#857 Ultrasonic Piezo Polarity Tester

Using an Arduino to automatically determine the polarity of ultrasonic piezo transducers.

Here’s a quick demo..

Notes

Ultrasonic piezo transmitters and receivers such as the TCT40-16 are inherently polarized. Correct polarity may be significant depending on the application. See LEAP#856 TCT40-16 for more details.

While a simple multimeter test can usually determine polarity, this project expands on an idea found at https://www.instructables.com/Acoustic-Levitator/ to create an automated test bed powered by an Arduino.

The project relies on the fact that when sampling the voltage:

• if the piezo anode is sampled with piezo cathode connected to ground: the voltage will steadily rise
• if the piezo cathode is sampled with piezo anode connected to ground: the voltage will always be ~0V
• if there is no piezo connected to the sampling pin: the voltage will fluctuate randomly

Some caveats:

• I’ve noticed the behaviour can be quite erratic if powered from a noisy power supply (e.g. a wall wart) or under-powered power bank.
• Generally works perfectly if powered from my computer USB.
• If don’t get a clear reading at first, can help to flip and retest the piezo orientation a few times until it “settles” on a stable behaviour

Circuit Design

Designed with Fritzing: see PiezoPolarityTester.fzz.

Setup for testing on a breadboard:

The Sketch

See PiezoPolarityTester.ino.

Some points to note:

• I am using an Arduino Nano here, but any Arduino model would work just as well
• it sets the ADC sampling rate to ~9.6KHz
  • ADCSRA register is setup with ADC prescaler of 128
  • with 16MHz clock in the nano, this sets ADC Clock Frequency to 16MHz/128 = 125kHz
  • 13 ADC clock cycles are required to complete one conversion, so sampling rate is 125kHz/13 ~= 9615Hz
• it uses a rough variance calculation to determine the polarity:
  • takes 5 readings, each reading is the average of 32 samples
  • calculates the average variance from one reading to the next
  • categorizes the result:
    • very low variance: piezo is inverted
    • moderate variance: piezo is not inverted
    • high variance: probably no piezo connected
• sets the LEDs accordingly:
  • both off: no piezo detected
  • will light the LED matching the positive terminal (anode) of the piezo

Breadboard Test

Works nicely with a reliable indication of which terminal is positive.

Protoboard Build

To make it a little more convenient to use, I transferred the circuit to a small 3x7cm protoboard. I planned out a rough layout:

As built. I fashioned clips from some component leads to “spring-load” the piezo under test.

I stuck the board to a 3x7cm piece of 3mm MDF with hot glue to protect the underside from shorts and make it sit on a bench nicely.

Under test:

Credits and References

• https://www.instructables.com/Acoustic-Levitator/ - source of the original idea
• LEAP#856 TCT40-16