This circuit also detects IR, providing LED indication. The circuit is used to indicate humidity in bench tests. We found it in the Radio Electronics magazine of February 1990.
This 80's circuit has been winning. It is a 1 000 gain sample and hold circuit and an offset...
The purpose of this 1-transistor amplifier is to allow the use of a milliammeter in an application...
We found this circuit in an old General Electric documentation. In fact, PUTs are not common...