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LDR circuit

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by bananasiong, May 11, 2006.

  1. philba

    philba New Member

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    yes, that's a good one - basically my 3rd option, if you hadn't noticed. though I think hysteresis isn't necessary.

    by the way, while it's kind of silly for a mower. some autos have tri-level instrument lighting based on ambient light. it's backwards from the intent here but it makes sense - bright lighting when it's bright, middle during "tween" times and dim when it's dark.
     
  2. bananasiong

    bananasiong New Member

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  3. docel

    docel Banned

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    Silly it may be, but quite a bit of applications! LG TVs use this for ambient light controlled Brightness/contrast, with an enhanced cost, ofcourse.
    Also in many instrument/annunciator display lamps to save battery power during day-light hrs.
    "Silly" might mean " :rolleyes: Whatever-for in a Lawn mower?!" kind of thing. :p
    But this thread has helped many, as I can see.....for many other applications.
     
  4. dave

    Dave New Member

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  5. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    It's been used by a great many TV's over the years, the first I can recall were some Pye sets from the 1950's, but that's not to say they were the first?. However, it's always been a really useless feature - but it's very cheap to do!, and it's an extra gimmick - just make sure you can turn it off.
     
  6. dknguyen

    dknguyen Well-Known Member

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    Yes, a photos resistor is the same thing as an LDR. (It's resistance lowers with increasing light levels, right?)

    I think the circuit with the resistor, LDR, and LED use the least power just because it has less components(plus it also adjusts brightness automaticall). I also think the relay circuit only has on/off and no "middle" brightness level.
     
  7. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    No, it's a wasteful 'circuit', because it uses MORE current when the LED is dark than when it's lit - it's also not likely to work very well, and not at all unless you can get a really low resistance LDR, most will be too high to work. There's not much to admire about it at all, except it's lack of components - which probably isn't a good thing?.
     
  8. dknguyen

    dknguyen Well-Known Member

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    Why doesn't the relay circuit cause a short when the relay is closed?
     
    Last edited: May 14, 2006
  9. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    How can it cause a short?, the contacts aren't even shown connected to anything.
     
  10. bananasiong

    bananasiong New Member

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    mayb he is saying the NC to GND, NO to Vcc, COM to LED and to GND.
     
  11. dknguyen

    dknguyen Well-Known Member

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    Short

    Well if the relay is closed, doesn't that create a straight path for current to flow down the right branch from +6V -> relay -> transistor ->GND. This would only happen if the gate voltage was pulled high, but since no current will flow through the right branch, I can't figure out whether or not the gate potential will be at GND or +6V to switch the transistor on to cause the short.

    Maybe I am misunderstanding the relay symbol?
     
    Last edited: May 14, 2006
  12. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    A relay isn't a short circuit, it's a solenoid that moves a mechanical switch, and the solenoid has resistance - which is listed in the specifications of the relay - some can be quite high. But no matter what it's resistance, it's designed to work at it's rated voltage - so why should placing the relay across it's intended supply cause a short?.
     
  13. Hero999

    Hero999 Banned

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  14. bananasiong

    bananasiong New Member

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    can i switch a 3v relay with 5v supply? i do this because i'm worried the power of my battery might decrease. am i going to distroy the relay?

    must i connect a diode parallel with the coil of the relay? cathode to Vcc and anode the other end of the coil.
     
    Last edited: May 16, 2006

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