I got it partially working this morning. Had a faulty link wire on the breadboard.
Forgot to mention, my comparator circuit relies on the low impedance of this parallel circuit via the wiper of the 200ohm POT. Even I forgot this while testing this morning
. But anyway, after testing with it I think the diodes were in the wrong place.
First I had an issue with the LEDs. The transistors were allowing 3mA of Collector-Emitter current in its OFF state, enough for the LEDs to glow bright enough. So instead I've connected the LEDs to the outputs of the comparator. Each LED only passes about 8mA at 12V, so shouldn't cause any problems for the comparator with it's max of 16mA. And the darlington transistors will add bugger all to that.
I still have a problem with the other circuit in parallel through the 200ohm POT, and I'm concerned about this. I have no idea how this other circuit is wired... it's a standard electronic automotive temp gauge. When I remove power from the gauge through its +12V terminal, the gauge still operates, meaning it has to be powered through the comparator circuit via the 200ohm POT. Not sure if I'm harming it or not. Under normal circumstances there would be nothing operating in parallel with the gauge.
Is there anything I can do to prevent the gauge being powered from the comparator circuit? A diode between the gauge and the 200 ohm POT works, but adds too much impedance which throws the gauge right out in its reading.