Bat Detector

This is one of those fun projects that'll bring a smile to your face. When I heard my first Bat using this detector it reminded me of the thrill I had years ago when I heard my first radio station coming through on a homemade crystal set. I wanted to build this as I always enjoy sitting in my garden after sunset watching the local Bats flittering overhead and thought that hearing them as well would add a further dimension to the experience, which it certainly does. The function of a Bat detector is to pick up the ultrasonic sound that a Bat emits for its echo location which we can't hear and convert it down in frequency to something we can hear. There are various ways to achieve this but one of the simplest methods is frequency division. As its name suggests, it takes the high frequency sound that a Bat makes and divides it down to a frequency within our hearing range. Click the play button below to hear a sample of what a Bat sounds like when heard with this bat detector

Schematic

The circuit has three stages. A preamplifier to boost the signal from the microphone, a divider to down convert the frequency of the Bat to something we can hear and a power amplifier to drive the loudspeaker. The preamp is based on a standard dual opamp and although various types will work, I found the TL072 to be the 'liveliest'. It has two stages, each configured as an inverting amplifier. The first stage is set to a gain of 22 and the second is set to 68. Multiplying these two figures together gives an overall gain of 1496. Experimentation revealed that the circuit was more stable with less gain on the first stage and more gain on the second. Having too high an overall gain causes the circuit to break into oscillation and so the final figure chosen was a balance between stability and sensitivity. The output from the preamp is fed directly into the clock input of a CD4024 divider IC. This IC has outputs at differing division ratios but the one used here is divide by 16, which gives the perfect detection frequency bandwidth. For example, a 40 kHz signal at the microphone divided by 16 would become 2.5 kHz with 80 kHz becoming 5 kHz, both well within our hearing range. In fact, much higher frequencies could be detected but the limiting factor is the microphone. Microphones are generally used for audio and as such their spec sheets usually only show the frequency response up to about 20 kHz, though in practice, some mics will go much higher and can be useable at ultrasonic frequencies. Of course the output will be much lower, but the preamp should be able to compensate for this

Veroboard layout

When designing something like this you have to forget everything you know about Hi-Fi principles as they don't apply here. I wouldn't want to look at any of the waveforms in this circuit with an oscilloscope as I would probably be horrified. The thought of feeding an analogue signal directly into a logic chip is bizarre, but as the signal contains high and low levels it actually triggers the input quite nicely! Because the output level of a logic IC has a constant amplitude, what you hear will not be a perfect representation of the sound a Bat makes (more complex heterodyne detectors are available that convert amplitude as well, and are also tunable to distinguish between species) but overall the results are pretty good. As there is plenty of level from the divider IC, rather than using a full blown audio amplifier I just fed the output into two transistors in push pull configuration. In fact, I had to add a volume control as it can go quite loud when driving an 8 ohm speaker, which incidentally remains completely silent until a Bat is detected. Because the CD4024 draws very little current and the transistor output stage is biased as class B, the overall quiescent current is mainly just that of the opamp (less than 4mA at 9V). I just managed to squeeze it all onto a 9 x 25 hole veroboard so I've shown the cuts and links below in more clarity as it's quite densely packed. Note on the main layout above that some of the pins on the CD4024 DIL socket are shown as White dots instead of Black dots to indicate they've been snipped off and removed

Track cuts and links looking at underside (cuts underneath, links on top)

It doesn't matter how good the electronics is if the microphone used doesn't respond to ultrasonic frequencies, so selecting a suitable microphone is very important if the detector is to work properly. First I tried a dedicated ultrasonic transducer like the type used in old fashioned TV remote controls, but didn't have much success. Maybe it was a dud! I also tried a couple of electrets... the Panasonic WM-61A which although now discontinued, is still available from a few eBay sellers at a reasonable price (beware of fakes though!) and the Primo EM258 which is in production, inexpensive and suggested as an equivalent to the Panasonic. Although these worked, they just didn't have the range or sensitivity of my final choice which was a MEMS type microphone (Micro-Electro-Mechanical System). These relatively new devices use micro sensors fabricated from silicon and can work well up into the ultrasonic region. The one chosen for this project is the Knowles SPU0410LR5H-QB which although being about twice the price of the electret inserts, is still not overly expensive and given the boost in performance is well worth it. All these mics require a voltage supply to work as they have an integral preamp stage. This voltage varies depending on the device but is normally around 2 to 3 volts. With an electret microphone you can usually get away with just a feed resistor from the main power rail, but a MEMS microphone shouldn't exceed it's maximum rating. A perfect solution to power the Knowles is to utilize the 2.5 volts developed across a blue LED as I'm doing here. A nice bonus from this is that it can also double up as a power indicator by drilling a hole in the case for it to shine through. The Knowles mic was purchased from micbooster.com (FEL Communications LTD.) who can supply it pre-mounted on a small PCB (believe me this makes life so much easier!). This PCB has the three connection points... supply plus (+), supply minus (-) and mic output and there's a handy mounting hole as well. If you want to try an electret mic, then just remove the LED and add another 4K7 resistor from the pad of the LED anode to the mic input. If you don't have a 'Test Bat' available, a good way to check if the circuit is working is to jangle a bunch of keys in front of the mic. This will produce a squeaky, clicky, raspy sound (well that's the best I can describe it anyway) but an actual Bat will sound much more easy on the ears! Other test sounds are the letters 'S' and 'F' (by coincidence my initials)

The enclosure chosen for this project is a 140 x 66 x 28mm Hammond 1593YBK. It's big enough to fit all the components inside, small enough to be pocketable, has a battery compartment and a recessed lid for an overlay if you want to personalise it. The best choice of loudspeaker is a 40mm low profile Mylar type. In some instances, feedback may occur at high volume. If so, this is easily cured by placing a small piece of cotton wool just behind the microphone. The 5K volume control pot is a switched type and is mounted on a small piece of veroboard. This sub assembly and the main board are connected together using 7 way Molex KK type PCB headers and housings, but for the microphone I found 3 way JST XH types to be more suitable. An overlay for the enclosure can be downloaded here, a copy of which can be used as a template to mark the holes for the loudspeaker before drilling. A small hole for the LED will also be required. The main overlay is printed onto self adhesive matt photo paper (set print height to 5 inches) and then covered with clear sticky back plastic film. Once the overlay is cut out and fitted, the speaker holes can be punched through using a metal spike. The LED simply shines straight through the paper

Wiring diagram showing the interconnections

LINKS:

micbooster.com for Knowles SPU0410LR5H-QB and Primo EM258
micronic.co.uk for Panasonic WM-61A
rapidonline.com for Hammond 1593YBK