This circuit was found in a 80's Popular Electronics. The sensor can be any phototransistor. The circuit is indicated for an audio link modulated in pulses.
We found this CMOS circuit in a 1978 publication. It allows you to use a standard calculator...
This circuit was found in a 1960s book of projects by Gernsbak. The circuit uses germanium...
This circuit was found in a 1985 documentation. It provides an audiovisual indication of the...