working on a comparator circuit which measures two different voltages on two seperate op amps then using a NAND gate , if both op amp outputs are same then output drives LED (going to be a 400 ohm relay)

 

oups got to quick
it works in LT spice

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How about using two open-collector voltage comparators like LM339s? Set one up to switch high when it's input is >3V; set the other up to switch high when it's input is >4V. Wire the two outputs together to a common pull-up resistor. The combined output will be high if and only if both comparators are above their respective trip points.

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Mike ML.

 

besides I have several LM324's on hand that I want to use.
have similar project using same LM342 (had 20 units given to me)

 

hi
You could use a couple of diodes and resistor to make an OR gate to replace the logic ic.

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Eric " Good enough is Perfect "
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Looks like he used two NAND gates to make an AND gate, not a OR gate, both inputs need to be logic 1, or did I miss something?

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Ok, so three diodes then :-)

 

Two diodes can be used to make either an OR or an AND, whats the problem.?

Why dont you suggest to the OP a way of solving the problem rather than nit picking my humble submission.

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Eric " Good enough is Perfect "
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hi Deb

Please post your LTS asc file and I will add diodes as required.

Confirm the operating sense of the relay drive transistor that you require.

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Eric " Good enough is Perfect "
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Sorry Eric, no offense to you, you did say or gate. Not to negate your input. My apologies.

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Not quite, two diodes reversed bias with pullup will make And gate.

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consider this.
IF the OP had wanted the relay to denenergise when either of the comp outputs had gone low then it would be an OR gate.

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Eric " Good enough is Perfect "
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I was under the assumption that both inputs must be true, else relay is off. Actually after re-reading OP I wonder if he wants XOR function? I did not mean to offend, often forum communication creates misinterpretation.

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Why do you think you are offending me.

Like you I dont not know for sure what the OP wants to do.
I assumed that the mention of two diodes would start bells ringing in MrDebs head, as a possible solution.

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Eric " Good enough is Perfect "
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in order for relay to energize
hope I got this right??

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