Found in a 90s Popular Electronics, this circuit excites a 40 kHz ultrasonic transducer. The transistor can be a BD135 or other medium power.
This simple oscillator with two general-purpose transistors has its frequency given by capacitor...
This flood or humidity alarm was found in an English publication in 1991. It can still be fitted...
We found this interesting configuration in a 1976 technical documentation. Equivalent diodes can...