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Phototransistors and op amps

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Sling

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Hi I'm trying to make a circuit which uses the KTIR0821DS opto-coupler with phototransistor in order to detect whether a square on a card in light or dark, and output a high or low voltage accordingly.

Ultimately I aim to feed the output into a comparator in order to process a high or low final output. The included circuit is able to detect the difference between a light or dark square, however the difference in voltage is far too low to do anything with, only about .1 to .3 volts.

I have been trying (and trying and trying) to use a 741 op amp to amplify the voltage output, and therefore the difference in light or dark squares, but I have been having no progress at all. (I'm a bit of a novice with electronics but willing to learn.) I keep trying different ways of connecting the op amp to this circuit but nothing works.

Can't seem to upload attachments from my PC so I put the circuit on a file hosting site : PicFront - Card_Reader_Circuit.jpg
 
The output of a photo-transistor is much too weak to drive a resistor as low as 75 ohms. Use 10k to 100k ohms then the photo-transistor drives an opamp or another transistor. The opamp or other transistor turns on the indicator LED.
 
The output of a photo-transistor is much too weak to drive a resistor as low as 75 ohms. Use 10k to 100k ohms then the photo-transistor drives an opamp or another transistor. The opamp or other transistor turns on the indicator LED.

I tried using higher values than 75 ohms in the beginning, but even with 1k ohms there is a maximum of 0.02 volts difference between light and dark squares. I don't think I'd be able to amplify that enough to differentiate between the two.

The datasheet recommends Vce=2v and If=4mA for the photo-transistor component, which gives a resistor value of about 75 ohms. These conditions do seem to make the difference between light and dark squares at least semi-distinguishable ... how can I properly amplify this though?
 
A photo-transistor has a fairly low output current, maybe 500uA which is 0.5mA when the light is very bright.

But the current will probably be much less so the collector resistor must be at least 40k ohms.

The photo-transistor collector voltage is supposed to swing from its supply voltage (4V or more) down to 0.1V.
Maybe you connected your photo-transistor upside-down?
 
A photo-transistor has a fairly low output current, maybe 500uA which is 0.5mA when the light is very bright.

But the current will probably be much less so the collector resistor must be at least 40k ohms.

The photo-transistor collector voltage is supposed to swing from its supply voltage (4V or more) down to 0.1V.
Maybe you connected your photo-transistor upside-down?

ROFL

Seriously mate, connect it upside down ? LOL

Not sure how to answer that one!

Suffice to say, everything I said in my previous post is correct. I may well be doing something wrong, but I DO know the difference between a collector and emitter! :)

Hehe .... upside down ... :)


....


Well I'm assuming you are not familiar with this particular opto-coupler/phototransistor audioguru, yes the voltage swings from 0v to about 4v but thats the difference between blocking all light and shining a light onto it. For this, the circuit needs to detect the difference between a light or dark square on a card.

Would really love to hear from someone who is perhaps more familiar with this type of phototransistor!
 
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You might want to think twice before ridiculing someone who is trying to help you. We have no idea what your level of expertise is.
 
Seriously mate
My wife is my mate, not you. The mates for guys are girls, not guys. Maybe you are ..., never mind.

I'm assuming you are not familiar with this particular opto-coupler/phototransistor audioguru, yes the voltage swings from 0v to about 4v but thats the difference between blocking all light and shining a light onto it. For this, the circuit needs to detect the difference between a light or dark square on a card.[/quote]
Then the contrast is not good enough.
 
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My wife is my mate, not you. The mates for guys are girls, not guys. Maybe you are ..., never mind.

I'm assuming you are not familiar with this particular opto-coupler/phototransistor audioguru, yes the voltage swings from 0v to about 4v but thats the difference between blocking all light and shining a light onto it. For this, the circuit needs to detect the difference between a light or dark square on a card.
Then the contrast is not good enough.

Sorry not looking for irrational or emotional replies ... I need useful advice. Please don't take offense at my post, just because you are incorrect.

Oh, and the contrast is fine. Black and white. I used a permanent black marker for the black (heavy on the ink), with white paper with no black ink for the light.

*Sigh*

:)
 
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You might want to think twice before ridiculing someone who is trying to help you. We have no idea what your level of expertise is.

It was not my intention to 'ridicule' anyone. That was my honest reaction to his suggestion that I had got the opto-coupler/phototransistor 'the wrong way', and I think it was a reasonable assumption that he may be a veritable expert in many areas of electronics but maybe he just wasn't familiar with this type of component.

Is my response really so hard to understand?

I would really like some advice, and I apologise to audioguru if he took offense. As I say, that was not my intention.

He didn't really have to post anything, but I am grateful for any input, naturally.
 
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Is your device exposed to ambient light? How much current are you driving through the LED? What is your supply voltage?
 
Is your white/black surface spaced at 1mm (0.040") away from the sensor, as shown in the data sheet?

Note in fig. 7 how the response degrades if your distance isn't accurate.
 
Does anyone see the difference between Fig. 8 and Fig. 9, other than the location of the origin of the horizontal axis?
 
Figure 8 describes the on/off border when the card is moving parallel to the row of pins; Figure 9 when the card is moving perpendicular to that.
 
Many black markers are shiny and reflect light very well. Then the contrast between black and white is low.
 
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