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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Flood Alarm (CB12183E)
This circuit activates a relay when the water reaches the sensors. The circuit is from a magazine...
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The noise comes from the PN junction of a common silicon diode. Since no gain is given by the...
Audio Preamplifier (CB16297E)
We have here an interesting audio preamplifier, which can be useful to those who are looking to...