I cannot make this circuit work. Please help. I took the circuit idea from a comparator WEB tutorial (that probably I did not undestood). The LED must turn on when I connect +5V to point A. Should it?

Thanks a lot.

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Should work. Check your wiring, or perhaps your LED is backwards.

 

Thanks but I believed the LED is correctly oriented as it is. Anyway I change its orientation an it remain on all the time (even without any reference voltage).??? Any ideas??

Thanks.

 

Hi,
I assume you have pin 2 as the output.

Which pin are you usin as the reference voltage.?

You say the LED, glows which ever way round its connected.?

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hi,
This drawing shows the states of the LM339.

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That's impossible, if the LED cathode was connected to +5 your LED will never light. You've wired up something wrong.

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Blueroomelectronics thanks for the reply.
Can you check the circuit diagram and tell me the correct connection of the LED? I try by connecting it the other way and it stay led all the time (even without the reference voltage or input voltage). I have seem many circuits on the web for comparator and the LED is always connected backwards???? This really confused me.

ericgibbs -> thanks eric . Pin 5 is the reference voltage (2.5 V) . Pin 4 is the input voltage, 5 volts in this case. Pin 2 is the output. So in this scenario the led must turn on when Pin 4 received the 5 volts.

Thanks

 

hi,
When pin 4 goes higher than +2.5V the output will go low and the LED will light.

Try connecting pin 4 to about +3V, rather than directly to +5V, some comparator input/outputs will misoperate at voltages close to their supply voltage.

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Eric " Good enough is Perfect "
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I just looked at the data sheet. I think you need a pull up on the output pin 2. In other words, put a 10K res from pin 2 to +5V.

 

Forget what I just said, I realize I was wrong. oops.

 

Mikebits - thanks.
I will try that when I get home. The pull up will be from pin 2 to 5v and the LED will be tied to pin 2 and ground. Correct ? (This is very different to the circuits I have seem on the web and also on a very old radio shack F.Mims book I have.)
Thanks again.

 

Mikebits .So I should not try this. Any suggestion? Should I try ericgibbs suggestion on the 3v instead of 5v.?

Thanks to all.

 

hi,
If you connect the LED that way, it will work the other way around, that is the LED will go OFF when the Vin > +2.5V.

Try a lower voltage than +5V on pin4.

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Eric " Good enough is Perfect "
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Is it possible that the circuit is breaking into an oscillation when the voltage is removed from pin 4. When the 5v jumper is pulled away, the pin 4 is left floating into a Wide band high gain circuit. Maybe a cap on pin 4 to ground would help??? What do ya think?

 

Why not kick off the LM339? Use an LM358 instead. Works reliably.