Digital Frequency Division Bat Detector: Counter and Audio
Counter, filter and audio output stages
Our original design idea was to provide output to a small speaker, and with this in mind we chose the LM386 audio amplifier which can provide 0.5W into an 8 ohm load. However on test we discovered acoustic feedback which meant the detector could not be used to drive a loudspeaker. In view of this we present an alternative output stage using transistors to make a class B push-pull amplifier optimized to drive headphones or earbuds. Class B is chosen because its much more efficient than Class A; and in the absence of a signal no current is drawn from the supply.
The circuit uses a HEF4024 binary counter to divide the frequency by a factor of 2,4,8,16, 32 etc as required. We opted for a division ratio of /32 which will bring a 48kHz signal down to 1.5kHz as human hearing is most sensitive in this region. You could change this to /16 by taking the counter output from Qd on pin 6.
The circuit requires only simple filtering. At the input we need to restrict pickup at audio frequencies that could affect operation of the circuit. C1 couples the signal from the sensor to the preamp input. The value of 1000pF is calculated to give 6dB/octave of bass cut attenuation below 16kHz. At the input to the Schmitt C2 at 470pF gives a further 6dB / octave. In this way the detector is protected from low frequency signals affecting the triggering.
At the output we need to eliminate the high frequency harmonics to give a more nearly sinusoidal output, and reduce the possibility of feedback and oscillation.
C3 at 6200pF has Xc=47k at 600Hz and provides a small amount of bass cut. C4 at 10nF has Xc = 2k2 at 6kHz and provides top cut. This reduces the higher frequency harmonics from the square wave signal, giving a smoother sound and reducing the possibility of acoustic feedback causing oscillation.
This plot from MultiSIM shows the amplitude and phase response of the filter for an input signal of 4V peak. The filter shows -3dB at 500Hz and 8kHz. The maximum output signal is about 175mV near 2kHz. This will be amplified to 3.5V peak i.e. 7V pk-pk output.
For our prototype we chose the ubiquitous LM386 which has proved reliable and robust in the past. It provides a gain of 20 with pins 1 and 8 open circuit. The R9 resistor is only needed for simulation and is not used in the final circuit.
We discovered on test it was not possible to prevent audio feedback causing oscillation. Since the counter output is an even sub-multiple of the input, the sound from the output contains harmonics which include the input frequency. When these are amplified by the input circuit they cause oscillation.
We decided it was not practicable to offer output to a loudspeaker, and headphones would need to be used.
To reduce current requirements and in keeping with our design objective we offer the following alternative output stage.
Alternative Digital Output Circuit
This amplifier is a class B amplifier using a complementary pair of transistors TR1 and TR2. C2 = 100uF and is wired directly across the collectors of TR1 and TR2 to prevent the output signal being coupled to the power supply rails. C3 = 4.7uF which has a reactance of 7 ohms at 5kHz. This acts to further reduce the high frequency components in the output. C4 = 100uF blocks DC to the headphones. (Xc3 = 8 ohms at 200Hz.) The output voltage from the counter provides the bias voltage for TR1 & TR2 which are configured as emitter followers. The audio filter described above is omitted from this circuit, with the 1k0 resistor and C1 providing a low pass filter at 3.3kHz.
Suggested transistors: not critical, but for example a 2n3702/2N3704 could be used.