Simpliest/Best LDR Switch?

Discussion in 'General Electronics Chat' started by vindicate, Sep 8, 2009.

  1. vindicate

    Thread Starter Active Member

    Jul 9, 2009
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    What would be the best/simpliest way for an LDR to switch a max of 500mA? Op-amp? Transistor? Something else?
     
  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    One transistor (and a resistor). It will still have a small zone of linear operation but I have used one transistor for LDR switching.

    If you add a second transistor and 2 more resistors you can dial in some hysteresis to give fast switching and remove the linear zone.
     
  3. vindicate

    Thread Starter Active Member

    Jul 9, 2009
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    To control 500mA with a 9v psu, you would need 2 transistors though, correct? Because the current goinging into the base will be so small(because the resistors are large).
     
  4. vindicate

    Thread Starter Active Member

    Jul 9, 2009
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    Also I read something about the transistor overheating when it is forward active(or w/e you want to call it when it's no saturated). I'm going to guess the transistor won't ever get saturated since the base current is so small.

    Am I wrong?
     
  5. THE_RB

    AAC Fanatic!

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    Depends on the transistor, I use the BC337-40 which are rated for 500mA and have a gain of 400 and are usually over 400.

    Also it depends on whether you want to switch the load on when it is light, or dark, ie whether you put the LDR at the top of the voltage divider or at the bottom.

    What's the load??
     
  6. vindicate

    Thread Starter Active Member

    Jul 9, 2009
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    I bunch of white LED's that I'm going to use to light an arcade marquee. It will light when the room is dark. So I'm pretty sure that means the LDR goes at the bottom of the divider.

    The transistors I have are model number pn222a.
     
  7. THE_RB

    AAC Fanatic!

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    Maybe use the transistor to operate a small relay with (say) a 80mA coil?

    The relay gives you hysteresis (sharp on/off switching), plus ability to switch more current and it will solve most of the problems of that transistor.
     
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