Reversing resistance?

Discussion in 'The Projects Forum' started by amc, Apr 24, 2007.

  1. amc

    Thread Starter New Member

    Apr 23, 2007
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    0
    I noticed the following thread. I am having an almost identical problem. My CdS cell behaves exactly the opposite of what I want. However, my circuit does not employ the 555 timer. Anyway...what is a good and simple way to "reverse resistance." (See below.)

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    Old 10-29-2006, 02:44 AM
    Tezz Tezz is offline
    Junior Member

    Join Date: Jun 2004
    Posts: 6
    Default Reversing Resistance?
    Ok, I'm not really trying to reverse resistance but here's the problem I've got with an assignment question. The question itself is measuring light, so when light falls below a certain level an LED flashes.

    I've got a CdS cell running into a NPN transistor (acting as a switch depending on the current coming from the CdS cell) which then triggers a 555 timing chip to control the LED flashing. Now all that works fine except for the fact it's doing the exact reverse of what it's supposed to do. ie. When there is plenty of light (ie. low resistance in the CdS cell) the LED flashes, but when there is low light it doesn't flash. Is anyone able to give me a pointer on how I could reverse this?

    I'll keep reading up on it but any pointers would be appreciated.
     
  2. windy

    Member

    Apr 19, 2007
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    is your NPN set up to drive or sink the 555 input? That should give you enough input to make it work.
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    If your CDS cell is connected in series with a resistor as a voltage divider, you just need to swap the positions of the resistor and the CDS cell. You might have to adjust the value of the resistor to get the correct threshold.
     
  4. amc

    Thread Starter New Member

    Apr 23, 2007
    3
    0
    Thanks Ron. I'll give it a go!
     
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