I'm building a rudimentary sonar using piezoelectric discs and I wanted to test it to see if a receiving disc could hear a sending disc. I have the sender hooked up to a ATTiny85 that is sending a pulse train (yellow on the oscilloscope output), and I watch the purple (received) signal. On some other questions here, people suggested I put a 1 megaohm resistor across the receiving disc. When I look at the scope output, it gives a very strange sinusoid while it's clamped in my little testing rig (4 arm soldering helping hand thing). If I take the receiving disc and put it on a table instead, the waveform looks more normal like the 3rd image.

Is the 2nd image a result of a capacitive effect while my helping-hand is gripping the disc? The metal of the claw is encased in some plastic so the metal doesn't touch the disc directly.

My final image is the disc itself with some added wires to it. I did an abysmal job soldering because I'm not that good at it. My question here is, can bad soldering like this introduce capacitive effects of it's own?

My final question: if I wanted to use the signal from one of the images, should I use an op amp to amplify the signal before I send it into an Arduino or some chip with an analog input? I'm thinking of doing that but wanted some quick feedback. The scale on the scope for the purple signal is 5mV per square.

220 ohm resistor

1 megaohm resistor in clamp

1 megaohm resistor on table

enter image description here

  • \$\begingroup\$ What is the scale of the time axis? What is the frequency of the sinusoid? \$\endgroup\$
    – AJN
    Nov 2, 2020 at 1:54
  • \$\begingroup\$ The frequency is supposed to be ~32 kHz but for some reason the scope reports at the top left 500 hz. The time axis I think is 500 us per cell. \$\endgroup\$
    – mj_
    Nov 2, 2020 at 2:02
  • \$\begingroup\$ The yellow wave occupies 4 squares each 0.5 ms wide. So it is 500 Hz. The pink wave seems to be 5 times yellow's frequency, i.e., 2.5 kHz. There is no 32 kHz to be seen any where? \$\endgroup\$
    – AJN
    Nov 2, 2020 at 2:14
  • \$\begingroup\$ Aha, I had a prescalar in the ATTiny85 chip of 64. So the yellow wave is supposed to be ~500Hz. \$\endgroup\$
    – mj_
    Nov 2, 2020 at 2:40
  • 2
    \$\begingroup\$ 500 Hz is far too low for a piezeo to be efficient. Try 5 KHz, maybe even 15 KHz. Clamping the disc at one point on the edge is going to horribly distort its vibration modes, what exactly that does is hard to predict but probably not good. Try to find something you can glue it to to by the edge - a 3d printer would be great to make a custom diameter mount but you might even roll a multi-layer paper tube around something (presuming you don't have a lathe...). Even just carefully supporting it by the wires would be better than the way you have it calmped. \$\endgroup\$ Nov 2, 2020 at 3:49


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