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.
Using an operational amplifier, the figure shows how to convert the waveform to low frequency...
This high quality circuit with complementary symmetry was found in an Electronics World of 1966....
This circuit was obtained in an old 1983 magazine, but it can still be easily assembled. The...