In the figure we have a basic shield circuit with TTL to TTL optical isolator. For the emitting LED the voltage can be different from 5 V.
This interesting circuit for testing NPN and PNP transistors for general use and medium power was...
We found this circuit in a 1986 documentation. It uses only LEDs, batteries, resistors and the...
Published in Radio Electronics in January 1963, this circuit uses 5 transistors of the time. The...