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Photo Resistor/Resistor value — Parallax Forums

Photo Resistor/Resistor value

NWUpgradesNWUpgrades Posts: 292
edited 2012-06-14 08:51 in General Discussion
I built a simple Photo resistor circuit that turns on LED's when it gets dark. I used a 1K resistor and have tried several others but have a bit of an issue. I would like the LED's to turn on just as IR LED's turn on at dusk for a CCTV camera. Using the 1K resistor the lights are still on unless I point a light directly at the Phototransistor. I have tried several combinations of Photo resistors/resistors that come with the What's a Microcontroller kit and can not seem to come up with the right combination. Does any one have any suggestions as to which resistor would be best for this project?

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2012-06-13 20:08
    A photoresistor is not the same as a phototransistor. Unfortunately, the very wide sensitivity range of photoresistors corresponds to a very wide resistance range, anywhere from megOhms to hundreds of Ohms, typically 4 orders of magnitude. There's also a bit of variation from device to device. Best thing is to measure the resistance of the photoresistor you want to use over the light intensity range you want to use. Remember that the color sensitivity of a photoresistor is very different from that of a phototransistor.
  • ercoerco Posts: 20,259
    edited 2012-06-14 01:55
    Are you making a voltage divider from a photocell and one fixed resistor, and feeding that directly into a Stamp pin? The high/low transition is about 1.4 volts, so you should tailor your parts to generate that voltage. It will definitely be sensitive (possibly oversensitive) to the light switched on, so watch out for feedback & oscillation.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2012-06-14 02:05
    There are several ways to tweak the photo resistor. The most tedious might be to do it all in your software.

    Consider that you can add resistance by putting a variable resistor in series.

    Or, you can reduce resistance by putting a variable resistor in parallel.

    With a physcial variable resistor in place, you can adjust the level with just your fingers or a small screw driver.
  • NWUpgradesNWUpgrades Posts: 292
    edited 2012-06-14 08:51
    I am not using a stamp or software. This is for my IR Illuminator from an earlier post. I have solved this by using a 10K Resistor. This seems to work on my test board. If I have any issues I will try Loopy"s idea of a variable resistor. How do I mark this thread solved? Thanks for the help guys.
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