Improved optoisolator

A recurring problem in the design of optoisolator circuits is arriving at the right compromise between insensitivity to spurious signals, component tolerances and longterm stability. One aid in minimizing this problern is replacing the usual collector resistor of the optoisolator by a current source as shown in this article. The resulting circuit is simple, inexpensive. uses readily available components and, last but not least, has good reliability.