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Piezo electric transducer used as a receiver

  1. May 6, 2007 #1

    Is it possible? I've found quite a few piezo electric transducers on rs.com but I wonder are they only good for transmitting or could they be used as a receiver with a really good pre-amp?

    Can anyone tell me? If so will they lose some of their response at low frequencies when used in this manner?
  2. jcsd
  3. May 7, 2007 #2

    This is a circuit I'm working on for said hydrophone above. I have been informed it is acceptable to use my proposed unit.

    I have blueprints for a circuit using op-amps but was wondering if the circuit below would be any good?

    It is a bias applied to a transistor using a voltage divider. I was wondering how to apply the dc bias to the voltage divider on the left without affecting the incoming signal. Could I just couple the top junction of the voltage divider to a dc voltage? Would that give me a quiescent point dependant on the voltage divider that would simply show a rise or drop from half the applied dc voltage for the incoming signals, or am I barking up the wrong tree here?

    The bit on the right is just a low pass filter. I'm listening for low frequency sound.

    http://img205.imageshack.us/img205/470/hydcircuitby8.jpg [Broken]

    I am to be honest trying to avoid a power supply which involves anything more than a few batteries.

    Perhaps I could use another transistor as an emitter/follower between hydrophone and voltage divider to lower the impedance of the signal on the voltage divider side and yet get the signal input impedance high so it didn't take much current to drive it?

    I wonder how that is possible though given how would it cope with signals close to or below ground. I can't remember if an emitter/follower easily allows negative signals?

    I'd prefer that if it is possible as I don't yet understand op amps that well.

    Please if my explanation doesn't make sense tell me and I will post another picture thanks.

    If I'm talking garbage due to my freshness to the subject please let me know also :wink:
    Last edited by a moderator: May 2, 2017
  4. May 7, 2007 #3


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    Staff: Mentor

    Can you describe a bit more about what your transducer/receiver element is? If it's a piezo, it will have a limited bandpass in its sensitivity, and you need to match that in terms of bandwidth and impedance in your amplifier.

    The emitter-follower that you show will buffer the input voltage and amplify current a bit, but will do nothing to amplify the input voltage (which I would guess to be fairly small and in need of amplification). You can AC couple the transducer into the base bias of the emitter-follower transistor, but again, pay attention to impedance matching this input bias circuit to the output impedance of the transducer.
  5. May 7, 2007 #4
    If you can use circuitry with op amps its more design-controllable. What about standard microphone amplifiers chips, designed for running from piezo electric audio mikes?

    The frequency range limits could be adjusted by altering any capacitor sizes..

    I seem to remember piezo voltage output is proportional to frequency, except around the mechanical resonance bands.. so if you use piezo for subsonics (< 20hz) then you need extra amplification. Hence why the low pass filter, to equalise output at various frequencies.

    If its a thin disc piezo then the mechanical resonance to vibration may be in the khz range, thats something to check on (e,g tweeters are used in cheap speaker boxes for high frequency, the rs ultrasonic piezo's resonate at 30khz or so, thats in the design)

    Op amps are not that complex to use, just remember the '0v' is the signal reference pioint, the + and - voltage lines are just for the chips, referenced to the 0v line.... most dedicated chips have good application notes to go with them.

    Selective audio filtering is often easier with op amps, and some are very low quiescent power (like 200 microamps per chip) so you don't need mega battery packs. Just don't remove the negative side battery and leave the positive one connected.. they'll probably get hot...:blushing: I know..
  6. May 8, 2007 #5
    Thanks for the replies, I'm right in the middle of a nightmare job on the car and promise I'll have a detailed read and response later. Thanks again for all contributions.


    I've read the replies thank you again.

    Berkeman, would it be possible to negate worrying too much about bandwidth responses by placing the low frequency filter at the first stage after the transducer and then using a large op-amp for amplificiation? Furthermore would it still be worth using an emitter follower to keep the input signal source stiff?

    By the way I bought Horowitz and Hill's book. Cracking read just a bit hard. 3 weeks I'm still on page 74 lol!

    Hareslade, I've had a quick read of Clayton's Op-Amps book and they don't look too bad actually as you say. Given that I will likely need good amplification is it possible to get such high amplification from a realistically priced shelf bought op-amp?

    I'll try and find a link to the transducer in question. Thanks again.


    New idea. The 5V applied to go against the signal at the last stage by the output is to cancel the bias signal from the emitter follower. This new design uses an op-amp to boost signal strength whilst still running into a high impedance so as not to make the signal source to 'un-stiff'. The 5V biased emitter follower (using a voltage divider for input) drops the signal impedance before it's sent through the 5V 'bias remover' and the output.

    http://img519.imageshack.us/img519/2677/hydcircuitvu5.jpg" [Broken]

    I'm suspect about the 5V bias remover. I think it would not work but I don't know why. If anyone does please tell me and I'll get rid of it, and the emitter follower too and just rely on a good op-amp and low pass filter.
    Last edited by a moderator: May 2, 2017
  7. May 10, 2007 #6
    I got a good book by Clayton on Op-Amps and I understand them enough to use one now. Quite interesting. I suppose there's little point messing around with anything else really when they're so cheap and readily available. I'll just put the output through an op-amp then through a low pass filter and then into the headphones.

    It's the most sensible choice for now I reckon until it becomes necessary to clean it up more.
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