I'm new to circuits and in a side project I'm trying to design a DC circuit without microcontrollers that will turn on an LED if a certain voltage range is detected. For example the LED turns on if there's a voltage over 1.2V on a certain node. I'm not sure what component or setup will be able to trigger at a threshold. I'm not familiar with most EE components beside the very basic ones (resistors, capacitors, inductors), but I would definitely like to know which ones I would need to look into to make this.

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    \$\begingroup\$ Check out a component called "comparator". \$\endgroup\$
    – Justme
    Oct 16, 2020 at 13:45
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    \$\begingroup\$ Googling 'voltage comparator example circuit' or 'voltage comparator application note' will give you exactly what you need, you'll find example circuits you can build. \$\endgroup\$
    – TonyM
    Oct 16, 2020 at 13:56
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    \$\begingroup\$ Looking into this would definitely work. Thank you! \$\endgroup\$ Oct 16, 2020 at 13:56
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    \$\begingroup\$ I'm new to circuits Excellent, we all were one day I'm trying to design Not a good idea! Instead, save yourself from lots of disappointments and just rebuild a design someone else already made. You cannot design a circuit without some experience. You get the experience by repeating what others do, maybe making a small change here and there. It is even be OK, to get a read-made module just to get the job done. Leave circuit design for much-much later. \$\endgroup\$ Oct 16, 2020 at 13:59
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    \$\begingroup\$ @TonyM Procedurally I disagree with deleting the question. If you claim that this has already been covered, then it should be closed as a duplicate, not deleted. \$\endgroup\$
    – Reinderien
    Oct 16, 2020 at 21:05

1 Answer 1


While Reinderien's answer is technically correct, you might want to have more flexibility when defining your voltage window. You might find the following circuit more satisfying:


simulate this circuit – Schematic created using CircuitLab

This contains 2 essential building blocks: comparators, and AND gates. Vref (high) and Vref (low) are defined by potentiometers (R1 and R2). They're essentially knobs you can use to tune voltages.

OPA3 and OPA4 are voltage comparators; OPA3 is essentially a '<' operator and OPA4 is essentially a '>' operator. The component on the right is an AND gate. If both the input voltages coming in are high, the Vout is high. If one or neither is high, Vout is low. The result is the circuit implementation of the following pseudocode: Vout = (Vin > Vref_low && Vin < Vref_high).

  • \$\begingroup\$ Great start but you can simplify this window comparator quite a bit. Many comparators have open-drain outputs (e.g. quad LM339) to make window comparators easier and save a chip here, as commonly shown in example circuits. Using them, each comparator's inputs would be swapped over and their outputs tied together, driving the the LED cathode with the anode pulled high. Will upvote if you update both your schematic and text as such. \$\endgroup\$
    – TonyM
    Oct 17, 2020 at 7:23

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