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Need Photoresistor Help

ltmhallltmhall Posts: 102
edited 2007-03-28 14:55 in BASIC Stamp
Will· photoresistor activity 5 in the Robotics with the Boe Bot manual work with an ordinary
0.01 ufd capacitor or does it have to be a Parallax capacitor ?

Also what is the average maxium range of a·photoresistor ?

Comments

  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2007-03-13 17:42
    Hello,

    The capacitor value will affect the RCTIME values, but it doesn’t have to be our brand. As for range, a CdS photocell doesn’t work like that…There is no range per se…It’s resistance is affected by the amount of light hitting the surface. Since that could be the sun I guess you could say the range is 93 million miles! Take care.

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    Chris Savage
    Parallax Tech Support
  • Mike GreenMike Green Posts: 23,101
    edited 2007-03-13 17:45
    See this article: en.wikipedia.org/wiki/Photocell. You'll notice they quote a range of perhaps 100 ohms to 10 megohms depending on the light level. This is typical for photoresistors.
  • ltmhallltmhall Posts: 102
    edited 2007-03-17 18:24
    Will a larger capacitor increase or decrease the sensitivity of a photoresistor ?
  • Mike GreenMike Green Posts: 23,101
    edited 2007-03-17 18:29
    The capacitor doesn't affect the sensitivity of the photoresistor. The photoresistor has an extremely wide range of resistance and the Stamp has limited capability for measuring resistance. The capacitor value essentially sets the "range of interest" so that you can get the most resolution within the limited range. The Stamp actually measures the time it takes for the capacitor to charge or discharge through the photoresistor. The time for a unit of 1 varies from Stamp model to Stamp model. For the BS2, it's 2us with a maximum time set by the use of 16 bit words of 65535 x 2us = 131ms.

    Post Edited (Mike Green) : 3/17/2007 6:34:13 PM GMT
  • ltmhallltmhall Posts: 102
    edited 2007-03-28 14:34
    Is there any type of circuit I can build using a pot and a photoresistor that could increase the sensitivity of a photoresistor ?
  • Mike GreenMike Green Posts: 23,101
    edited 2007-03-28 14:55
    No

    Photoresistors are already pretty sensitive. You just have to measure the low light and high resistance end of the resistance/light curve rather than the high light and low resistance end. You do this by using smaller capacitors and longer RC times. You can use a pot (or a fixed resistor) in parallel with the photoresistor to emphasize (and possibly make easier to measure) a particular range of resistance, but it won't make the photoresistor more sensitive.

    Alternative measuring devices include www.parallax.com/detail.asp?product_id=27924 which outputs a pulse train whose frequency depends on the light level. Look at some of the articles with links on this page. Perhaps this might work better for you.
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