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Build a Simple Bat Detector
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Tony Messina - Las Vegas, NV

Ever since my grade school years I've been fascinated with the way a bat uses ultrasound to "see" in the dark. The bat's echolocation skills let it avoid small obstacles, and even catch insects, while in flight. I learned that bats are, ultrasonically speaking, very loud --- so loud, that some of them have special ear flaps that close when they generate an ultrasound pulse so they won't make themselves deaf. I thought something this loud should be easy to detect !

On retirement, I decided to undertake the development of field instruments for bat research. One of the first devices I designed used a simple and inexpensive circuit to make an ultra-portable, personal bat detector. A picture of my original prototype unit is shown at the left. It is small in size, lightweight, easy to build, and can cost as little as $25 in parts. It turned out to be an amazingly effective circuit so I dubbed it the Simple Bat Detector.

I originally published the circuit for the Simple Bat Detector in 1997. Since then, I've received emails from all over the world, corresponding with many who have built the detector from the information published on this page.

In 1998, I designed a circuit board for an enhanced version of the detector, and started making kits and assembled detectors available for those who had trouble finding parts, or were unable to assemble electronics. I've been thrilled by the many comments of folks who have used the Simple Bat Detector to hear a bats echo call for the very first time.... and have come to admire and respect the bat as I do.

So far, the Simple Bat Detector has found its way to: Australia, Belgium, Canada, the Canary Islands, the Cayman Islands, Costa Rica, England, Finland, France, Germany, Ireland, Italy, Japan, Poland, Mexico, the Netherlands, New Zealand, Scotland, Singapore, South Africa, Spain, Sweden, Switzerland, and all over the United States.

 


How the SIMPLE BAT DETECTOR works...

The Simple Bat Detector is a frequency division type device. Frequency division type detectors allow you to hear ultrasonic sound by digitally scaling the frequency down into the human hearing range. For instance, a western pipistrelle bat emits ultrasonic sound in the range of 53 to 91 kHz. If you divide that frequency by 16, the new frequency range is 3.3 to 5.7 kHz, easily within our hearing range. Because the division is done digitally, all amplitude information is lost. Ultrasonic sources processed by the detector convert to sounds like geiger-counter clicks and chirps.

The basic circuit of the Simple Bat Detector is shown in the schematic diagram to the right. It is essentially composed of 3 integrated circuits, or ICs. The signal from an ultrasonic transducer is fed to IC-1, an LM386 audio amplifier, which is configured to provide a signal gain of 200. The signal is coupled to IC-2, a second LM386, by a .05 uf capacitor. IC-2 is configured to provide an additional gain of 20, for a total system gain of 4,000. The output of IC-2 is direct coupled to the input of IC-3, a 7 stage CMOS digital divider circuit. The input stage of the divider acts as a zero crossing detector, triggering on the negative transition of the signal from IC-2. The divide by 16 output is connected to a potentiometer, which serves as an audio level control. A high impedance ceramic earphone is connected to the output of the level control. The 10K level control is a small printed circuit pot that is set and forgotten. The detector circuit is powered by a nine volt battery. ( The numbers next to the IC nodes refer to the pin numbers of the IC's. Note the additional pins listed at the bottom of the schematic that need to be tied to ground. )

A major advantage of a frequency division detector is that it is a wide band device ... that means it will let you hear all detectable bat sounds without the need to tune the detector to any particular frequency. Heterodyne detectors, which process ultrasonic sound in the analog domain, only convert a small range of frequencies at any given time - you must select which frequencies to listen to. If you tune up around 60 kHz to listen for a pipistrelle, you may not hear the big brown bats flying nearby. The frequency division detector works in the digital domain, converting the full spectrum of sound that the transducer is able to detect. So you get to listen to all of the ultrasonic sounds around you, without missing anything due to unfortunate tuning choices. I feel this no-knobs-needed characteristic of the frequency division detector makes it a great choice for the casual bat observer, and student.


Building a SIMPLE BAT DETECTOR...

Although I have designed a circuit board for an enhanced version of the Simple Bat Detector, it is important to note that the basic circuit shown above is very effective - and parts are available world-wide. Many have built the detector in the same manner as I did with my original prototype - on perf board - as shown to the left. If you don't want to use the perf-board technique, the circuit can always be assembled on the enhanced PCB - as shown to the right. The circuit assembly is then mounted inside the case with hot-melt glue.

The ultrasonic transducer is simply mounted to the front panel by drilling 2 small holes for the transducer pins, and bending the pins over after inserting them through the panel.

Keep all wire lengths short, as very high gains are being generated in IC-1 and IC-2. The picture on the right also shows a method for installing an external volume control, if you feel one is needed. The wiring of the earphone jack is also a little tricky, as it is doubling as the power switch ... so pay careful attention to the schematic above.

In order to accomodate various transducers, you can easily tailor the gain of the two LM386 IC's. An additional resistor can be added in series with the 10uf tantalum capacitor to reduce the gain of IC-1. A resistor / capacitor combination can also be added between pins 1 and 8 of IC-2 to increase it's gain. In this way the total system gain can vary anywhere from 400 to 40,000. Be sure to keep the positive side of the capacitor towards pin #1 on the LM386.

If you have experience in electronics repair or assembly, you can usually build a Simple Bat Detector in a single evening. I will always try to have a supply of circuit boards and components kits on hand for the Simple Bat Detector, so if you have trouble finding parts - email me. Also, if you don't solder, don't despair, completely assembled detectors are also available!

As far as bat detector cases go, you can use the enclosure on the parts list, or go creative ... This is an area where you can cut cost with a little imaginative engineering. Here are a couple of prime examples...

To the left is a picture of a Simple Bat Detector that was cleverly designed by Hisafumi Tateno, in Japan, using a plastic jar as the case. A film container was employed as a battery holder. This design has the added benefit of letting you see what's inside, as well as listening to bats !

The picture to the right is a detector I built for Jackie, my spouse. Being married to a batty person like me, she deserved something special. I used an off-white enclosure, and added a cloisonné bat pin that I had found in a gift shop.

I would be interested to get other pictures from Simple Bat Detector builders who have dared to be different. I could always set up a gallery of the most interesting designs!

The instructions on this site have been made as complete as possible to allow anyone to build the detector, with whatever local resources that can be found. A resource list has been compiled with part numbers, component sources, and email contacts in several countries. The list is always changing based on information I get from Simple Bat Detector builders via email. If you construct a Simple Bat Detector, be sure to send me an email and let me know how it worked out for you.

 

Updates on the Simple Bat Detector ...

How does the Simple Bat Detector do in the field ? Here are the Field Reports.
Here's a bat call recorded with a Simple Bat Detector ... BatCall1.wav (64KB).

Where to find more information about Bats:

Other Websites on Building Bat Detectors:

 

Tony Messina - Las Vegas, Nevada - email: T-Rex@ix.netcom.com

First published September, 1997 - Last update: May 17th, 2001