This circuit was found in a Popular Electronics from the 90's. The CMOS integrated circuit used is not very common nowadays. The range depends on the infrared LEDs. R7 can be changed.
This power supply, found in an Argentine publication from the 1970s, uses a wire potentiometer of 25 W or more...
This circuit was obtained from an English magazine from 1979, but can be easily assembled today...
The oscillator shown in the figure generates a 1 kHz tone that is reproduced on the speaker with...