Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Detecting Lasers with photodiodes---Circuit layout?

  1. Oct 17, 2015 #1
    For a certain science fair working model, what should I use to detect the collimated light of a HeNe laser(in the red;653nm if that is necessary)?After some surfing on the net, I found that I should use a 'photodiode'.
    The desired result should be solely to detect whether the laser's light is falling on it or not.What electrical device should come into play if I was to employ it in this science project?
    Note--- This project is actually similar to a burglar alarm, so upon detecting that an object is crossing the laser's beam,a buzzer should ring an alarm.
     
    Last edited: Oct 17, 2015
  2. jcsd
  3. Oct 17, 2015 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    It depends on your setup.

    If the laser is powerful enough, a simple "supply voltag, buzzer, photodiode, resistor" circuit could be sufficient. You probably want to amplify the signal with a transistor, however. How does the buzzer work?
     
  4. Oct 17, 2015 #3

    davenn

    User Avatar
    Science Advisor
    Gold Member



    there's zillions of ideas on the net .....

    http://www.bing.com/images/search?q=photodiode+light+detector+circuit&FORM=HDRSC2


    Dave
     
  5. Oct 17, 2015 #4

    davenn

    User Avatar
    Science Advisor
    Gold Member

    you also need to consider some narrow band optical filtering if this is going to be used in daylight
    as any ambient light will be picked up by the photodiode


    Dave
     
  6. Oct 17, 2015 #5

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Differential measurements (laser on/off) are an alternative, but I don't know if HeNe lasers can be switched on and off frequently without issues. Works fine with LEDs over short distances. It requires coupling the source and the receiver, of course. Which leads back to the point "we don't know the setup".
     
  7. Oct 17, 2015 #6

    davenn

    User Avatar
    Science Advisor
    Gold Member

    personally, I wouldn't even be considering a HeNe laser or even a laser diode UNLESS the distance between the source and detector was 10's of metres
    HeNe lasers are dreadfully power hungry, run hot and have a very short life span ( when run continuously) compared to laser diode or IR LED
    For anything up to around 10 - 20 metres, standard IR diodes ( focused) and use an appropriate IR receiver diode.
    eg

    Factory-wholesale-38Hz-IR-receiver-Module-8-30M-transmission-distance-20mA-2-5-5-5V-plastic.jpg
    these have a + and - supply and an output

    This will make daytime use so much easier

    Dave
     
  8. Oct 17, 2015 #7

    meBigGuy

    User Avatar
    Gold Member

    Not sure why you want to use visible red light. Is that just because it is what you have, or is it part of the demo?

    For the basic design steps, you have to consider:
    A. You need to deal with ambient light vs. laser intensity.
    B. You need to deal with interfacing between the photodiode and whatever you want it to "trip".
    C. You may want to consider advanced techniques to improve #1.

    Assuming the laser is always on (#C is discussed later), you probably need to have a threshold calibration to set a threshold for the laser light level.
    The laser needs to be bright enough to make a significant difference to the ambient light. This is essentially your signal to noise level.

    A. There are several simple optical methods to help you out with ambient to laser ratio:
    1. Get a filter for the laser frequency to put in front of the photo diode. <---- Probably Required
    2. Put a neutral density filter in front of the laser to avoid saturating (overloading) your detector. <--- Kludge
    3. Add a lens system that focuses a narrow area on the detector. <--- Good
    4. Something as simple as a cardboard tube can reduce adjacent light sources <--- quick and simple

    B. There are many different circuits ranging from very basic and insensitive, to very sensitive. The link from davenn shows they range from simple transistor based circuits to opamp based circuits. The most basic opamp principle for a photodiode is shown below. Measuring the voltage output from such a circuit would allow you to experiment with The ideas in A and give you some idea of what the final design needs. The value of Rf depends on the diode and light levels.

    Essentially when light falls on a reverse biased photodiode, the leakage current increases and the opamp output voltage changes to supply the current.
    This circuit is an over simplification to show the principle.


    th?&id=OIP.Mf70f06d13bd9fcbd781cafcd0c42f4deo0&w=300&h=300&c=0&pid=1.9&rs=0&p=0.jpg

    You may want to search for a module. Such as
    http://www.lightinthebox.com/lm393-...-for-arduino-free-dupont-cables_p1023968.html <--- looks adequate!!
    https://www.amazon.com/RioRand-Photodiode-Photosensitive-Detection-Brightness/dp/B00JRNN1FW

    Although, I didn't see schematics for either of those.

    C. The next level of sophistication involves chopping or modulating the light at a particular frequency and building your detector to filter for that frequency. I think you need to understand all of the above and determine a true need before you head down the modulation path.
     
  9. Oct 17, 2015 #8
    Could you suggest any other laser,not necessarily in the visible spectrum but definitely in my low budget???
     
  10. Oct 17, 2015 #9

    davenn

    User Avatar
    Science Advisor
    Gold Member

    reconsider what I said in my opening comments in post # 6


    Is there really a good reason why you want to use a laser ?
    if so use IR one instead
    you still haven't commented over what sort of distance you want this to work


    Dave
     
  11. Oct 17, 2015 #10

    meBigGuy

    User Avatar
    Gold Member

    Why a laser as opposed to an LED? You really have not explained what you are really trying to do, which makes it hard to focus on the correct advice.
     
  12. Oct 18, 2015 #11

    davenn

    User Avatar
    Science Advisor
    Gold Member

    Yes, this is what we have all been asking
     
  13. Oct 20, 2015 #12

    rbelli1

    User Avatar
    Gold Member

    Paradox, can you describe the experiment and its objectives? If the goal is for object detection then just about any other light producing technology from candle on up would be easier and more cost efficient than HeNe laser.

    BoB
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Detecting Lasers with photodiodes---Circuit layout?
  1. Layout verification (Replies: 2)

Loading...