piezoelectric driver circuit from square wave to sinusoidal

Thread Starter


Joined Jun 5, 2016
have started a project using these ultrasound sensors and the following circuit to drive them (c/- Patrick Mitchell and HobbyTronixStore) as my eletronic theory is about 30 years old and hazy

R1 - 220R
R2 - 220R
R3 -390R
R4 - 5k6
R5 - 1k8
R6 - 1k8
R7 - 10k
Q1 - 2N2222
C1 - 0.01uF...with a VCC of 9volts

This works fine and produces a square wave output of roughly 8.5 volts but there are a couple of things I am not sure of.
The 555 output is roughly 8.5 volts too so am not sure what the transistor is doing there apart from acting as a 1:1 driver.

I want a sine wave output, so connected up a signal generator to resistor R4 and found the output from the transistor has been saturated so that I am still getting a square wave....How would I produce something similar with a sine wave input to the transistor.


Joined Jun 4, 2014
As the ultrasonic devices are fairly sharply tuned (narrow bandwidth) the actual ultrasound produced will be a fairly good sine wave with square wave drive. Are you sure you need an better sine wave sound output?

Two ways of getting sine wave drive occur to me:
1. Connect a sine wave input to a linear amplifier (e.g. speaker amplifier, perhaps LM386) with output connected to the Piezo device.
2. Feed the square wave drive to the piezo through an LC tuned circuit or transformer. The tuned circuit will limit the bandwidth even further.


Joined Apr 24, 2011
As stated you would need a power linear amp to drive with a sine wave. Q1 is connected to act as an on off switch.

However, I have never seen these driven with a sine wave. I did lots of work in one place with 1 and 3 MHz crystals and we always used square wave drives, but got back very clean sinusoidal echos.

There is something out there called the "Ping))) sensor." If you google that term you will see lots of examples, some with schematics too.

Thread Starter


Joined Jun 5, 2016
Thanks guys for all your suggestions...have tried making a 1 watt amplifier with a LM386 based on the circuit below (9V) but while amplifying the voltage I am not getting a sound out of the transducer ??? Does it need a further transistor driver on the output?? Tried sine and square wave as input @ 20K.. Need some help.


Joined Jun 4, 2014
I am not getting a sound out of the transducer ??? Does it need a further transistor driver on the output?? Tried sine and square wave as input @ 20K..
Your original post linked to 40kHz transducers. They won't do anything useful at 20kHz.

You might also need to change to change the components on the LM386 output. The coupling capacitor could be a lot less than 250uF and the inductance of that capacitor may be a problem with the capacitive load of the transducer.

Thread Starter


Joined Jun 5, 2016
Thanks again guys...tried 40k and no luck..went back t the original 555 circuit and got a signal. I think I will give up on these transducers as I believe they are nothing more than glorified buzzers. Will try the disk piezoelectric transducers as I am after a modulated sine wave transfer.


Joined Jun 19, 2012
You might try sweeping the transducer with a range of frequencies to see if there is a resonance point.

Some ultrasonic transducers are designed to perform on resonance, which can be a very specific frequency.
Off resonance a few percent- and they perform terribly.


Joined Jan 22, 2018
I understand SuperRoo's concern about driving a piezo with a square wave. I recently put together the acoustic levitation kit (40KHz, totally silent !) and landed on this thread because I wanted to find an explanation for how I got such beautiful sine waves from square wave excitation (still searching). The best way to characterize the output was to directly connect a small piezo transducer to oscilloscope probes and bring this probe transducer close to a source transducer. Best of luck !