Yet another analogue ultrasonic bat detector design.
By Guy Fernando
Created Jun 2026 - Last modified Jun 2026
Bats navigate and hunt using echolocation, emitting short bursts of ultrasonic sound and listening for echoes reflected from nearby objects. These calls are typically above the range of human hearing, often between 20 kHz and 100 kHz, making them inaudible without specialised equipment. An ultrasonic bat detector converts these high-frequency sounds into frequencies that can be heard through a loudspeaker or headphones, allowing the listener to observe bat activity and, with experience, identify different species from the characteristics of their calls.
Several methods are commonly used to make ultrasonic sounds audible, including frequency division, time expansion and heterodyning. The latter is perhaps the simplest and most widely used technique in portable bat detectors. A heterodyne detector mixes the incoming ultrasonic signal with a locally generated ultrasonic frequency. The difference between these two frequencies falls within the audio range and can therefore be heard directly.
The detector described here is a compact analogue heterodyne design intended for hobbyist construction. It uses modern, readily available components throughout and operates from just two AAA cells. The design goal was to achieve good sensitivity and audio fidelity over a tuning range of approximately 15 kHz to 80 kHz while keeping the circuit simple, inexpensive and easy to build using conventional through-hole components and veroboard construction.
The completed detector is housed in a compact hand-held plastic enclosure measuring approximately the size of a small television remote control. A MEMS ultrasonic microphone is mounted at the top of the case behind a foam windscreen, while a front-mounted loudspeaker provides audible monitoring of detected bat calls. The front panel contains three rotary controls labelled Gain, Volume and Tune, together with a power switch and power indicator LED.
In operation, the Tune control is adjusted to select the ultrasonic frequency of interest, while the Gain control allows the sensitivity to be optimised for local conditions. The converted audio can be listened to through the internal loudspeaker or through stereo headphones connected to the 3.5 mm headphone socket. The detector is powered by two AAA batteries, providing a simple and portable solution for outdoor use.
The detector employs the conventional heterodyne principle to convert ultrasonic bat calls into audible frequencies. The circuit is divided into five main functional blocks: microphone amplifier, ultrasonic filtering, local oscillator, commutating mixer and audio amplifier.
Ultrasonic signals are received by an SPU0410LR5H MEMS microphone. This device was selected because of its useful sensitivity at ultrasonic frequencies, low operating voltage and widespread availability. The microphone output is AC-coupled into a variable-gain pre-amplifier built around one section of an MCP6024 quad operational amplifier. The gain can be adjusted over a wide range to accommodate both weak and strong signals.
The amplified signal then passes through a second-order Sallen-Key high-pass filter with a cut-off frequency of approximately 15 kHz. This filter significantly attenuates audible sounds such as wind noise, speech and handling noise while passing the ultrasonic frequencies of interest. A further stage of gain is provided by the filter amplifier itself.
The local oscillator is implemented using another section of the MCP6024 configured as a relaxation oscillator. The Tune control adjusts the oscillation frequency over a range of approximately 10 kHz to 70 kHz, covering the echolocation frequencies used by most British bat species. The oscillator produces complementary switching signals which drive a pair of analogue switches within a 74HC4066.
Rather than using the now obsolete SA602 or SA612 Gilbert-cell mixer found in many traditional bat detector designs, this circuit employs a commutating mixer. The amplified ultrasonic signal is alternately switched between positive and negative signal paths under control of the local oscillator. This technique effectively multiplies the incoming signal by a square-wave reference and produces the desired difference-frequency products while requiring only a single inexpensive and readily available integrated circuit.
The mixer output is fed to an active low-pass filter which removes the higher-frequency switching products and passes only the audio-frequency difference signal. This stage also provides additional gain and helps improve the overall sound quality of the detector.
The recovered audio is routed through a volume control and then to a TDA2822 stereo audio power amplifier. Although designed as a stereo device, both amplifier sections are used to provide sufficient output power for the internal loudspeaker and stereo headphones. The detector can therefore be operated either through its built-in speaker or through headphones when quieter operation is preferred.
Power is supplied by two AAA cells providing a nominal 3 V supply. The MCP6024, 74HC4066 and TDA2822 all operate reliably at this voltage, eliminating the need for voltage converters or additional power-management circuitry. The result is a simple, low-power design using only three integrated circuits and a modest number of discrete components.
The detector was constructed entirely using veroboard and through-hole components. To make efficient use of the limited space available within the enclosure, the electronics were divided between two stacked circuit boards. The lower board contains the main signal-processing circuitry, while the upper board carries the front-panel components including the Gain, Volume and Tune controls, power switch and indicator LED.
Interconnecting wiring between the two boards was kept as short as practical to minimise unwanted noise pickup. The loudspeaker is mounted directly inside the enclosure lid, while the MEMS microphone is positioned at the top of the case behind a small foam windscreen. Despite the compact dimensions of the enclosure, all components fitted comfortably with careful planning of the mechanical layout. The completed instrument is robust, lightweight and well suited to portable field use.
This project demonstrates that an effective ultrasonic bat detector can be constructed using a small number of readily available components and straightforward construction techniques. By combining a modern MEMS microphone, a quad rail-to-rail operational amplifier, a commutating mixer based on the 74HC4066 and a low-voltage audio power amplifier, it is possible to achieve a useful tuning range of 15 kHz to 80 kHz while operating from only two AAA batteries.
Although many modern bat detectors rely on digital signal processing and microcontrollers, there remains considerable value in a simple analogue approach. The circuit is easy to understand, requires no firmware development and can be built using commonly available through-hole parts. The use of a commutating mixer also avoids dependence on obsolete mixer integrated circuits that are increasingly difficult to obtain.
The detector has been successfully tested using laboratory ultrasonic signal sources and has demonstrated good sensitivity throughout its tuning range. Future field testing will determine its effectiveness in detecting and monitoring local bat populations. Regardless of the outcome, the project provides an interesting example of modern analogue signal processing applied to a practical wildlife observation instrument.
This website is powered using ultra low power green locally based servers.