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I am designing a circuit with not a lot of knowledge in electronics and I am close to finishing it. The finishing part remains.

So, I have got a logic 1 or 0 from comp op amp and based upon that one of two LEDs should be on. One LED is always on. First one if its HIGH, second one if its LOW.

Both LEDs shouldn't be on at the same time.

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    \$\begingroup\$ What is your question? \$\endgroup\$ – Andy aka Jul 21 '18 at 11:15
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    \$\begingroup\$ You're "designing" yet I do not see a schematic. You're just describing what you want. That's not "designing a circuit". And if you say: "Oh, yeah, I'm supposed to design it" then it would be a homework question so I'd ask: "what have you tried?". \$\endgroup\$ – Bimpelrekkie Jul 21 '18 at 11:51
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You can make a NOT function via a transistor (e.g. see here). Excerpt:

A simple 2-input logic NOT gate can be constructed using a RTL Resistor-transistor switches as shown below with the input connected directly to the transistor base. The transistor must be saturated “ON” for an inverted output “OFF” at Q.

enter image description here

Than you can lead your output signal (for HIGH) to a resistor and the HIGH LED, and the same output signal through the NOT circuit and a resistor to the LOW LED.

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