This bat detector uses cheap off the shelf components, is easy to build and gives good performance. The heterodyne principle is used in this detector. Incoming ultrasonic bat calls are fed into a self oscillating mixer Q2 via the ultrasonic microphone and preamplifier Q1. Several preamps were tried ( cascode fet,cascode bipolar and combinations of ) but surprisingly this simple common emitter preamp gave the best results. The wanted IF signal ( 455 kHz is used here ) is passed through a ceramic filter FL1. This filter provides excellent selectivity and keeps any local oscillator signal out of the IF amplifier ( the local oscillator signal is only 20 kHz away from the IF frequency at the low end of the band, IF transformers on there own struggle to keep the LO out of the IF amp at the low frequency end of the band ). The filter used here was taken from a scrap FM CB set and has a -6dB bandwith of roughly +/- 3kHz, most CB and PMR type filters ( AM or FM ) will work here fine. A simple two stage IF amplifier follows the ceramic filter and provides the bulk of the gain in this circuit, Q3 employs a small amount of emitter degeneration to raise its input impedance giving a suitable match to FL1. No special IF transistors are used, BC107c's are adequate but be sure to use the C gain group as the very simple bias networks are calculated for this Hfe range. The IF transformers are 455kHz types, however you can use domestic broadcast set types ( these usualy operate at 468- 475kHz ) providing you put a 15pF capacitor in parallel with the main winding, as was done here. A diode detector follows the IF amplifier and is also used for AGC of the first IF stage, the diode has a forward bias current of 10uA to reduce distortion at very low signal levels. Demodulated audio is fed to the LM386 power amplifier via a volume control. A few notes: The oscillator coil used is a long wave osc. coil giving a local oscillator coverage of 475kHz - 600kHz which provides a incomming ultrasonic frequency coverage of 20kHz to 145kHz. It may be possible to pull down a Medium wave oscillator coil by placing extra capacitance across the coil and screwing the core all the way into the can - room for experimentation. The circuit was designed specificaly to run off four AA cells ( 6 Volts ), battery life is very good. Tuning capacitor VC1 was obtained from the GQrp club. If you find your circuit has too much gain then increase the resitor value across pins 1 and 2 of the LM386 accordingly. In use you should find this detector has a usable range of at least 10 - 15 metres depending on the ultrasonic mic used, a old TV remote type is used here, however Maplin and other suppliers stock suitable units ( be sure it is a receiving transducer and NOT a transmitting type ).
Barry Zarucki M0DGQ
HOME