The Digital "Bat Scanner" Detector
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As a rule, heterodyne bat detectors only allow the user to monitor a single range of frequencies at a time. If you are not constantly tweaking the tuning, you may easily miss a bat. I've built a number of heterodyne detectors, and find that they are more pleasing than a frequency division detector for listening to bat calls ... and more sensitive. But it can be a pain to be constantly tuning them so you don't miss a bat.

Then my friend Frank, who has worked with me on several bat projects, found a way to program a PIC microprocessor to generate continuous frequencies in the ultrasound range. From this discovery came an idea for a bat detector that had the ability to scan for bats ... automatically !! This scanning heterodyne bat detector I call the
"Bat Scanner".

The idea behind the scanning heterodyne bat detector was to use a PIC microprocessor to generate the mixing frequency for an otherwise standard heterodyne detecting circuit. The program that was developed allowed the PIC to enable the heterodyne detector to scan frequencies between any two predefined decades, from 10kHz to 80kHz, in 2 kHz steps. It was also possible to turn off the scanning and manually set the frequency if desired. The PIC also provides the detector with a digital LED display of the frequency that the detector is tuned to.

But there was still one more feature to be added ... a special detection circuit to let the PIC determine when there was a sound. This made it possible to automatically stop or pause the scanning when a sound was detected ... pretty cool :-)

Here is a simplified block diagram of the scanning heterodyne detector:

An electret microphone signal is fed to a two transistor preamplifier which selectively amplifies the frequencies in the ultrasound range. The preamplified ultrasound is fed to an NE612 mixer IC that converts the ultrasound to audible sound, using the reference frequency from the PIC processor. The audible sound is fed to an LM-386 amplifier IC and speaker, as well as back to the PIC so that it can detect when sound is present to pause or stop scanning. Three buttons on the PIC processor allow selection of the tuned frequency, as well as enabling the scanning mode and allowing the setting of various operating parameters.

 

Frank worked on the code for the PIC and made up the first prototypes ... and with a little tweaking they worked well.

My job was to design the circuit board, and put some final tweaks to the circuitry. We now have a second generation, 4 layer circuit board. The board has heavy traces, a split power plane, ground plane, solder masking on both sides, and a very readable silk-screened parts layout.

We, and others, have constructed many Bat Scanner detectors over the last year. The circuitry, and PIC firmware, is fully operational, tested, and debugged.

The idea of a scanning heterodyne isn't really new. There have been a couple of expensive commercial detectors that have had scanning features. But this detector is relatively low in cost, less than $100 to build, and has a very efficient design.

Here is a sample bat call with feeding buzz recorded from a BatScanner

     

Notes on Construction ...

The scanning heterodyne bat detector is not as easy as the Simple Bat Detector to construct. The circuit board has many more parts, and many of them are quite small. The case requires a lot more in the way of tooling to prepare it for assembly.

Circuit board construction requires a decent temperature controlled soldering iron with a small chisel or conical tip. Good lighting, and a magnifying glass are essential for working with some of the small parts. You also need to be mindful of ESD precautions when handling the semi-conductors - especially the PIC microprocessor chip.

Download and look over the manuals listed below to see all the suggested steps for construction of the bat detector. If this looks like the type of project you would like to take on, then email me :-)

If you are in the USA, and are interested in building one of these detectors, email me to see what's currently available. I will try to have complete parts kits made up, so you only need to add is some solder, and a 9 volt battery. All of the manuals for constructing and using the Bat Scanner detector are listed below for downloading and viewing. Be sure to go through each of them to see what is involved in building one of these detectors.

Manuals:

Diagrams:

If you are in Europe, and want to build a scanning heterodyne detector, you should email Frank to see if he has any kits available. Frank also has a web page with more information on the detector at: http://cms.zwergfledermaus.de/


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