IR LED - photodiode project challenges

Discussion in 'The Projects Forum' started by cce1911, Jun 18, 2013.

  1. cce1911

    Thread Starter Member

    Jun 11, 2013
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    I'm trying to build a variation of this project: I really just want the detector portion of the project so I can detect a small fast object moving through a "screen". I have built the attached circuit on my breadboard including the exact components specified. When I power up the circuit, the output LED stays lit continuously. If I shade the LED/PD section of the circuit from ambient light, the output LED will go out, but then nothing triggers the PD output into my amplifier circuit.

    The IRLEDs output at 880 nm while the PD peak sensitivity is 940 nm. I was thinking this was far enough into the IR spectrum that ambient light would not play a major factor. I'm a ME not a EE, so my EE skill are pretty weak! Any assistance would be greatly appreciated.
    -Capel
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Your circuit is AC coupled, so its designed for an AC signal so you need to be waving your hand over the light sensor temporarily blocking the receiver, this should create a series of pulses and then the Green led will lash on /off.

    The comparator is set for about 1.67V at U3 pin 10 (resistors R3,R4), and the signal is superimposed on a DC voltage of 2.5V set by pin 5(R6,R7),
     
  3. cce1911

    Thread Starter Member

    Jun 11, 2013
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    Dave,
    I think I understand the theory of operation, but I'm struggling when it comes to breaking the problem down into the individual pieces to troubleshoot. If I assume that the circuit design from the R10/C6 node downstream is correct, then I should reach a steady state at the PD output and the green LED should not be lit. That is not what's happening.

    It's not obvious to me what the voltage should be at the R10/C6 node by looking at the QSE773 datasheet. Regardless this voltage should remain constant until the light on the PDs are momentarily blocked. This momentary decrease in the reverse current develops a very small change in the voltage across R10, which is the signal you mention above. So I'm trying to figure out why I'm not getting a steady state voltage at the R10/C6 node.
    -Capel
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    So what are the voltages at these pins, 1, 7, 8, 14 with the hand waving, and without waving.?
     
  5. cce1911

    Thread Starter Member

    Jun 11, 2013
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    I will fire up the oscope when I get home this evening and see what I can find out.
     
  6. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    What are you using for the unregulated voltage source?
     
  7. cce1911

    Thread Starter Member

    Jun 11, 2013
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    A 9volt battery.
     
  8. tracecom

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    Apr 16, 2010
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    I think the N&V article calls for 12V in the form of 8 AA cells; the 9V battery may not be up to the task.

    I would start by checking the voltage out of the battery under load. Then, I would look at it to see if there was any induced AC component. Then, I would look at the LEDs through a digital camera to see if they are lit. Then, I would check the current through the LEDs and see how close it is to 46mA. Then, I would look at that on my scope to see if there is any AC component.

    All simple stuff, I know, but that's where I have lots of problems, i.e., the simple stuff.

    If that's all okay, then you can move to the sensor side and do the same things again.

    ETA: I don't see a .1μF cap on the input to the LM317; it needs one there.

    2nd ETA: The datasheet for the LM78L05 calls for a .33μF cap on the input and a .01μF cap on the output.
     
    Last edited: Jun 19, 2013
  9. cce1911

    Thread Starter Member

    Jun 11, 2013
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    I pulled out a 1.4 amp-hr 12vdc battery for testing and I get no AC component, at least the oscope can't find any in "auto run" mode.

    I took a picture of the IR LEDs when powered up and you can see from the photo that they are on. The current from the LM317 was measured at 56 mA and I did put 0.1uF cap on the input. The voltage measured at the input to the first IR LED was 5.9 v, 4.5 v, 3.0 v and 1.5 v respectively.

    Per the schematic, I used 1.0 uF cap on both the input and the output of the 78L05. I will have to get some more caps to try the datasheet suggestion of .33μF cap on the input and a .01μF cap on the output.

    I measured the following voltages:
    Vcc = 5.0 vdc
    PDs (R10/C6 node) = 4.1 vdc
    pin 1 = 2.5 v
    R6/R7/C8 node = 2.5 v
    R4/R3/C5 node = 1.57 v

    Pin 7, 8 and 14 had square wave forms pin 14 varying between 120 and 150 hz. Pin 8 varied between 220 and 270 hz and pin 7 ranging between 240 and 340 hz. This is while the sensors are exposed to ambient light.

    When I cover the sensors or turn off the lights, The green LED goes out and the wave forms take a strange shape as can be seen by the photo of pin 7 (shaded).

    As you can see from the IR_LED.jpg, I've got the components pretty close together. I'm not sure if this is having a negative impact. I'm also not 100% clear on which surface of the PDs is the sensing surface. From the datasheet screenshot, it would appear that surface B is, but it may mean surface A. (BTW, I put the letters on the screenshot. The datasheet just had the note with the arrow leader.)
     
  10. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    So, that problem is resolved, apparently with a better unregulated power source.

    I think that you have that part correct. Maybe there is a wiring error in one of the op-amps.

    I will study the rest of the circuit, but could you post a photo of the breadboard for it?
     
  11. cce1911

    Thread Starter Member

    Jun 11, 2013
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    I'm not sure what the R10/C6 node voltage should be or how to measure the very small change that takes place when the small fast object "shades" the PDs. As is obvious from the pictures of the oscope on pin 7, I'm still getting some sort of transient signal.

    I will take a picture tonight and post it.
     
  12. tracecom

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    Apr 16, 2010
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    I have breadboarded the circuit from point PDs to the green LED, and it works as I think it should. Point PDs is held low by R10 unless the PDs are conducting, in which case point PDs is high. Whenever I take point PDs from low to high, or from high to low, the green LED pulses on briefly; otherwise, the LED is off.

    Because you have a high at point PDs, I think your breadboard is okay up to that point. I suspect a wiring error on the LM224; it is a little difficult to keep all the connections straight.
     
  13. cce1911

    Thread Starter Member

    Jun 11, 2013
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    I will double check the wiring on the LM224.

    The theory of operation is that the PDs are conducting all the time. When an object passes between the LEDs and the PDs, the PDs will be "shaded" ever so slightly and there should be a very small voltage drop at point PDs. It is this ~12mV drop that I'm trying to turn into a TTL signal.

    BTW, thanks for all your help!
     
  14. tracecom

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    Apr 16, 2010
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    You are welcome. I believe the circuit will do what you want. Please post your conclusions.
     
  15. cce1911

    Thread Starter Member

    Jun 11, 2013
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    Well, I finally had some time to spend on my project, but still haven't made much progress. I built a frame to put the LED & PD's on. I've triple checked the wiring of the LM224 and physically measured the resistance of every resistor in the circuit. Maybe I can't see the forest for the trees? Attached are some photos.

    breadboard_w_sensor.jpg shows an overview of the project. Note: the PDs are not connected in this photo.

    breadboard2a.jpg shows the LM224 wiring with the 7808 regulator providing Vcc=5.0vdc. The red/black pair coming in from the right is the PDs in parallel. Point PDs is where the black wire punches in.

    measurements1.jpg shows the measured dc voltage at different points in the circuit.

    IR_LEDs.jpg shows the LM317 constant current source and IR LEDs emitting. The other photos are pretty self-explanatory.

    From the dc measurements, it appears that I have the circuit wired correctly. I'm still stumped with regards to the photodiodes. They are wired in parallel, but what should the voltage drop across them be? What is the best way to capture the transient signal at point PDs?
     
  16. LDC3

    Active Member

    Apr 27, 2013
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    If you have incandescence lights in the room, then the IR sensor will pick up the light. Incandescence lights heat up the filament and the output goes all the way through the IR (decreasing in intensity).
    Also, you should use the same wavelength for both the LEDs and the sensor (it works better that way).
    I suspect that you are not seeing a change because of ambient light and because the change in light is too small to register.
     
  17. cce1911

    Thread Starter Member

    Jun 11, 2013
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    Where the pictures were taken, in my basement, the only light was fluorescent and I even turned that off. When I stick my whole hand in the frame I'm not getting a response. I'm still not confident which face of the photodiode is the receiver.

    The PD has a peak sensitivity of 940 nm and the IR LEDs emit at 880 nm. So we are talking about 60nm difference. I'm not sure I can find a closer match.
     
  18. LDC3

    Active Member

    Apr 27, 2013
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    I guess you need to know where to look. At Digikey, I found 342 IR LEDs with wavelengths of 880, 890, 935, 940, and 950 nm and 202 phototransistors with the same wavelengths.
    Here is a pair for 940 nm:
    http://www.digikey.com/product-detail/en/WP710A10F3C/754-1600-ND/2769807
    http://www.digikey.com/product-detail/en/PT15-21C/TR8/1080-1380-1-ND/2676114
     
  19. cce1911

    Thread Starter Member

    Jun 11, 2013
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    Thanks LDC, I'll try to do a better job of research before I make a comment like that again:) What do you think about the PD? (There is a picture in post #9 of this thread.) As you can see from the first photo in post #13, the PDs are facing down so no direct light is hitting them. I've also included a close-up of the PDs in the frame.
     
  20. tracecom

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    Apr 16, 2010
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    Is there 5VDC between pins 4 and 11 on the LM224?

    Is there a current limiting resistor built into the LED on pin 14; if not, you have to add a series resistor of about 560 ohms. (And your LED might be shot, as well as your LM224 might be damaged.)

    In one photo, the 220k resistor that is supposed to be going from pin 12 to ground doesn't look to me like it's connected to pin 12; it appears to be connected to an empty row of pins between the capacitor leads. In another photo, it looks ok; check it.

    The 470pF cap that is supposed to go from pin 12 to 8 looks like it's going from 13 to 8.
     
    Last edited: Jul 1, 2013
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