This circuit appears on this site from different sources. This is from documentation from the 1980s. The operating system used is uncommon and can be replaced by a more modern equivalent.
This circuit is from a publication in 1972, originally made with germanium transistors, but can be...
This circuit is from an April 2000 Poptronics. The magazine no longer exists, but the circuit can...
The circuit shown in the figure is suggested by Maxim (www.maxim-ic.com) and makes use of the...