Bat Detector kit

[TheWelly888]

My old schoolmate's 6 year old daughter is fascinated by the bats flying around their house; a few months ago I rashly offered to build a bat detector out of my electronic junkbox!

I'm going round to their house in a couple of weeks time and my mate emailed me,"She has been asking 'Is the bat detector ready yet please?' " I had to reply, "Probably not"

So I had a look at some old electret microphones in my junkbox - sadly they were dead! So I had to find some alternative that's likely to work. Most ultrasonic sensors appear to be specified for a narrow bandwidth and one needs something wider than that so I searched for a suitable kit ( I'm not buying a ready made one as it's no fun for me! )

I found this one supplied by Magenta Electronics ( UK based ) http://www.magenta2000.co.uk/acatalog/Bat_Detector_Mk_2.html It is based on a VFO and a SA602 mixer to an audio amp, a nice simple looking circuit.

It comes with a ready drilled case and a stick-on label - [Dave's voice] beauty! [/Dave's voice]

First picture is the components - only misgiving is the fact that the ICs have been stuck in polystyrene.
Second is the rather crudely drawn circuit diagram which is only just clear enough. Naturally I have downloaded the datasheets for the mixer and audio amp ICs.

Let's see how I get on.

[saturation]

I like it!  A novel ideal, pick up the ultrasonics of bats and convert it to a human audible frequency.  Maybe you can modify it so you have an option to switch to flash an LED to save battery power, rather than playback the down-frequency of the bat sonics.

[migsantiago]

Hi!

Some questions, what's a VFO? What's the audio frequency that a bat emitts (ultrasound of course)? 

[David]

Hi!

Some questions, what's a VFO? What's the audio frequency that a bat emitts (ultrasound of course)? 

Variable frequency oscillator...I think!

[saturation]

More on bat detectors:

http://www.openobject.org/objectsinflux/?p=16

Bats can emit up to 100kHz, depending on species.

Ultrasound is anything over 20kHz.

Yes, VFO from the schematic looks like a variable frequency oscillator.

[Matt]

This is pretty much one of the "industry standard" bat detectors:
http://www.titley.com.au/anabat-bioacoustics-anabat-bat-detector

I was lucky enough to participate in a National Science Foundation (US) sponsored research program studying bat ultrasound.  This was a joint effort between the biology, computer science, and electronics programs at my old school.  Another student and I comprised the electronics group.  We were able to design and build small transmitters, weighing <1.5g that were placed on actual bat subjects out in the field.  The transmitters consisted of an ultrasonic microphone feeding into a VCO, which transmitted the ultrasound from flying bats to a receiver.  The receiver was then fed into a DAQ connected to a laptop to record the bat calls. The recording and analysis software was written completely from scratch by the computer science guys.  It was a very fun project to be a part of.  Not only did we get to design and build the hardware, we also went out into the field with the biology guys to help capture the bats and run the recording setup. The research is still on-going, but before I had to leave the project, we were able to obtain some very interesting results.

-Matt

[DJPhil]

This reminded me of something a while back. Turns out scientists used a similar (identical?) device to make the ultrasonic vocalizations of rats audible.

The video's worth a peek, just a few minutes. Gives me a warm fuzzy every time.

[TheWelly888]

Good to see interest in this project!

Progress so far...

Before I populated the pcb, I decided to screw together the pcb and the case to check clearance for the loudspeaker. Then I found a snag, the self-tapping screws were just too fat for the moulded pillar in the case front and cracked them. ( photo 1 ) These will eventually disintegrate so I found some countersunk bolts , plastic spacers and nuts in my junk-box ( they took hours of finding cos I have so much crap!!! ) then using the moulded pillars as guides drilled through the case front and using a craft knife countersunk the holes just enough to allow the bolt heads to sit flush with the surface to avoid making bumps under the overlay. ( see photo 4 as well. )

The pcb is then populated with lowest components in first then finally the tallest electrolytic caps. ( photo 2 ) The ultrasound sensor is soldered to it's separate mounting board which will be secured using sticky foam pads and connected to the pcb via a co-ax lead. ( photo 3 )

Before gluing the speaker in place, I lined up the cone of the speaker with the four holes and marked all the way round the rim. UHU adhesive was applied to the edge of the rim and the speaker stuck down after lining with the mark. ( photo 4 )

[TheWelly888]

Yes! The kit is complete and it works! ( photo 1 )

I wired up the pcb to the battery contacts, speaker, power-on LED. I left off the sensor at this stage

I first fitted the VFO IC ( TL071 op-amp ) and checked that it oscillates and hooked up the output to my scope and determined the frequency range. It goes from 20.8kHz to 147kHz and the waveform is triangular.

Next are the audio amp and the mixer ICs. I temporarily connected a signal generator to the input via a co-ax microphone lead and inputted ultrasound frequencies to it, it was very sensitive and I suspected I was hearing clipping so I decided to secure the sensor.

The instructions stated, "..take care to fit the microphone board so that the microphone is central in it's hole and does not touch the case." Obviously to avoid the sensor picking up handling noise! Because the sensor is secured to the case with 2 layers of sticky sided foam, I needed to get it right first time so I wrapped the sensor with a strip of scrap paper and pushed the lot through the hole, the paper acting as spacer ( photo 2 )

It was easy to assemble the whole case apart from a small mistake on my part. I covered the back of the speaker with a piece of 30 thou thick plasticard to prevent the solder joints touching the speaker but the clearance inside the case is so tight that it wasn't possible to get the rear half of the case on! I sanded the plasticard down and the case fits! Should have used 10 thou plasticard!

The battery holding foam block was stuck in and the unit powered up from a PP3 battery, the foam holds the battery and prevents rattling though the battery lid is rather fiddly.

Testing...

Rubbing dry finger tips together got a response, so does rattling keys. Key rattling probably produces broadband ultrasound so I tuned the VFO across the band and noticed that the peak sensitivity is at 40kHz which is what I expected for that type. I also pointed it at a CRT television set and computer monitor and could "hear" the line coils!

I was concerned that the unit would pick up the 60kHz MSF atomic clock radio signal but I did not hear any emf pick-up at that frequency.

To check the range, my father operated a ultrasound distance measurer whilst I moved backwards and the detector was able to detect the measurer at a range of at least 30 metres at 40kHz and was able to pick up 20 metres when set to 100kHz! A pretty good range for something inexpensive!

It is nearly 9pm BST at the time of typing and it's getting dark soon, so hopefully a bat will fly about within the hour.....

My conclusion, it's a nice easy project to put together but then I have a lot of experience of soldering and electronic assembly. The pcb mounting was the only real problem with this but I was able to overcome it with some thought. I might get one for myself or I'll design a better one using a sensor with a broader frequency response - most probably a condensor microphone.

Overall THUMBS UP!

[saturation]

If you hear a bat with it let us know, and recorded the output if you can.  Enjoy, and good work.

[TheWelly888]

Sadly there weren't any last night and I'll be lucky to hear one before I hand it to my friend's daughter on Saturday. But I enjoyed putting it together.