A simple bat detector using frequency division to map the bat spectrum into the audio range was evaluated by Michael Dixon for his HND project for 1995/96. This used a piezo transducer as the microphone, and was based on a design in Electronics Today International in May 1992.
Although the work proved valuable the detector did not actually work, and an examination of the original circuit from the magazine showed some catastrophic design errors. Further work by N. Wakefield resulted in the correction of these faults, and the discovery that the output from this system was useful in detecting the presence of ultrasound, but gave no indication of its amplitude.
The design was therefore extended to incorporate an active detector and low pass filter to measure the amplitude, and mixing this with the divided signal using an Operational Transconductance Amplifier (OTA) to reinstate the amplitude from that of the incoming sound. This signal is then fed to a loudspeaker via an audio power amplifier stage.
All of these stages have been thoroughly tested and found to work effectively.
This circuit clearly has potential, but this is severely limited by the transducer. An alternative transducer will need to be sourced which is capable of detecting over most of the range from about 10kHz to 200kHz. This would then be divided by 16 (or 32) to map the signals into the range 625Hz to 12.5kHz (300Hz - 6kHz). Possible solutions are a small electret microphone such as are used in hearing aids, or a high frequency capacitor microphone.
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