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In ultrasonic distance measurement a train of pulses is sent using typically a 40Khz piezo transducer. After some time the same piezo transducer is used to receive the pulses and distance calculated by Time Of Flight.

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The pulses are generally created by MAX232 to generate a high voltage of 10V needed to generate pulses of sufficient intensity

I was wondering if just 1 pulse could be used. Instead of 10V, we could use say 20V to generate a single pulse. On way back, just detect that 1 pulse.

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  • \$\begingroup\$ Maybe it could. But how it would be simpler to detect a single pulse from noise, instead of detecting a tone burst of 40 kHz which provides redundancy to minimize errors? \$\endgroup\$
    – Justme
    Commented Mar 8, 2021 at 17:06
  • \$\begingroup\$ A train of pulses need pattern matching. A single pulse does not. A single pulse could be falsely triggered so use a higher intensity pulse to improve signal to noise ratio. The receive circuit could be used to just stop a h/w timer in an MCU so its certainly easier than pattern matching at high speed in MCU. \$\endgroup\$
    – wonderfuliot
    Commented Mar 8, 2021 at 17:08
  • \$\begingroup\$ IMO, the best way is to use a PRBS sequence modulated carrier of 40kHz. Then use cross correlation to detect target and eliminate the dispersed echo. \$\endgroup\$
    – Marko Buršič
    Commented Mar 8, 2021 at 18:12
  • \$\begingroup\$ There is no train, and there is no pattern. It is a burst of a single-frequency sine wave. Because the transmitted burst is at one specific frequency, a simple bandpass filter can separate it from background noise way better than an impulse, which the world is full of. \$\endgroup\$
    – AnalogKid
    Commented Mar 8, 2021 at 18:17

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I was wondering if just 1 pulse could be used.

  • Trying to emit a single ultrasound pulse from a highly resonant piezo transducer will be impossible. Try it and see. To do so you need a wideband transducer and, as far as I'm aware, they are not common at 40 kHz.

  • Trying to get a highly resonant piezo transducer receiver to work with a single incoming ultra sound pulse will be problematic to say the least.

Piezo transducers are highly resonant. If you have a decent data sheet that has got a model of the equivalent circuit, this can be simulated very easily.

Here's some data from a Murata transmit/receive pair (redlines added by me): -

enter image description here

If you then modelled it with an RLC and fed it a step or pulse you would seen the transient response graph oscillating with a general exponential decay like this: -

enter image description here

Picture above taken from this interactive RLC bandpass calculator. Notice how in the lower graph that one step of voltage as an input produces a succession of decaying sinewaves.

And this is just from what is produced in the air from the transmitter. The combined effect of transmitter and receiver will have approximately double the Q factor and the received sinewave will take even longer to decay.

That's why many cycles are used to activate the piezo transmitter rather than a single pulse. But, if you can find a wideband transducer, the Q factor will be a lot lower and a single pulse is more attainable but, the downside is that the receiver sensitivity falls through the noise floor unless you use high power transmit pulses.

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