I'm trying to build a driver circuit for an ultrasonic transducer for fog making. The circuit is required to generate a 1.6 MHz signal to drive the ultrasonic transducer.

I have found a circuit which seems to be working from here. The author of the video claims that he designed the circuit. Here is the circuit, ( The buzzer symbol is used for the ultrasonic transducer)

Ultrasonic fogger circuit

I have built the circuit and it works to some extent. I was able to capture an output waveform using my USB Oscilloscope. enter image description here

The circuit generate a good 1.7 MHz sine wave. However, I am unable to understand what type of oscillator is this. It seems to be a Colpitts oscillator, but I cannot find how to match this with a Colpitts oscillator.

I need to understand the operation of the circuit and how to change/calculate frequency values.


Here is the waveform between base and emitter. It has the same frequency enter image description here

  • 1
    \$\begingroup\$ That is just so wrong. Take a look at the waveforms in the emitter and base of Q1, these are probably going above the rail. I believe it is a class C stage of sorts. And the gain is coming from driving Q1 into reverse active region. I guess the RTC generates the input to be able to start the oscillation. \$\endgroup\$ – Edgar Brown Dec 1 '18 at 15:21
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    \$\begingroup\$ 4 volts peak to peak seems a low yield from a 32 volts supply. It's not a colpitts and it was probably slung together hit and miss until something worked and then the designer said freeze. \$\endgroup\$ – Andy aka Dec 1 '18 at 15:26
  • \$\begingroup\$ @Edgar Brown I have added the waveform between base and emitter. Can you kindly explain me what you meant by 'driving Q1 into reverse active region' ? \$\endgroup\$ – Anuradha Dec 1 '18 at 16:07
  • \$\begingroup\$ @Andy aka Yes it's indeed a low yield having such a low peak to peak voltage. That is an another problem I am trying to solve. Since the amplitude is low, the transducer can 'shake' the water but cannot produce any fog. Also the frequency seems to be wrong. The transducer is rated at 1.6 MHz. I have too suspected that the circuit is not exactly a 'designed' one but made by trial and error. But it seems to be working. That's why I'm interested. \$\endgroup\$ – Anuradha Dec 1 '18 at 16:13
  • \$\begingroup\$ I would suggest that you post a screen shot of the circuit shown in the video in case you have misinterpreted it. I honestly cannot see this producing a reliable oscillation. \$\endgroup\$ – Andy aka Dec 1 '18 at 16:31

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