Hack Together Your Own Bat Signal

Bats use echolocation to see objects in front of them. They emit an ultrasonic pulse around 20 kHz (and up to 100 kHz) and then sense the pulses as they reflect off an object and back to the bat. It’s the same type of mechanism used by ultrasonic proximity sensors for object-avoidance. Humans (except perhaps the very young ones) can’t hear the ultrasonic pulses since the frequency is too high, but an inexpensive microphone in a simple bat detector could. As it turns out bat detectors are available off the shelf, but where’s the fun in that? So, like any good hacker, [WilkoL] decided to build his own.

[WilkoL’s] design is composed primarily of an electret microphone, microphone preamplifier, CD4040 binary counter, LM386 audio amplifier, and a speaker. Audio signals are analog and their amplitudes vary based on how close the sound is to the microphone. [WilkoL] wanted to pick up bat sounds as far away as possible, so he cranked up the gain of the microphone preamplifier by quite a bit, essentially railing the amplifiers. Since he mostly cares about the frequency of the sound and not the amplitude, he wasn’t concerned about saturating the transistor output.

The CD4040 then divides the signal by a factor of 16, generating an output signal within the audible frequency range of the human ear. A bat signal of 20 kHz divides down to 1.25 kHz and a bat signal of up to 100 kHz divides down to 6.25 kHz.

He was able to test his bat detector with an ultrasonic range finder and by the noise generated from jingling his keychain (apparently there are some pretty non-audible high-frequency components from jingling keys). He hasn’t yet been able to get a recording of his device picking up bats. It has detected bats on a number of occasions, but he was a bit too late to get it on video.

Anyway, we’re definitely looking forward to seeing the bat detector in action! Who knows, maybe he’ll find Batman.

27 thoughts on “Hack Together Your Own Bat Signal

    1. … for example, the Zenith “Space Command” remote control. We had one (1960’s), and from the factory, the tuner only handled channels 2-13 (U.S., VHF). If I remember correctly, ít had a provision to add a tuner for UHF (special 14th position on the tuner). The alternative was a Blonder-Tongue UHF adapter. Don’t remember for sure, but I think it output on VHF frequencies.

  1. WilkoL here. Thanks for having my simple detector on Hackaday! It is correct that there aren’t that many bats here, but I have detected bats on several occasions. I just can’t get it recorded…

    1. Hi WilkoL, thanks for the correction! I’ve updated the article accordingly. Let us when you’re able to get a recording. We’re pretty excited to see it.

  2. There was an instrument that HP made a long time ago, called an ultrasonic translator leak detector. The principle was, since the pitch of the sound air going through a small hole makes is inversely proportional to the diameter of the hole, if a leak is small enough, you can’t hear it. So they made this “translator”, which is really just an oscillator and mixer, kind of like a superheterodyne radio, only acoustic. It shifts the spectrum of the sound down by the frequency of the oscillator. Because this doesn’t use counters, the sound doesn’t have to be at digital triggering levels, so it is very sensitive. It had more than one transducer – I think one was a contact probe, and another was a directional microphone, probably not important for bat detection, but real useful for pinpointing (as it were) small gas leaks.

      1. Well, the pictures I’m finding for the 4917, 4918, and 4905, all look alike, and look like the instrument I used. But this was 1980 or so, and I don’t remember exactly which model we had. It may be that they differ by how far they translate the signals, for different applications.

        But HEY! Here’s a video showing a 4918A in action!

    1. Yes, but sound quality wasn’t the goal in this one. The output of the divider is a squarewave anyways. I made this one to warn me when bats are around. They are rather rare here and I didn’t want to stare at an empty sky all night. I’m building a heterodyne and dividing detector that should sound better.

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