A chain of 100 LDR's - please help!

Discussion in 'The Projects Forum' started by Chris-S, Oct 26, 2013.

  1. Chris-S

    Thread Starter New Member

    Oct 26, 2013
    Hi all, firstly my electronic skills are medium and my circuit design skills are almost none.
    I have an arduino project to detect 100 light sensors, so my first thought was to use the resistance or voltage checking facility..

    But the help I need is about the chained circuit, something like: If each activated LDR in the line bypasses a 100k resister, i could simply measure the total resistance to find out how many sensors are activated (or not activated, i don't mind calculating it backwards)

    Here is about as far as i got with my circuit, no idea if i need the 1m ohm resister to make it complete.. can someone help me with this design?
    Many thanks!
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    With 100K resistors and 100 sensors your off resistance is 10 Mega ohms. That's so near an open circuit you may want to rethink the overall scheme. Breaking the chain into several smaller chains would help.

    You say "activated (or not activated)" like it is a digital state. There's a lot of gray area as it goes between full light and full dark. If that covers several sensors your results get skewed.

    Your statement about each "LDR in the line bypasses a 100k resister" makes sense. Your schematic does not make sense.
  3. MrChips


    Oct 2, 2009
    One technique for measuring the resistance of 100 LDRs would be to wire them in an 10 x 10 matrix and interrogate each by scanning in the same manner that one would scan keyboard switches.
  4. John P

    AAC Fanatic!

    Oct 14, 2008
    What exactly does "detect 100 light sensors" mean?

    Detect that any one is seeing light?
    Detect that any one is not seeing light?
    Detect how much light is falling on the most brightly illuminated one?
    Measure the light on each one individually?
    (many other options)

    More info needed.
  5. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    From his post #1 I think he just wants to know "how many" are illuminated.

    There must be a simple trick? Assuming the LDRs are almost open circuit when dark, and very low resistance when illuminated, you could run all 100 LDRs in parallel, with a fixed resistance in series with each LDR.

    Imagine it as 100 switches if you like with 100 series resistors, the whole lot in parallel. If you connect to a regulated voltage, the overall current will be directly proportional to the number of closed switches.