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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

Tester for ultrasonic devices

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This circuit was designed to test an ultrasonic rangefinder but could also be used to test car reversing sensors or any other device based on 40kHz ultrasonic transducers. It uses a 40kHz transducer as the input device and its received signal is amplified by op amp IC1 which has a gain of 100, as set by the 33kW and 3.3MW resistors. Its output signal is then squared up by IC2a, the first gate of a 4093 quad Schmitt trigger.

IC2a is biased by trimpot VR1 to a point just below that required to give a positive output at pin 3, when no signal is present.

When a signal is received, the resulting 40kHz square-wave from IC2a is rectified by diode D1 which charges a 4.7nF capacitor to detect the very short bursts of 40kHz signal. The resultant positive signal is fed to Schmitt trigger IC2b which switches its pin 4 output low when signal is present. This is inverted by IC2c which then gates on oscillator stage IC2d. This runs at about 5kHz to drive a piezoelectric transducer.

The current consumption is only a few milliamps and can easily be supplied by a small 9V battery.

Graham Jackman,
Oakleigh East, Vic. ($40)

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