Photo Cells Help

Discussion in 'General Electronics Chat' started by khanz, May 1, 2008.

  1. khanz

    Thread Starter Member

    Mar 24, 2008
    Hi all,
    I am designing a circuit, which is light dependent. Please look at the diagram and guide me, if there are some mistakes, I want to know the variation in voltage through the LED, but it's not working, what's the problem, Please help me in finding the solution.
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    You won't see a variation in voltage in the LED. They are current-dependent devices. After enough voltage has been put across the LED for bias it into operation, the LED intensity is totally dependent on current.

    Your CDS cell has some minimum resistance spec. To get variance in the LED, enough voltage has to be present to forward bias the LED and then push curent through it. If your CDS cell goes to 1000 ohms under full light, then you would need about 12 volts across the LED and CDS cell for roughly 10 ma current in the LED.

    Using a potentiometer to pick a voltage off a source will be unsatisfactory. The voltage at the wiper will vary as he conduction of the CDS cell varies after the LED starts to conduct. You would be much better off using an active voltage regulator.
  3. khanz

    Thread Starter Member

    Mar 24, 2008
    How I will use a voltage regulator and which one you would prefer, Should I replace the potentiometer with the voltage regulator or what, Please if you could suggest me one, it would be much easier for me. Thanks in advance.
  4. SgtWookie


    Jul 17, 2007
    Try something more like the attached.

    The 0.62v in the equation for R2 is due to the Vdrop across the transistor. YMMV.