Ambient light intensity indicator

Discussion in 'The Projects Forum' started by robjarron, Oct 20, 2009.

  1. robjarron

    Thread Starter New Member

    Oct 20, 2009
    I'm currently working on a project to indicate 3 levels of ambient light intensity using 2 LEDs as the indicators.
    Good light - Both LEDs on
    Medium light - One LED on
    Poor light - One LED flashing at about 1 Hz

    At the moment, the best solution I can come up with is using a LDR to assist turning on 3 NPN transistors at different light intensities.
    First transistor turns Both LEDs on in series,
    Second transistor turns 2nd LED on,
    Third transistor turns a 555 astable timer on, with the output going back to the 2nd LED.
    ... I'm sure this is probably not the greatest solution... does anyone else have ideas or suggestions how I could tackle this problem?
  2. Darren Holdstock

    Active Member

    Feb 10, 2009
    It sounds like you're relying on the gains of the transistors to set the various thresholds, in which case you'd be better off with an op-amp window detector circuit as hfe characteristics shouldn't be relied upon.

    Another point to note is: Don't illuminate your light sensor with your LEDs...
  3. someonesdad

    Senior Member

    Jul 7, 2009
    Another technique to consider is to use a photodiode. I use one as a quickie light detector when firing up the spectroradiometer is a pain. It's some generic part I had laying around; chances are I got it at Radio Shack.

    I stuck the diode into a home-made holder with a female BNC connector and potted it in adhesive so it's pretty much bomb-proof. I just measure the voltage across the diode with a digital voltmeter. It will output 0-10 mV in a dark room, 100 mV seeing the light from my monitor in a dark room, 200 mV from my fluorescent lamp, and 500 mV up against a small incandescent light bulb in the same lamp. It also measures fine in sunlight too (if I recall correctly, it's around 0.5-0.6 V in sunlight).

    In other words, if you have something to calibrate it with, I figure it would be a good sensor for what you want. You could use an LM339 IC with four comparators on it to turn on at the different voltage levels. With some trimpots for each comparator input, you should be able to set it to turn on the LEDs where you want. It looks like each comparator can reasonably sink 10 mA, so you could light an appropriate LED without a driver transistor.
    Last edited: Oct 21, 2009