How do I know if my photo diode is "lighting"?

Discussion in 'General Electronics Chat' started by spinnaker, Apr 1, 2015.

  1. spinnaker

    Thread Starter AAC Fanatic!

    Oct 29, 2009
    4,887
    1,012
    I have never used photo diode / transistors before.

    I have a OP906 photo transistor with a bpv11f photo diode. My transistor circuit is based on this:

    transistor.PNG

    Except I am using a 1K resistor. My supply is 3.3V. I know the transistor circuit is working because I use a remote and I can see transitions at A3.


    My photo diode circuit is a follows. If I am reading the datasheet correctly my forward voltage is 1.2V with an If of 1ma.

    diode.PNG

    If I do my math correctly that means a 2K current limiter. I am using 2.2k since that is what I have on hand.

    I have the LED in very close proximity to the transistor but I am not using a lens. Doesn't matter if I interrupt the light path or disconnect the LED, I see no transition in voltage at A3.

    Do I need a lens?

    Am I using right diode?

    What else could be wrong?

    How do I know if the LED is actually "lighting"?
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,446
    3,361
    A photo-diode is not a photo emitter. What you need is an LED. If you want an IR LED you can use a digital camera or a smart phone camera to view the IR LED.
    Just view the LED on the camera screen and you will observe if the IR LED is on.

    (btw, your two links above are the same)
     
    Hypatia's Protege likes this.
  3. spinnaker

    Thread Starter AAC Fanatic!

    Oct 29, 2009
    4,887
    1,012
    Thanks. Cool tip on the camera thing. Works great with my remote. Guess I am off to order some IR LEDs.
     
  4. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    Just to let you know. The OP906 photodiode has a maximum response at about 880 nm and the BPV11f photodiode has a maximum response at about 940 nm.
     
  5. spinnaker

    Thread Starter AAC Fanatic!

    Oct 29, 2009
    4,887
    1,012
    OK then it sounds like I need to learn more about matching photo diodes. Exactly what does that all mean?
     
  6. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,058
    3,820
    You need the LED iR emitter to be the same wavelength as the phototransistor absorbs. 880 nanometers is one common wavelength and 940 nanometers is the other.
     
  7. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    If the IR diode and the phototransistor are close together, it probably won't matter if they match; you will still get a signal. If there is some distance between the 2 (or something that absorbs the light), a unmatched pair might not show any signals where matched pair would.

    A matched pair will give you the longest working distance between diode and transistor.
     
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,058
    3,820
    Also the angle of emission and reception are really narrow (20 to 40 degrees). So, when you are really close, (cm) you really have to align them. It is almost easier to put the LED and phototransistor parallel to each other and measure reflects nice off of a white piece of paper.

    Room light can interfere a lot. That is why they are normally pulsed at 38k and the received signal is filtered with a band pass filter. Also, if you use a higher value resistor on your phototransistor you get high voltage gain(switch) and a lower voltage will be more analog (distance sensing or amplitude modulation).

    Watch out for reflected light. Put some heat spring tubing around your sensor, even paint the pcb black so the backside of the phototransitor doesn't pick up unintended IR. Again, the pulsed signal is best.

    If you have a Microcontroller. You can measure the ambient IR, the. turn on the Led IR emitter, wait about 25 microseconds and measure again. Works well. Simple. Other methods with RC filters take up a lot of board space, drilling holes, ...
    RadioShack sells the emitter /phototransistor pair for about $3 if your looking to do it immediately. The phototransistor has the smoked lens and emitter is clear.
     
  9. spinnaker

    Thread Starter AAC Fanatic!

    Oct 29, 2009
    4,887
    1,012
    Darn was just at the Shack did not know they had pairs. I picked up both versions of the IR diodes as I could not remember what photo transistor I had at home.

    My intended use is a home sensor for a propeller clock, so I am guessing sampling will be out of the question?
     
  10. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,058
    3,820
    Last edited: Apr 2, 2015
  11. spinnaker

    Thread Starter AAC Fanatic!

    Oct 29, 2009
    4,887
    1,012
  12. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,058
    3,820
    Yes, it is sampling. I was thinking about something else as I read your post - samples from RadioShack, I don't think they are into that so he must be talking about something else. Now I know.
     
Loading...