Industrial Controls - Digital Input Signal Conditioning

Earl Foster

I am having a problem with experiment #2 exercise 4 page 49-50.· It wants me to calibrate the presence of an object by measuring between the collector and emitter of the phototransistor and recording the readings - which I have done.· But then it wants to adjust the potentiometer to provide proper reference voltage which is halfway between my high and low reading.· The problem is that the voltage does not adjust while measuring across the collector and emitter of the phototransistor.· It does adjust when I measure between pins 3 and 8 of the LM358.

I have rechecked the circuit and it appears to be correct.
My phototransitor Voltage with no object: 4.55vDC
··········································with Object: 4.10vDC

Both cases the LM358 output voltage is 0vDC when measuring pin 1 of the LM358 to ground.

I am looking for clarification on this part of the excerise because it seems that I am doing something wrong.

Any thoughts?
Thanks

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"Don't ask yourself what the world needs - ask yourself what makes you come alive, and then go do it." - H.T.Whitman

Martin Hebel

Hi Earl,

What you need is to adjust the potentiometer so that it is at a voltage of 4.3V as referenced to ground (Vss).· This is the reference voltage.· When collector voltage is above 4.3V, the output (pin 1) of the LM358 should be near 5V.· When below 4.3V, the output should be 0V. When the fan is running, it will produced pulses that should be read as counts by the BS2.

-Martin





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StampPlot - GUI and Plotting, and XBee Wireless Adapters
Southern Illinois University Carbondale, Electronic Systems Technologies

Earl Foster

The measurement points for adjusting the reference voltage is what my question is really all about.· I am unclear where I should be taking my measurement while adjusting the pot.

Martin Hebel

Measure off the potentiometer itself or at the input to the op-amp (pin 3?- not in front of me).· You are setting up a reference to compare the photo-transisitor's output to.

-Martin


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StampPlot - GUI and Plotting, and XBee Wireless Adapters
Southern Illinois University Carbondale, Electronic Systems Technologies