distinguishing light surface from dark

Discussion in 'The Projects Forum' started by jcm2112, Jan 18, 2011.

  1. jcm2112

    Thread Starter New Member

    Jan 11, 2011
    6
    0
    Hi everyone,

    I'm working on a similar project, but I need to be able to distinguish between a shiny bright surface and a duller, dark one, also from ~10 cm away. My plan is to use a laser pointer and couple this with a phototransistor so that the laser will reflect off of the bright surface only and be detected by the sensor.

    I was going to just use a regular laser pointer and then a separate detector, but does anyone know of an inexpensive laser diode + detector combination (i.e., one component)?

    Thanks!
    jcm
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
  3. jcm2112

    Thread Starter New Member

    Jan 11, 2011
    6
    0
    Sorry about that, and thank you for your reply. I'll make sure to check the frequency.

    beenthere, off the top of your head have you ever worked with (inexpensive) components which combine both laser emission and detection in one unit? While individually I have found a laser led and phototransistor for under $2, my concern is that it will be difficult to set the angle correctly off of a surface 10 cm away. However, all the parts which combine these 2 (and therefore are designed for this type of accuracy at this distance) that I have found on digikey and Mouser are very expensive.

    Thanks again
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    I have never messed with such. The problems are pretty severe - consider approaching a surface at a shallow angle. You would need to be doing a scan to detect before a collision.

    You're probably thinking of an IRLED,as they are cheap and it's easy to obtain phototransistors. The surprise is that the colors we perceive may or may not be reflective/absorbent to IR frequencies. I might suggest you get an emitter and receiver and do some experimenting before settling on the particular laser.

    Even with spot using while light, you would still need to allow for ambient illumination to be sure the detection was valid.
     
  5. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    A white LED combined with a photoresistor will work for a 3" distance.

    Get the background reading of the voltage across a photoresistor, then turn on the white LED, and get reading of voltage across photoresistor.

    If the voltage changes significantly, it is most likely a white surface
    If not, it is most likely a dark surface.

    CdS Photoresistors are somewhat slow to react, on the order of fractions of a second vs milliseconds or less, so if you need high speed, this wouldn't be what you are looking for.

    This method is fast enough to keep line following robots on the edge of a black line on a light background though, when the speed is around 1 ft/second.
     
Loading...