I need to reverse LDR !!

Discussion in 'The Projects Forum' started by myamzen, Nov 19, 2009.

  1. myamzen

    Thread Starter New Member

    Apr 26, 2009
    Hi everyone,

    I am doing a light and motion sensor circuit for my final project.
    I was only able to find an LDR that has less resistance as it's exposed
    to light. However, I am trying to do the opposite. One way I thought
    of was using an inverting op amp (which turned out to be a stupid idea :D )

    Anyway, I am trying to get less resistance in the dark so my
    LED's would light up the room!! I tried to connect two of my LDR's
    in parallel and that did not work out T_T

    SO, what do u think might do the trick?????

    Thanks for helping me out :)
  2. hgmjr


    Jan 28, 2005
    If you post your present schematic, we can tell you how to rearrange the components to invert the sense of the LDR.

  3. hobbyist

    Distinguished Member

    Aug 10, 2008
    Heres one idea for an inverter.

    Light hits the LDR and as resistance gors down, transistor conducts heavily.
    Turns off LED.

    Light NOT presenrt on LDR then transistor shuts off and LED comes on with current suplied through it's resistor.

  4. someonesdad

    Senior Member

    Jul 7, 2009
    In the attached picture, assume R0 = 1. Also assume that LDR = 1 when the room lights are on and 10 when the lights are off. Then, if V = 1, you'll get V0 is 1/2 when the room lights are on and 1/11 when the room lights are off.

    So the lesson is to remember i = V/R. R is the independent variable (the LDR), so combine it with a voltage to get a current that has the behavior type you want. Then you can either use that current directly or convert it back to a voltage as I did in the picture by dropping it across R0.
    • ldr.jpg
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  5. KL7AJ

    Senior Member

    Nov 4, 2008
    Why was that a stupid idea?
  6. BMorse

    Senior Member

    Sep 26, 2009
  7. hgmjr


    Jan 28, 2005

    You could sure save us a lot of guesswork if you would provide us with a schematic of your original schematic.

  8. Dragon Tamer

    New Member

    Nov 20, 2009
    One way you could try to make the circuit respond to the dark (remembering that you just can't change the way a part operates) try switching the photo sensor with the grounding resistor on the base (as it gets darker it keeps less current from going to ground). another relay effective circuit that I've used (I made an entire control system for a shoe box radio using this circuit multiplied several times), it uses a comparator to look at the reference voltage.


    One thing that I have found is that if you remove the relay, and use a single transistor (or darlington array) it works just as well and... IT'S QUIET!!! And it can handle pretty high currents (my 2N3904 NPN transistors stood up to 100mA of current without giving out) You can also use the two resistors that are before and after the transistor you can modify the voltage output of the transistor, remember the formula R1+R2=R total, V total/R total=I total, sence the current through the 2 resistors are the same the you just solve for I total x R(1 or 2 depending on the value you are looking for although the resistor twards the Vdd is the bes choice), this same formula applys to the non inverting input of the op-amp. (just remember that the op-amps output is high if the inverting input[-] is lower than the non-inverting input[+]), for my circuit I think I made a few changes. IMPORTANT!!! MAKE SURE YOU ADD CURRENT LIMITING RESISTORS ON THE NON INVERTING INPUT OF THE OP-AMP, EITHER POSITIVE OR NEGATIVE END, OTHERWISE YOU WILL BLOW THE LDR OR THE POT!!! Another note, the lower the pot value, the more sensitive the circuit will be. If you decide that you do want to use the relay make sure that you modify the voltage for the rated voltage of the relay.
    Last edited: Nov 20, 2009