How to Make a Proximity Sensor Using Piezoelectric Ultrasonic Transmitters and Receivers

Created by Ben Roloff, last modified on Jan 22, 2019


PUI Audio Ultrasonic Receiver

PUI Audio Ultrasonic Transmitter

LM555 Timer

LM324 Op-Amp



1 X 0.1 μF Capacitor

1 x 0.01 μF Capacitor

2 x 0.001 μF Capacitors

1 X 3.3 nF Capacitor

2 x 1kΩ Resistors

1 x 2kΩ Resistor

1 x 5kΩ Resistor

1 x 12kΩ Resistor

1 x 390kΩ Resistor

5 x 1MΩ Resistors

2 x 4.7MΩ Resistors


Proximity sensors are common thing used in electronics. You can find infrared sensors in most kits. They work by powering an infrared LED and sensing the reflection with a phototransistor. Another way to do it is with ultrasonic sound. This can be achieved with piezoelectrics.


Piezoelectrics work by producing an electrical charge by being introduced to a mechanical stress. The specific piezoelectric benders in the sensors react to a 40 kHz signal. So when the piezo bender is driven by a 40 kHz electrical signal it will vibrate and produce an ultrasonic sound. When it is vibrated at a frequency of 40 kHz it will produce an electrical signal.

Piezos are used for various things in the electrical world. The ones from PUI Audio are simple transmitter and receivers. They can be used in various ways. Here we have made a simple proximity sensor.



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The circuit is split into two parts. We have a transmitter circuit and a receiver circuit. The transmitter circuit centers around a 555 timer. The timer is used to produce the 40 kHz signal. It is setup in astable mode which means that it does not take an outside trigger, but instead uses feedback for a trigger. To figure out the frequency of the output in astable configuration you use the equation f = 1.44/((R2 + 2*R1)*C2). Using 1k for R2, 5k for R1 and 3.3nF for C2, we get 39.669 kHz. This is close enough to the desired 40kHz to still work. The signal is then used to drive a transistor circuit, with a resistor pulling up on the collector.


Image from

The receiver circuit is a signal processing type circuit. The user has a few options depending on what they need to do with the signal. For this one we are just turning it into a simple analog signal. The processing starts out with increasing the gain of the signal. This is done by passing it through two op amps. We then send it through a diode to act as a rectifier. This makes sure we have a positive output. It is then sent through a comparator so that it does not output on a low signal. I did change one part for my circuit, on the comparator I had the feedback loop connect to the negative terminal with the diode output.


This is a simple way to setup the PUI Audio Piezoelectric Ultrasonic transmitters and receivers in a proximity sensor. There are many different ways you can use these and really it is up to the imagination, but this is a good place to get started.


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