Answer 3—R2 is required because the input is not necessarily pulled to ground by the external connection. If it’s just a switch connected to the battery, it is left floating when the switch is open. Without R2, the current supplied by R3 would be enough to keep both transistors turned on, and the output would never go low once it had gone high.

R5 is required because of the forward voltage drop of the optoisolator LED. When the input and output are low, there can’t be any significant current through R3 without R5, and the hysteresis effect would be nullified. R5 makes sure the output is pulled all the way to 0 V.

Contributor: David Tweed

Published January 2004