Help with transistor circuit

Discussion in 'General Electronics Chat' started by dreed75, Apr 18, 2004.

  1. dreed75

    Thread Starter New Member

    Apr 18, 2004
    3
    0
    Hi, I have this circuit that I am using to test an optical read head that uses 9 little solar panels that each give off about .4-.5 volts depending on intensity of the light source. My problem is that I want to hook it up to LEDs that will light when it "sees" a voltage from the solar panel. In my circuit, I have a 9V battery that is hooked up to 9 transistor's collector sides and then the emitters are hooked to the LEDs. The solar cells have their positive side going to the base to switch on the 9V to the LEDs. The problem is that in testing, the transistor doesnt open up til they recieve about .8V so my circuit doesnt work. Does anybody have any recommendations as to what I should use like an OP-AMP or a different type of transistor? Thanks alot.

    David
     
  2. dreed75

    Thread Starter New Member

    Apr 18, 2004
    3
    0
    Here is a picture of it simplified with just one LED and one transistor as an example.
     
  3. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    hi,

    here's some food for thought, try to see how an light dependent resistor (LDR) works when connected to an opamp or a transistor.
     
  4. bipin

    Well-Known Member

    Mar 21, 2004
    80
    0
    Hi dreed75,
    I feel the easiest way to get rid of this turn on problem is to switch over to an op-amp where you can carefully bias the inverting terminal to .1 volt or even lesser(based on the light sensitivity required) and connecting the photo cell to the non-inverting terminal. use the op-amp in open loop so that you can have the full gain operation and so the two mode operation of the led (on or off). dont forget to connet a series resistor of 1k ohm or so to the out put of the op-amp.

    if you still want to continue with transistor you can biase the transistor base with 1meg fixed, 220k variable resistor and your photocell as potential divider network(three in series) and adjust the pot to get the on off sensitivity.(drop across the 220k variable + the cell voltage should go to the base) .
    you might have to use a darlington pair arrangement in this case.
    best of luck
    bipin
    india
     
  5. dreed75

    Thread Starter New Member

    Apr 18, 2004
    3
    0
    Thanks bipin, your last idea was what I thought of today but I still didn't know if it would work but since you suggested it, I am sure it will :). Thanks again!

    David
     
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