Reversing Resistance?

Discussion in 'Homework Help' started by Tezz, Oct 29, 2006.

  1. Tezz

    Thread Starter New Member

    Jun 23, 2004
    6
    0
    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. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    Can you supply a circuit, please?
     
  3. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Have you considered inverting your switch output with a second transistor?
     
  4. Tezz

    Thread Starter New Member

    Jun 23, 2004
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    0
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Your CDS cell gets more resistive as the light level falls, so your 555 will shut down. Be a good idea to put a 330 - 560 ohm resistor in series with the LED, too.
     
  6. Tezz

    Thread Starter New Member

    Jun 23, 2004
    6
    0
    Yep, that's exactly what my problem is :D
     
  7. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    You may be able to get rid of the transistors. Try this.

    Supply the 555 V+ with 9V as normal. Connect your cell between -ve supply and Reset pin. Connect a resistor between Reset and V+.
    You will need to adjust the resistor value to suit.
     
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