Phototransistor problem ?

[smiles]

phototransistor

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Above is the reflection sensor that I get problem with the IR detector.
I have the two machine with the circuit design is not quite the same at the sensor part (but they are of the same manufacturer, the same model and appearance, location of connectors almost the same). Those sensors used to discover paper but one machine get error in detecting paper. I permute between good machine and error machine and find that these two sensors still be good (the sensor in error machine still work well in good machine).
The emitter part in sensor both supplied by 5V, but it 's different in detector part, that is:
In good machine, the phototransistor is supplied by about 4.98V and when I apply my hand up and down in front of it than the voltage on it swing between 2 and 4.98V.

In error machine, the phototransistor is supplied by 3.98V and when I apply my hand up and down in front of it than the voltage on it swing between 3.98V and about 0.2V below it (~3.78V).
I wonder that if I increase the voltage applied to phototransistor up to 4.98V then the swing distance of voltage increase ?
One problem is that the that voltage supplied (in error machine) is from pin E of transistor NPN model BC847B. When I measure on it I see that pin B is 4.32V, pin C is 5V (Vcc), is that logical ? Suppose that pin C just only connect to detector part of sensor.
Thanks.

 

[thomaslg]

If the sensor module works fine in another machine then there should be problem with other part of the circuitry in the faulty machine.

Disconnect the output pin of the sensor module from the faulty machine and check the voltage swing by blocking and unblocking the light source, if the result is normal then you should check components of the faulty machine's circuitry connected to this sensor module's output.

Regards.

 

[FvM]

If I understand right, you said, that the connected circuits are not identical. If so, how do you expect to know if different voltages observed at the pins are normal operation or fault?

At least, you must understand the exact circuits and there differences. Generally, it's either a faulty part or a design flaw. Without knowing the intended circuit operation, you can't decide.

 

[smiles]

Thanks ! Maybe I just need to pay attention to error one. Here is the circuit that I can see till now and the voltage is unchanged (when I apply my hand in front of the phototransistor) in the part from the left plate of capacitor C3 to the left (capacitor are SMD type so don't sure the value, no text on it).
Hope you see something abnormal with it :|
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