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[SOLVED] need help in figuring out the proper voltage at the output in the circuit

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kdg007

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lm339 as comparator.jpg

i am getting output as 2.8v at the pin 2 of the comparator in the practical design but its showing 5v in the pspice. i want to get 5v but when i ON the signal generator its reducing to 2.8v.can anyone please help me figure out how to get the 5v on the output when i give the signal ?

thank you.
 

resistor R6 is not there in the circuit, schematic mistake.
 

it may not be the problem but you would be better off with a higher load resistor than 1k - the chip is only guaranteed to sink 6mA and you are asking for 5mA.

Also, assuming your photodiode is really a photodiode and not a phototransistor, it would also need to be at the top of the resistor chain not the bottom as the comparator will work to V- but not V+. I guess it depends on exactly what level you need to trigger at though.

Keith
 
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Apart from some confusion in the LED drive circuit (you can't drive a LED with a voltage source), if you you are driving the LED with an AC waveform, you get an AC waveform at the output. 2.8 V seems to be just an average value. But it's not clear what you exactly want to achieve.

I remember, that I previously suggested to exchange the position of photodiode and diode load resistor, which would allow the comparator to work down to zero photocurrent. This is a simple concession to limited LM339 input voltage range, which includes negative but not positive rail.
 
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lm339 as comparator.jpg


sorry ,i am new in pspice. here is my complete circuit.I am expecting the square wave at output (pin 2) with 5v.but i am getting 2.8v.
-
Keith - what value of the load resistor shall i take ? will 2k be ok ?
=
FvM - yes, as you suggested,I made the changes but still i am getting 2.8 to 3.1 v at the output.
 
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I think it would be worthwhile checking your circuit carefully. You shouldn't really be getting intermediate voltages out of the comparator. One likelihood is that you have an oscillation. If you have an oscilloscope, look at the output. Also, force the comparator inputs to + and - rails to see if it works and eliminate the optical part. The resistor you use for the photodiode resistor chain depends on the sensitivity you need but as you have a pot there it is not important.

I am assuming we are talking here about problems with the physical circuit not a simulation.
Keith
 
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yes,i am having trouble with the physical circuit.I have for the oscillations.There are no oscillations when i switched off the frequency generator.But when i switched on the frequency generator(LED ON) ,there is a little bit of oscillation(0.1v) at 50mv/div in CRO.I am assuming that was noise.
 

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