Found in a PE Hobbyst Handbook of 1991, this circuit was originally proposed to time photographic baths in developing rooms but can be used for other purposes. The circuit beeps at 30-second intervals.
This circuit was obtained from a Poptronics in June 2001. The magazine no longer exists, but the...
We find this circuit in a 1980s documentation. The circuit uses uncommon components today which can...
This circuit has a very high input impedance and a very low output impedance. We found it on a...