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Ultrasonic Transducers

Discussion in 'General Electronics Discussion' started by Gav1985, Jun 2, 2013.

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  1. Gav1985

    Gav1985

    18
    0
    Mar 12, 2013
    Hi guys, in need of some assistance

    I'm trying to build an ultrasonic ranging module using 40kHz transducers.

    I've attached my schematic below. I don't think it's the emitter that's causing me issues. I've tested it down at 4kHz and its produced a nice ringing sound and it's definitely working. So I'm going to assume it's operating 100% at 40kHz.

    I've also attached a waveform of what the Schmitt Trigger should be producing at 40kHz.

    The receiver circuit is based on two CEAmps. And I've set up a 10mV peak pulse input with an interactive button to simulate "echo" being detected by the receiver.

    I'm also using 1n5817 schottkys instead of BAT42s for the peak detector as thats all ive got atm.

    When I try to test the circuit, i'm not getting any response at the receiver end so far as I can tell.

    Does anyone have any advice? Will this work as intended?

    Thanks all.
     

    Attached Files:

  2. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    A couple of comments.

    The electolytic capacitors are the wrong way round but this could just be a drawing error. At 40kHz, you could go for smaller non-electrolytics.

    The led shoud be connected in the collector not the emitter of the transistor. You may not be getting sufficient voltage (measure it) at the high frequency where the gain is less.
     
  3. Gav1985

    Gav1985

    18
    0
    Mar 12, 2013
    The electrolytics were a drawing error. ;p

    So, I tried adjusting the value of the resistor R13. In the schematic, its 82k which according to the Proteus oscilloscope, gives me a 40kHz output.

    I swapped it for a 470k and the transducer became audible, so, suggests a lower frequency and the receiver circuit started working. Its detecting echo up to around 1m.

    ??????Huh??????

    Surely it shouldn't be operating at a lower, audible frequency as the transducers are meant to be tuned for optimum operation at 40kHz?
     
  4. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    I do not understand how the circuit is supposed to work. I would think that there should be a short pulse and then a circuit to measure the time delay on reception.

    Thumping a resonant transducer at a low frequency will give a short train of high frequency output at its resonant frequency.

    Have you heard SONAR pings on the films to detect ships? If you are detecting a shorter distance, then the pings can be closer.
     
  5. Gav1985

    Gav1985

    18
    0
    Mar 12, 2013
    Yeah that's how its supposed to work. I'm currently breadboarding the circuit to try and get it working.
    The aim is to have my MCU control the bursts and measure the distance.

    I should add, I've used a button as an "pulse" device so the emitter isn't constantly on. I know it's not as efficient as using a MCU. There is a response with the 82k resistor but the response is better with the high resistor, low freq.

    They way I understand it, the closer to their operating freq (40k), the greater the SPL and therefore the detector will generate a bigger input the amp stage. Yet, it seems the other way round atm.

    I'm confused ... lol.
     
    Last edited: Jun 2, 2013
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,270
    Nov 28, 2011
    You would be better to generate the whole transmit signal in the micro. If it has a timer with an output compare feature, you can make it generate a frequency of 40 kHz exactly, or very close, from any clock frequency, and the frequency will be crystal-accurate, unlike the simple design you've drawn.

    Also, you can achieve a higher output level using a bridge driver made from MOSFETs or transistors, or just gates (several gates "in parallel" will reduce the output resistance).

    As you know, you can't just drive the transmitter with a constant tone. But that will be enough to provide an input to the amplifier. If you're not getting anything on the charge pump output, I'd guess there's something wrong with the receiving transducer and/or the amplifier.

    With R5 and C6 as shown, the time constant of the smoothing circuit that follows the charge pump will be 0.1 seconds. This is much longer than it needs to be, and will affect the accuracy of the detected envelope. The envelope will also be affected if the amplifier clips, which will happen when the input signal is strong. For these reasons you can use an adjustable-gain amplifier controlled by the micro. The output of the envelope detector can also be handled by the micro on an ADC input.
     
  7. poor mystic

    poor mystic

    1,066
    31
    Apr 8, 2011
    :) Hi
    I can't see how the circuit is supposed to work either. Why the 2 identical transistor stages, with no gain?
    How does the transmitter circuit work, with its input seemingly taken from an external generator connected to an amplifier's output?
    And Kris' comment about the time constant of the detector.
    I think it a mockery of a circuit.
     
  8. sirch

    sirch

    109
    1
    Dec 6, 2012
    FWIW here's the circuit I have been using to drive a 40kHz ultrasonic transmitter. The inductor needs to to be sized to be resonant with your piezo transmitter's capacitance. I drive this from a Arduino at 40kHz
     

    Attached Files:

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