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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?.

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schematic

simulate this circuit – Schematic created using CircuitLab

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.

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  • \$\begingroup\$ Nice circuit and it works but, is there any other way to achieve the same result but going with Lm339 or any similar route instead transistors?. \$\endgroup\$ – Chris Steinbeck Bell Jul 28 '16 at 1:34
  • \$\begingroup\$ You can replace Q1, D4 and the component to their left with a LM339 \$\endgroup\$ – jp314 Jul 31 '16 at 21:28
  • \$\begingroup\$ I am still a newbie with these things, mind being a little bit specific (do you mean R3, R2 and D6 as well, as they are in the left). In other words, can you redraw the schematic so could I understand?. \$\endgroup\$ – Chris Steinbeck Bell Oct 24 '16 at 16:11

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