4 in 1 ), the alarm will be switched on under the following four different conditions:
1. When light falls on LDR1 (at the entry to the premises).
2. When light falling on LDR2 is obstructed.
3. When door switches are opened or a wire is broken.
4. When a handle is touched.

The light dependent resistor LDR1 should be placed in darkness near the door lock or handle etc. If an intruder flashes his torch, its light will fall on LDR1, reducing the voltage drop across it and so also the voltage applied to trigger 1 (pin 6) of IC1. Thus transistor T2 will get forward biased and relay RL1 energise and operate the alarm. Sensitivity of LDR1 can be adjusted by varying preset VR1. LDR2 may be placed on one side of a corridor such that the beam of light from a light source always falls on it.

When an intruder passes through the corridor, his shadow falls on LDR2. As a result voltage drop across LDR2 increases and pin 8 of IC1 goes low while output pin 9 of IC1 goes high. Transistor T2 gets switched on and the relay operates to set the alarm. The sensitivity of LDR2 can be adjusted by varying potentiometer VR2. A long but very thin wire may be connected between the points A and B or C and D across a window or a door. This long wire may even be used to lock or tie something. If anyone cuts or breaks this wire, the alarm will be switched on as pin 8 or 6 will go low. In place of the wire between points A and B or C and D door switches can be connected.

4 in 1 Burglar Alarm

These switches should be fixed on the door in such a way that when the door is closed the switch gets closed and when the door is open the switch remains open. If the switches or wire, are not used between these points, the points should be shorted. With the help of a wire, connect the touch point (P) with the handle of a door or some other suitable object made of conducting material. When one touches this handle or the other connected object, pin 6 of IC1 goes ‘low’. So the alarm and the relay gets switched on.

Remember that the object connected to this touch point should be well insulated from ground. For good touch action, potentiometer VR3 should be properly adjusted. If potentiometer VR3 tapping is held more towards ground, the alarm will get switched on even without touching. In such a situation, the tapping should be raised. But the tapping point should not be raised too much as the touch action would then vanish.

When you vary potentiometer VR1, re-adjust the sensitivity of the touch point with the help of potentiometer VR3 properly. If the alarm has a voltage rating of other than 6V (more than 6V), or if it draws a high current (more than 150 mA), connect it through the relay points as shown by the dotted lines. As a burglar alarm, battery backup is necessary for this circuit. Note: Electric sparking in the vicinity of this circuit may cause false triggering of the circuit. To avoid this adjust potentiometer VR3 properly.

Cut Phone Line Alarm Schema

Burglars, rapists and other criminals will often cut a phone line before they break into a home. The cut phone line will prevent many alarm systems from alerting the police. The battery powered circuit below will sound an alarm when the unit no longer detects the voltage present on an active phone line.

Cut Phone Line Alarm Schema

A bridge rectifier front end makes the circuit phone line polarity independent. It shows four 1N4004 diodes being used but you can also use a standard low current bridge rectifier package with a 200v rating or more. The bridge is wired so the output voltage goes negative relative to circuit ground. The phone line will typically be somewhere around 30v when in standby mode and drop to something less than 15v when a phone is connected to the line. In the circuit, this translates a negative voltage fed to the gate of the 2N7000 FET, which kept the transistor turned off. When the phone line is cut, the voltage fed to the gate of the FET will swing to +9v, which will turn on the FET. When Q1 turns on, it also turns on Q2, which routes power to a 555 timer circuit. The 555 timer is wired as a 1Hz square wave oscillator, which drives the transistor Q3. That transistor drives an alarm beeper. With the components shown, the circuit draws nearly zero current in standby mode. Therefore a good quality 9v battery should last for many years.