Wilko Lunenburg's Simple Bat Detector Uses a Minimum of Parts and Requires No Programming

Tinkerer Wilko Lunenburg has published a guide to building a bat detector, which has just seven unique parts — not counting resistors, capacitors, and a project box for a housing.

"I made a simple bat detector, with just a handful of transistors, a binary divider and an audio amplifier," Lunenburg explains, "no microcontrollers, Arduino or even Raspberry Pi, so no coding required."

"Bats navigate with sonar, a kind of radar but with sound. Those sounds are short pulses of a high frequency, the wave length therefore is very small enabling them to 'see' small things, such a their food: insects. Those frequencies can range from 20 kHz up to 100 kHz. Humans cannot hear higher than 16 kHz, so we have to lower the frequency. When I built it I decided to use the dividing method of detection. With that method you amplify the sounds of the bats so much that it is clipped. It transforms from a sine wave to a square wave. This square wave still has the frequency that the bats use."

"This is done with a simple binary counter. I used the CD4040 which is somewhat overkill as it can divide in steps of 2 up to 4,096 and I chose the output that divides the input signal by 16. Only 4 stages of 12 of the binary counters are used. A bat signal at 40 kHz thus is divided to 2,500 Hz which is perfectly audible. And even a bat that produces sounds at 100 kHz can be heard at 6,250 Hz."

The finished project is about as simple as it could be: The bill of materials includes just one electret microphone, four NPN transistors, a CD4040 binary counter, one LM386 audio amplifier IC, a potentiometer, a switch, a loudspeaker, and a battery — plus some resistors, capacitors, and a project box to make the whole thing portable.

The full build guide — including some testing results, though sadly not yet any live recordings of frequency-divided bat sonar — can be found on Instructables.