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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 15th Oct 2019, 5:14 pm   #1
Pete_kaye
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Default Piezo element

Why are there 3 wires to an extract from a smoke detector ? I want to use in a simple crystal set with small amp.
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Old 15th Oct 2019, 5:52 pm   #2
merlinmaxwell
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Default Re: Piezo element

I don't understand your question. Any pictures?
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Old 15th Oct 2019, 6:17 pm   #3
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Default Re: Piezo element

My guess is that this is one of those piezo buzzer 'disks'. Some types have 3 connections, a common wire to the metal disk (and thus one side of the piezo-stuff), a large 'drive' electrode on the other side and a smaller electrode on the same size as the large one that can be used for feedback in an oscillator circuit. I seem to recall you can make an beeper with one of these 3-terminal piezo sounder, a transistors and a small number of resistors and capacitors.

If you want to use one as a speaker then use the wire to the disk and the wire to the drive electrode. Either ignore the feedback electrode or connect it to the drive electrode.
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Old 16th Oct 2019, 6:17 pm   #4
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Default Re: Piezo element

This is what I meant. How do I know which is which , other than by experiment?
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Old 16th Oct 2019, 6:30 pm   #5
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Default Re: Piezo element

The colours of the existing wiring and markings where they connect to the detector PCB (which I don't see in your picture) is usually the best clue. More often than not the 'main' connections to the sounder will be red and black and the feedback wire some other 'neutral' colour like white, green or blue.

Have a look at this article, which also includes a standalone circuit to drive such an element.

https://electronics.stackexchange.co...a-piezo-buzzer
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Old 16th Oct 2019, 6:36 pm   #6
TonyDuell
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Default Re: Piezo element

I would think the metal disk is the common connection, the large annular metal part is the drive electrode and the little 'dot' in the middle is the feedback electrode.

I am not sure how you are going to make connections to it (soldering wires to the electodes can damage the piezo-stuff) but I'd try using the metal disk as one connection and the 2 electrodes joined together as the other one.
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Old 16th Oct 2019, 7:31 pm   #7
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Default Re: Piezo element

I have a couple with wires on if you need one and several minature speakers if preferred for the cost of postage.

Andy.
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Old 22nd Oct 2019, 2:24 pm   #8
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Default Re: Piezo element

Thanks all of you . I have sorted myself out now.
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Old 22nd Oct 2019, 3:17 pm   #9
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Default Re: Piezo element

If you make an oscillator, or make a signal with software on a microprocessor and feed it to a piezo transducer, you run into the problem of making sure the drive frequency is on the resonance of the transducer for best output.

Using the transducer's resonance to control the frequency is an elegant way round this problem. All you need is an amplifier the small sense electrode drives the amplifier and the amplifier drives the big electrode. It goes unstable and ocillates bang on its own ideal frequency.

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