OK this is perhaps overkill.. see footnote. So I've got an indicator LED (bi-color) the input is the error term from a thermal loop, and I turn that into an LED current.
I was thinking it might be nice to turn up the gain on the current, but then I worried about burning out the LED with too much current from the opamp.* So I drew this up... and then added the diodes to stop the transistors "running backwards". And the cap to stop the oscillations when the transistors turn on.
Don't laugh too loud. Is there some easier way to do this?
George H.
*Right one easy answer would be to find an opamp with ~20 mA maximum current. (uA741 ??? :^) Do you have any suggestions? All our "in stock opamps are higher currents.