Ultrasonic repellent (CB9384E)

Found in Radio Electronics from the 90s, this circuit is based on the 555 directly feeding a small tweeter. An amplifier stage can be added.

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Proximity Relay (CB25343E)
This proximity relay circuit was found in the July 1990 Radio Electronics magazine. The supply...
Pulse Width Modulator (CB5364E)
This circuit is from an old operational amplifiers manual. However, the setting applies to modern...
Sine to square converter CA3140 (CB24971E)
Using an operational amplifier, the figure shows how to convert the waveform to low frequency...