This optical counter was found in a 1964 Electronics Design. It makes use of an uncommon optocoupler today, using emission lamp and a phototransistor.
The base of this circuit is 1/4 of an integrated circuit 4136 which consists of a quadruple 741....
This interface shield circuit is from the 1983 Intersil Databook. The components used are uncommon...
This circuit was found in a Texas Instruments optoelectronics documentation. The gain depends on...