This circuit uses an unusual sensor nowadays, but the configuration can be adapted to more modern types, as well as the operational amplifier. The circuit is from an 80's documentation.
This circuit was found in February 1976, Radio Electronics magazine. The circuit is powered by 12 V...
This circuit was obtained in a 1973 publication, being implemented with TTL technology. The...
Found in a 1969 documentation on neon lamps, it actually takes advantage of the durability of the neon...