Bats in the garden

Heterodyne DIY batdetector Since several years we saw that shortly after sunset several small bats were flying close to our house on the garden side. After some research I found out that it was relatively simple to design and build a heterodyne bat-receiver especially since I'm an electronics engineer that worked a large part of my cariere in cable television and am since a few years heavily involved in smart metering and powerline communication. Finding the time was rather difficult however ...This page describes the result of the effort with some pictures of the detector and a sound file with the recorded result which is quite impressive (being able to hear those small creatures that is) ...   Prototype "hardhat" helmet based bat-receiver. Parabolic microphone on the left, receiver circuit on the right. A simple low cost headphone is used. Receiver circuit based on a transistor pre-amplifier, Siemens SO42P double balanced mixer followed by a simple LM386 audio amplifier circuit. Oscillator coil is a 56 mH pot-core inductor with a middle tap. Prototype PCB The "hardhat" helmet used to mount the parabolic microphone and receiver to. This way it is very easy to track a bat, as the microphone automatically points to where hou are looking when following a bat in flight. As a microphone I use a simple and inexpensive 6 mm diameter elektret microphone capsule which seems to be sensitive enough to clearly receive sounds at frequencies up to at least 50 kHz. Sensitivity is such that you can hear bats at a distance of about 50 m. A recorded sound file of an active bat period: Recorded bat sound (mp3) . Recorded at 16 bit resolution and 44.1 kHz sampling frequency via a normal sound card in a PC. Best reception quality with these bats is with the local oscillator of the detector set to about 48 kHz. Details are shown in the pictures below: 1 minute 7 seconds recording of an active period Detail of single bat sonar area scan pulse Detail of a bat searching for bugs and then increasing the rate of firing of the sonar pulses when a bug is in his sight Detail of the burst of short pulses emitted when the bat is ready to catch a bug Detail of the apparently single cycle "clicks" transmitted by the bats when zooming in on bugs.