A simple car shock alarm circuit presented in this article can eb effectively used to alarm the owner each time the car comes across some kind of vibrational intrusion ether from a potential theft of simply a damaging hit from an external source.
Today most of the cars are equipped with this type of security feature where an alarm is raised in an event of a shock or a hit made over the car body. The article explains a very simple and cheap car shock alarm circuit which costs hardly 1/2 a dollar yet performs the actions reasonably accurately.
The working Principle of the Shock alarm
The principle employed here is pretty basic, a mic is used to sense the impact, the sensed output is amplified by a transistorized amplifier.
How the Circuits Works
Looking the circuit diagram, the whole functioning can be understood with the following points:
The mic stage consists of the mic itself, the biasing 2k7 resistor and the 47uF coupling capacitor.
T1 and T2 form the first amplifier stage and is the heart of the circuit.
The feedback 100K resistor plays an important function of keeping the amplification sustained at a stable rate.
When a shock impact is sensed by the mic, it converts the shock vibrations into tiny electrical pulses.
These pulses are suitably amplified to a a reasonable level and fed to the next stage where T3 further amplifies the signals to higher levels.
The 100uH inductor placed at the base of T3 ensures that T3 conducts only in response to legit shocks and not to stray RF pick ups or similar disturbances that might influence the operations and trigger false alarms.
The last output stage is used for amplifying the signals from T3 to the highest level which becomes appropriate for driving the connected alarm.
One big drawback of this cheap car shock alarm circuit is that it cannot distinguish between physical shocks and shock waves developed due to loud bangs or noises.
T1, T2, T3 are BC547, T3 is BC557, and T5 is TIP122. The mic is condenser type.