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When opening up a car alarm system I found this strange-looking component. I thought that it could be something which is light dependent, but the entire board is enclosed in a black plastic box and the alarm should trigger before the thief reaches the depths of the electric system and thinks of opening it.

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What component is this?

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It's a piezoelectric disc buzzer with a weight. It's being used as a vibration sensor, not a sound generator.

The weight amplifies the strain produced by vibration.

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    \$\begingroup\$ @DanielTork: To amplify the strain produced by vibration. \$\endgroup\$ – Ignacio Vazquez-Abrams Dec 10 '17 at 10:24
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    \$\begingroup\$ So, it triggers the alarm in the event of physical tampering, right? \$\endgroup\$ – voices Dec 10 '17 at 18:13
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    \$\begingroup\$ @tjt263: Not necessarily triggers, but it lets the device know that it's a possibility. There are various things that can cause vibration and it's up to the circuit to figure out what is going on. \$\endgroup\$ – Ignacio Vazquez-Abrams Dec 10 '17 at 18:48
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    \$\begingroup\$ The weight idea is really cool -- it lowers the frequency of oscillation that the piezo will be sensitive to into the range that the car might vibrate. \$\endgroup\$ – Dave X Dec 10 '17 at 20:10
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    \$\begingroup\$ @DaveX I'm with DaveX here. The weight modifies the resonant frequency; it doesn't amplify (how could it; it's passive). \$\endgroup\$ – Kaz Dec 12 '17 at 0:31
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My answer is an educated guess.

It looks like a vibrating plate that is activated when the vehicle is shaken or something like that. On the plate, in the centre, you find a piezoelectric element converting the mechanical movement into an electrical one. From there on it should be clear.

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Seems piezo electric component that vibrates when a specified vibration applied . It creates a necessary clock pulse required for the PIC IC to turn on then pulse conversion from digital to analog to creates sound . Only a guess from me .. thanks

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    \$\begingroup\$ Good suggestion, but usually a clock (often a crystal) deals with that. \$\endgroup\$ – wizzwizz4 Dec 11 '17 at 21:11

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