Connect with us

photodiode help

Discussion in 'General Electronics Discussion' started by johnzack, Jul 17, 2011.

Scroll to continue with content
  1. johnzack

    johnzack

    1
    0
    Jul 17, 2011
    I saw this circuit on a website for a line sensor using a photodiode

    The circuit is like this

    VCC----1Kohm-----(output signal)-----photodiode------GND

    But when I measure the output signal I always get Vcc(5V) , implying there is a 5V drop across the photodiode :confused:

    Is there any mistake that you can point out?
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,411
    2,779
    Jan 21, 2010
    The resistor looks like too small a value. The output will have a very high impedance and the change in voltage very small, so a multimeter will probably not be able to show it.
     
  3. daddles

    daddles

    443
    3
    Jun 10, 2011
    Photodiodes in current mode typically have small currents as signals; I have an IL1700 research radiometer and it measures currents from mA down to pA (IIRC; I'm too lazy to go find the manual); it's a general purpose tool for measuring photodiodes. However, you can also use them in voltage mode -- just connect the diode across your digital multimeter and read the voltage. I have a cheap Radio Shack photodiode I potted in a piece of PVC and stuck a female BNC connector on. This thing works great as an informal light detector; I can measure a few mV output in a nearly dark room up to 0.45 V when I point it at the sun.

    I just aimed this photodiode out the window and took a voltage reading every second or so. Here's the plot, which gives you an idea of the variability; the abscissa is time in seconds and the ordinate is voltage in V. I don't know why there's a negative slope, but I took the data at around 5:30 pm in the Northwest US around latitude 40 degrees north. It might be just the normal diurnal variation, but sunset isn't for another 3 hours.

    Now my interest is piqued because a lot of the solid angle this diode could see was the sky. I'm going to set up an experiment to monitor the output every second for a whole day and see what happens. The next few days are supposed to be sunny and I hope we don't get any thunderstorms.
     

    Attached Files:

  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,411
    2,779
    Jan 21, 2010
    Looks like it was just getting darker. And that's exactly what I'd expect at that time of day.

    As a photographer, I can tell you that most people will not notice a 3 stop change in light levels (unless they occur quite quickly). That's an 8:1 range of intensity. It's hardly surprising that you did not notice it was getting darker outside during the time of your measurements.
     
  5. daddles

    daddles

    443
    3
    Jun 10, 2011
    I agree that it's probably because of the sun going around the earth :p, but I was a bit surprised that it was picking this up with such a relatively clean signal (and it was only a fraction of a percent of change in the signal). Of course, I can't leave it alone because now I want to know, so I set up the diode clamped in a vise to look out the window for the next 24 hours and I'll have the data tomorrow. If I don't have a stroke or unless my wife kills me for mouthing off to her (or one of the cats disturbs the setup), I'll post the data.

    I was aware from past scientific and photographic work that it takes a goodly change for humans to notice light differences, but I didn't realize it was quite as much as 3 stops. This measured change was only a fraction of a percent of the signal, so I had no hope of detecting it by eye.

    Here's a tidbit from when I was a student. I was setting up a Fabry-Perot interferometer and I was in a dark room and looking at the sodium doublet. After a little bit I was a bit surprised to realize that my eye could rather easily see the color difference between these two orange lines (that's about half a nm difference in wavelength if I recall correctly). Of course, if you saw these colors one-at-a-time, you'd likely declare them identical. But if you can compare colors, luminances, etc., you can detect much smaller changes.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-