Simple circuit with Photointerrupter has got me baffled...

Discussion in 'General Electronics Chat' started by pureanalog, Oct 8, 2010.

  1. pureanalog

    Thread Starter Member

    Oct 8, 2010
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    Hi folks.

    I have this instrument which has a moving arm in 3 dimensions. I will save you with the malfunction's symptoms, but I narrowed down the problem to a small pcb on the arm which has 4 photointerruptor sensors on to help with positioning the arm.

    These 4 sensors are named Y home, X Sensor, Z Home, Z Sensor.

    There are another couple sensors on the arm system on a different place than here, but the problem is on this particular pcb.

    So here are some pics of the circuit itself. The schematic is quite simple and it goes something like this that I drew, as far as I could tell. Excuse me for not drawing it properly, but I am no professional :( :cool:


    [​IMG]

    These sensors are fed on one side with 1.15V approx each and they interrupt the 5Volts that are to be read on the other side.

    So when something is between the IR sensor and interrupts the lightpath, you have 5Volts reading on the multimeter.

    One of those sensors display 5 volts all the time, even though there is nothing in between the ir transmitter and receiver... No response from them whatsoever...

    Here is the confusing part. I tried swapping a working sensor with one that didn't work, and the problem remained where I put the "new" sensor. So the problem is in the particular positions of the circuit itself and not the sensor.

    The resistors are used to drop the 5V voltage to 1.15V to feed the IR transmitter. The are 330 ohms and they measure correctly on the circuit with the multimeter. The caps seem to be leak free. When I put the leads of the multimeter one way I have infinite resistance and when I put the multimeter the other way, I measure 1433 ohms.

    There is a 6 pin connector on the pcb as you see and it gets connected with a ribbon with 6 cables inside. These cables give all 5volts each and one is ground. The 4 pins with 5 volts are for the sensors, one pin with 5 Volts is for feeding the IR transmitters of the sensors and it is common for all sensors.

    So when the 5Volts from one sensor get interrupted, the instrument reacts and understands the command. I tried short circuiting those 5Volts individually on the ribbon cable and the instrument responds to those signals. So the problem should lie on the pcb I guess, right?

    I also checked the ribbon for bad connections etc, but all works well with it. It sits in tight and gives a good connection.

    Could somebody chime in and give me an idea of what could possibly go wrong with this circuit. It seems quite simple and yet I cannot figure it out.. :confused: :confused: :confused:

    Thanks in advance.
     
    Last edited: Oct 10, 2010
  2. pureanalog

    Thread Starter Member

    Oct 8, 2010
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  3. retched

    AAC Fanatic!

    Dec 5, 2009
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    Have you considered exorcism?

    ;)

    Try removing the part and checking for the 5v on the output pad.

    You may be back feeding 5v to the device instead of switching 5v with it.

    Your problem may not be that board.
     
  4. pureanalog

    Thread Starter Member

    Oct 8, 2010
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    Hopefully I will not have to resort to exorcism!! :eek:

    I will try that. But if not on that board, then where could the problem be to cause such a behaviour?

    I mean the ribbon that comes from the instrument's motherboard, gives all the correct voltages, 5 volts on 5 pins and 1 pin GND. So what else could be wrong and causing the sensors to respond problematically.

    I am sorry I am so un-educated with electronics...
     
  5. eblc1388

    Senior Member

    Nov 28, 2008
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    Please measure and post back the voltage ACROSS the four 330Ω resistors.

    This is how to test the function of the board.

    [​IMG]
     
    Last edited: Oct 8, 2010
  6. pureanalog

    Thread Starter Member

    Oct 8, 2010
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    well thanks, I will try measuring this way and report back. The resistor you drew parallel to the volt-meter, that should be 1k to 50k, what is it used for? why to I have to use it to measure really?

    by the way here is the schematic of the photointerrupter.

    http://www.optekinc.com/datasheets/opb818.pdf
     
  7. eblc1388

    Senior Member

    Nov 28, 2008
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    The resistor is needed only in case of DVM as they have very high impedance and slight leakage from the circuit would show up as non zero voltage.

    It would be confusing in times for people not accustom to the high impedance to identify between real circuit fault or leakage voltage.

    The addition of the resistor would counter the effect of this leakage current(if any) and provide a "cleaner" measurement.
     
  8. pureanalog

    Thread Starter Member

    Oct 8, 2010
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    For the moment I don't have a resistor handy so I did plain measurements to the points you have shown me.

    For point A I get 5 Volts when nothing is inbetween the IR sensor. When I block it I get 0 Volts

    For point B I get 5 Volts when nothing is inbetween the IR sensor. When I block it I get 0 Volts

    For point C I get 0,7 Volts when nothing is inbetween the IR sensor. When I block it I get 0 Volts

    For point D I get 5 Volts when nothing is inbetween the IR sensor. When I block it I get 0 Volts
     
  9. eblc1388

    Senior Member

    Nov 28, 2008
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    [​IMG]
    OK. This is where the problem lies.

    Have you measured the voltage across the R3 330Ω resistor? You would need to.

    Power off and use ohm range of meter to measure (see image)

    1. point C from the connector all the way back to the photo-interrupter soldering point

    2. between ground and the other photo-interrupter soldering point

    [​IMG]
     
  10. hgmjr

    Moderator

    Jan 28, 2005
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    Take a digital camera or cellphone camera and use it to view the two infrared (IR) emitters. These will be on the inner surface of the gap on the post marked with an "E" on the top. "E" stands for emitter and "S" stands for sensor. You should see a distinct glow from the infrared emitter when viewed on the camera screen. It appears that in this design the infrared emitters are alway illuminated when power is applied to the board. If you are unable to see any light using the camera then the IR emitter may have been damaged or there is an open connection between the +5 volt power through the current limiting resistor and into the IR emitter's anode pin. Or the connection of the IR emitter's cathode pin to ground is open.

    hgmjr
     
  11. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    When I look at the photo's I see some text on the side of the interupter.
    What does the text say at the side of the interupter?

    Bertus
     
  12. pureanalog

    Thread Starter Member

    Oct 8, 2010
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    Alright, here are the measurement results.

    -I measured across the 4 resistors like you told me.

    Across the resistor of the non-responding sensor I get 3.62 Volts.

    Across all the other working sensor resistors I Get 3.78 Volts

    -Then I measured using the ohm-meter range at the points 1 and 2 as you marked them on the diagram, and I get 0 Ohms resistance on both measurements.


    HGMJR, I tried using my cellphone cam, but I cannot see anything. I will try with my camcorder later.

    Bertus, the text on the photointerrupter says OPB818, 0814, MEXICO.
     
  13. eblc1388

    Senior Member

    Nov 28, 2008
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    Unsolder the 330Ω and measure its resistance.

    Confirm continuity from +5V to 330Ω, 330Ω to photo interrupter(+) and other end of photo-interrupter(E) to ground.
     
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  14. pureanalog

    Thread Starter Member

    Oct 8, 2010
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    I unsoldered it. The resistor measures the same as it does on the circuit. It measures 326 ohms, just like the other 3 resistors.

    Continuity from +5V to resistor, resistor to + and from theto E to the ground are all perfect.
     
  15. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    With the code OPB818 I could find the datasheet.
    (See the attached PDF).
    This gives you a lot of information on the connections and sesitivity.

    Bertus
     
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  16. pureanalog

    Thread Starter Member

    Oct 8, 2010
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    yes I have posted this data sheet in the beggining of the thread!
     
  17. eblc1388

    Senior Member

    Nov 28, 2008
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    The circuit connection checkout OK. The input LED terminal voltage not normal, LED output(or the lack of) not able to switch on output photo-transistor, output photo transistor appears to response to ambient lights when user trying to block the ambient light(0.7V -> 0V).

    The opto-interrupter(emitter side) is faulty and need replacement.
     
  18. hgmjr

    Moderator

    Jan 28, 2005
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    If you can't see any light output from the emitter side of the opto-sensor then either your sensors have failed or you have an open circuit in the emitter side of the circuit. I suspect that it is a failed sensor rather than an open circuit.

    For a sanity check, you can take your cell phone camera and examine any tv remote in your house. This will give you an idea of what to expect.

    hgmjr
     
  19. pureanalog

    Thread Starter Member

    Oct 8, 2010
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    Well, the strange thing is that this is not the case. Because I swapped the sensor with a working one from the other side of the pcb (all 4 are identical) and the problem persists on the particular position.

    Yes indeed the interrupter responds to signals, but not like the others that respond with 0/5 volts. I only does 0.7 volts...

    The photo interrupter is not faulty!! It was the first thing I checked by swapping it with a working one!!

    Now what could the fault be!!



    Regarding the camera. Probably my camera cannon "see" IR light. Because the other 3 sensors on the pcb are properly working and I cannot see them. Probably the IR emmiter is deep in the plastic and it cannot be seen if you are not in front of it...
     
  20. hgmjr

    Moderator

    Jan 28, 2005
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    I am inclined to agree with you that the aperture on the emitter sensor is so small that you cannot see it with your camera from an angle. You could get a small mirror and use it to achieve a more head-on view of the sensor.

    hgmjr
     
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