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Burglar Alarm for Car Stereos

2016-02-19 12:14  
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Burglar Alarm for Car Stereos circuit 300x150

We have another interesting application of 555. This is used as a burglar alarm for car stereos. Thieves have, some how, the knack of removing the car stereos even though all the car doors are closed and locked. If we forget to close any window, these people will have a field day. This circuit can easily be adopted as a domestic burglar alarm. The original astable oscillator of 1.8 KHz is still used but with some small tricks. As this alarm should alert passers by even on a busy road, more volume is developed by a Darlington pair. You may use this in the house as a burglar alarm or in the garden.

The circuit is shown in Schematic 12. Car stereos are normally kept in metal shell, which is clamped into the housing meant for auto stereo decks in the car. Body of the car stereo is normally connected to the ground that is the negative side of the battery in the present day cars. Shell is also connected to the car in some models by a separate wire coming from the wiring harness or the terminal block of the stereo connections. That suits us well. The alarm is placed at a convenient place and power is taken from the battery before the ignition switch so that this alarm is always powered. Now the reset terminal is pulled up by R3 (10k resistance), but it is also connected to the ground from the shell of the stereo deck so that the connection is made only when the stereo is inside. Hence it is effectively at the ground potential as long as the stereo is inside the shell. If any body removes the deck from the shell, reset pin looses ground connection and goes high. Alarm sounds.

To give a very loud volume, a Darlington pair is used now comprising of Ql, Q2. Darlington pair is a cascade of two transistors where the gain of individual transistors is typically multiplied. Collector load is shared by both transistors. It works as if single transistor of a very high gain is used.

You may use this circuit for protection of home.Use a very thin enamel wire (LI) of; say 36 gauge wire is connected between the reset pin and ground. This can be a long wire running invisibly across the door, window or across any other places you want to protect. You can even wire at around a fruit bearing tree in the garden. As the reset pin is shorted to the ground with this wire, oscillator will not start. If the wire is broken, reset is pulled up and the alarm starts.

The alarm in a suitable box is placed outside the stereo at a convenient place. Please see if the shell is already connected to ground or if it is making the ground contact from the casing of the stereo. The trick is to keep the reset pin at ground level during normal operation. If the contact is broken, alarm sounds. If the stereo casing is connected to ground, take the ground contact of reset pin from the shell or vice versa. If they are totally housed in plastics, use a very thin enamel wire and connect the shell and the casing. Connect one end to reset pin. Negative or ground connection is common with the automobile ground. Now if the stereo is removed from its position, ground contact of the reset pin is broken. It goes high and alarm sounds.If you are using this around the house, use a length of thin enamel wire. Fix it around the area as required. Scrap the enamel and solder the ends at Pin 4 and ground. Handle this wire carefully as it is likely to break while working. Construction is same as the 1.8K astable oscillator. Reset Pin (4) is pulled up with a 10K resistor.
This project may have to be left on for considerable lengths of time. Hence it will be useful to use CMOS version of 555, i.e., 7555 for this project as its standby current consumption is very low. The Darlington pair amplifies the output any way. CMOS version can work at higher voltages.Power is taken from the car itself. It should be taken before the ignition switch, directly from the battery as the circuit needs to be powered always.


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