A frequency division bat detector circuit with amplitude restoration

Sylvain Frenehard

The work done by N. Wakefield led to the identification of several disadvantages in the original circuit. Not the least of these was a tendency to oscillation, due to the very high gains involved. After considerable testing and redesign it was decided to reject the use of the LM381 or 387 audio preamplifier in favour of a circuit using LM353 dual operational amplifiers. The LM353 has a gain-bandwidth product of 10MHz allowing a stage gain of 50 with a bandwidth of 200kHz.

A further problem with the method of operation of the original circuit is that any input that is not a pure sine wave causes unpleasant scratchy sounds at the output, particularly with low amplitude signals. The amplitude restoration circuit works well, but it is felt that it will be an advantage to use a comparator with hysteresis to square the incoming signal.  An improved design for the audio output stage has also been developed, which offers good output without needing a large battery.  This uses two LM380 audio amplifiers arranged in a bridge configuration.

Sylvan has worked very hard and tested several different approaches, and we feel that the design choices are becoming much more clear as some alternatives are rejected. The problem of a suitable transducer remains. Tests on piezo-detectors show that they have a very narrow bandwidth, but they are relatively robust, sensitive, readily available, and cheap. The search for a broadband detector with good sensitivity goes on, so if you know of a possible sensor, please get in touch.

The new design runs from a 9V battery and provides a signal into headphones or a small loudspeaker. The PCB uses a ground plane as for radio designs to minimize oscillation, pick-up of radio signals and interference.

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