Here you will find some discussion and circuits at various stages of development for different types of bat detectors. Most of this follows work done by students at the University of Northumbria at Newcastle in the UK under my direction as lecturer or project supervisor.
The work on a simple frequency division detector led to the development of a product which was specified as having a good frequency response, and being a realistic project for an electronics hobbyist as using readily available components and with a low component count. This link opens in a new window as it presents lots of detailed information.
Frequency division detectors give an output which has an unpleasant "scratchy" sound as all amplitude information from the incoming signal is lost. A more advanced exercise was the development of a frequency division detector where the sound amplitude was measured and restored. Detectors of this type are now available commercially as for example the BatBox DUET.
As part of this work some experiments were carried out on sensors, and the results are provided here.
Its tempting to think that bat sounds could be recorded with a laptop by feeding the amplified signal from an ultrasonic sensor directly into the soundcard. New top-of-the-range cards like the Sound Blaster X-Fi Elite Pro are capable of 96kHz sampling rate; however this is still insufficient to record bat sounds directly, as it still has a frequency limit of about 48kHz. For more information on recording with sound cards click the link.
Of course it is perfectly practicable to use a laptop computer to record bat sounds directly by connecting the signal output from your detector to the "line in" connection on your laptop. Just a bit bulky expensive and cumbersome for field work!
The information and circuits generated by these projects are provided here for interest and not as final working circuits but rather as a basis for further experimentation; however where problems are known to arise they are flagged.