Here’s a novel way of listening to bats over the Summer. Put the receiver — powered by four AA (R6) cells — on a window-ledge, for example, preferably aiming the ultrasonic detector towards an outdoor light or some trees. Run out a few meters of cable so as to install the small loudspeaker inside the house. Wait for nightfall and, if there are bats around, you’ll hear a sound like bursts of crackling from the loudspeaker. Do note that bats won’t fly about under some weather condition (rain, strong winds, etc.)
The detector receives the ultrasonic signals, which are amplified by T1, T2, and T3, then sent to IC1 which is wired as a threshold detector. It converts the analog signal into digital pulses which it sends to IC2, which divides the signal by ten so as to make it audible to the human ear. The gain of the LF amplifier IC3 is adjusted automatically by transistor T4 and T5 depending on the amplification of the signal by T3, filtered by R7 and C14. The impedance between IC3 pin 2 and ground is what determines the amplifier gain.
The 40 kHz ultrasonic receiver used (MA40-R or SQ40-R) is available from Conrad Electronics (# 182281-62) or Farnell (# 213226).