# voltage comparator help please

Discussion in 'The Projects Forum' started by peterBC, Jun 24, 2012.

1. ### peterBC Thread Starter New Member

Jun 24, 2012
1
0
I am having trouble figuring out the operation of my voltage comparators.
I have both dual (LM393N)

my understanding is you would give it +5 vdc on VCC+, ground on VCC-
and then if the voltage on input 1+ is higher than the voltage on input 1-, then the output goes to 5 volts, if not, it goes to ground (0 volts)

the closest I have gotten is when the voltage is higher on one input, the LED on my output gets slightly brighter because it has a better ground. I have not been able to put voltage to the output.

Are there any rules of thumb for what voltage and amperage I should use to power it, and any minimum or maximum on the inputs. Do I need to use pull up or pull down resistors on any legs/connections

I am currently using a battery pack with some old AAs in it to get just over 5volts. I am testing it on a breadboard to see how it works before attempting to build a circuit.

Thank you very much for any help.

2. ### #12 Expert

Nov 30, 2010
16,023
6,537
Both of these chips are "open collector" outputs. You have to provide the positive voltage on the output with a pull up resistor.

It is considered helpful if you post the datasheets for the parts in question. It is almost always necessary to post a drawing for the circuit you are talking about. It is good to use "User CP" to add your location so we know where you can get parts.

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• ###### LM339.pdf
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3. ### crutschow Expert

Mar 14, 2008
12,529
3,067
If you are trying to light an LED, then place the LED and its series resistor between the +5V and the comparator output. That will cause the LED to light when the + input is more negative than the - input (connecting the LED in that manner reverses the input versus output polarity).

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