Diagram produced with Design Works Lite.
The circuit requires only simple filtering. At the input we need to restrict pickup at audio frequencies, that could affect operation of the circuit. 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.
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.
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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.
Diagram produced with Design Works Lite.
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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. |