This ultrasonic amplifier project comes in two parts, design, and testing. Today we will cover the design and rational.
Many animals emit high pitch noises, what we call ultrasonic, that is beyond the hearing range of humans. Two such animals I am interested in are rats/mice, and bats. I want to hear the noises that they emit.
The classic approach that many bat detectors use is to amplify the signal coming from an ultrasonic microphone, then just treat it as a square wave and divide it down into human hearing range. This seems a little cheap to me, as you are getting a severely distorted noise, that does not carry amplitude info at all. Imagine a speaker screeching in distortion because someone set it too high, that is essentially what you are hearing from this detector.
My thought was, why not do a FFT (or even better, an FHT) to get the spectrum of the sound, compress it if needed, and then shift it down into human hearing range. Lastly, we just do an inverse FFT/FHT, and voila, truthful sound at roughly the right speed.
ultrasonic amplifier Circuit Design
First things first, we need to amplify the signal from a ultrasonic microphone. The ultrasonic amplifier works in three stages, with the final stage having an adjustable 1-100x gain. When selecting the gain per stage, don’t forget to consider the gain-bandwidth product of the amplifier! The 1/2Vcc below is generated by a simple resistor divider, which works fine in this use case, as your op-amp input impedance is very high.
This signal is then fed to a single stage Sallen-key filter, to remove any signals above ~90kHz, and then into an ADC. Remember to always chose an ADC with a sampling frequency at least 2x your highest signal frequency, this is known as the Nyquist rate. In reality, you want a sampling frequency of ~5x or more, to get a decent picture of the sampled waveform.
The signal can then be read by a oscilloscope for testing, and fed via SPI to a microcontroller. I will be using the ESP8266 for the FHT and IFHT steps later, possibly showing the waveform in a webpage.
The circuit layout is a fairly straightforward 2 layer board. The only trick for this ultrasonic amplifier is that the microphone is mounted horizontally to save on board space.
Now to wait 2-4 weeks for delivery and assembly 🙂
As always, these projects are open source and usable for any reason (preferably with some attribution). You can find the designs on my Altium circuit maker page: