I want to make a simple circuit, it uses two comparators to drive two LEDs, it should turn the LED (green) if the voltage is higher than let's say +2.5V. If is lower then turn the first one off and on a second led (red). But here is the catch, it should be able to accept any voltages in the input signal between let's say +9v to -9v. How to achieve this in order to avoid blown up any component?.
When Q1 turns on, the red LED will short across the green+diode and turn on. The threshold for Q1 is ~ 0.65 V (at room temperature), D4 increases this to 1.3 V, and R2 & R3 increase this further by 2x to ~2.6 V. Adjust R3 to suit.
If temperature changes significantly, the threshold will change (decrease by about 0.1 V for 20 F/10C rise).
D6 protects against negative inputs.
If you need Red on when V is low, then swap the red and green LEDs, but use 2 diodes in series with the red one.