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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Alarm with parallel sensors (CB26226E)
This simple alarm with SCR triggers a buzzer. It was found in the Encyclopedia of Electronic...
Differential Input Instrumentation Amplifier (CB13220E)
This circuit is from an old publication from the 70s, but it can be implemented with more modern...
TLV354x CMOS Operational Amplifiers (COM179E)
January 2017 - The new Texas Instruments TLV354x series of operational amplifiers with 200 MHz...