This simple three-LED sensor uses an NTC at the input. The circuit is based on an uncommon integrated component today. We found it in documentation from the 80s.
This 1968 Texas Instruments circuit uses diacs of the era. SCRs can be modern equivalents according...
For the 400 to 500 MHz bands, this circuitry found in an 80's documentation uses a very critical...
This amplifier for FM antenna signals was published in Radio Electronics magazine in May 1963....