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?.



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.

| improve this answer | |
  • \$\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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.