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 circuit matches a source of high or medium impedance audio signals with a low impedance...
Found in a 1993 Electronics Hobbyst Handbook, this circuit uses components that are still common...
This pulse width-tuned circuit was found in the March 1996 issue of Electronics Now magazine. The...