Found in a 90s Popular Electronics, it serves both TTL and CMOS logic. The injector can provide 10 kHz or 100 Hz signals. The circuit drives a transducer and two LEDs.
This circuit is from 1988 documentation. It uses a range of 30 LEDs driven by 4017 integrated...
This circuit was found in Electronics Now, it can be used with universal phones on a common...
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