Alarm transmitter (CB21874E)

This circuit was found in an Australian publication from 1991. The circuit emits a tone as well as the picked up sound when triggered.

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5 MHz oscillator with 250 kHz variation (CB13852E)
We found this oscillator in a documentation for radio amateurs from the 1970s. The circuit can use...
Impedance Matcher (CB15540E)
Many oscilloscopes have high impedance inputs that make it difficult to observe high frequency signals...
Optical Communicator – Receiver (CB25536E)
This is the receiver for the previous circuit we found in the December 1992 Radio Electronics...