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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?

Daniel Tork
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3 Answers3

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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.

Daniel Tork
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Ignacio Vazquez-Abrams
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  • What is the role of the weight? – Daniel Tork Dec 10 '17 at 10:23
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    @DanielTork: To amplify the strain produced by vibration. – Ignacio Vazquez-Abrams Dec 10 '17 at 10:24
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    @DanielTork https://www.google.com/search?q=piezoelectric+vibration+sensor+car+alarm&num=100&safe=off&client=safari&hl=en-us&prmd=sivn&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjk486z2__XAhWJipAKHcsLB9MQ_AUIEigC&biw=1024&bih=627&dpr=2#imgrc=MOMGaS-EKDgwNM: – Thom Blair III Dec 10 '17 at 15:06
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    So, it triggers the alarm in the event of physical tampering, right? – voices Dec 10 '17 at 18:13
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    @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. – Ignacio Vazquez-Abrams Dec 10 '17 at 18:48
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    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. – Dave X Dec 10 '17 at 20:10
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    @DaveX I'm with DaveX here. The weight modifies the resonant frequency; it doesn't amplify (how could it; it's passive). – Kaz Dec 12 '17 at 00:31
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    I wasn't certain, which is why I said it in a comment. – Ignacio Vazquez-Abrams Dec 12 '17 at 00:32
  • @Kaz F=ma, the attempt to tamper causes an acceleration (more likely an impulse rather than a vibration), increase mass and you increase the force applied at the end of the piezo, increase the force/stress and you increase the movement/strain, increase the strain and you get a larger signal for a given acceleration. So it increases the output for a given input, which is the usual definition of amplification. – Pete Kirkham Dec 12 '17 at 17:35
  • @PeteKirkham I agree that the weight amplifies the vibrations, but I would explain in another way. The force you speak about acts rather momentarily and can be written as delta p(impulse)/delta t(time). The bigger is the mass, the bigger is delta p , the bigger is the force – Daniel Tork Dec 13 '17 at 07:13
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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.

Peter Mortensen
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Decapod
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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

Berto
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