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This circuit uses a 1458 dual op-amp to
form a radar detector. C1 is the detector of the radar
signal. The first op-amp forms a current-to-voltage
converter and the second op-amp buffers the output to
drive the piezo transducer. R5 sets the switching
threshold of the second op-amp; normally it is adjusted
so that the circuit barely triggers on background noise,
then it's backed off a bit. The response of the circuit
may be tuned by adjusting the length of the leads on C1.
For typical road-radar systems, the input capacitor's
leads should be about 0.5 to 0.6 inches
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