Light Dependent Resistor

Discussion in 'The Projects Forum' started by aespino88, Apr 14, 2010.

  1. aespino88

    Thread Starter Member

    Mar 31, 2010
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    hey guys, i was sittin on my patio this afternoon drinkin a beer when sudden inspiration hit me. i thought it would be cool if my beer lit up when i set it on my glass table. i know i how to wire the LEDs, resistor, transistor and photocell but the problem is the glass is tinted or smoked. so light under the table is already reduced. i would like the photocell to remain off until the beer or cup covers it completely. my questions are.......

    what size transistor should i use?
    what size resistors?
    and what size photocell?

    i will be using 5 LEDs wired parallel
    if you have any questions please ask. THANKS!!
     
  2. barrelracerdad

    Member

    Jan 29, 2010
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    0
    What size power supply are you using? You will need an optocoupler to switch the curcuit from on when it senses light to on when it is dark (dark on) Photo cells and resistor sizes can be found on a thread called fast horses (schematics also) for a light sensitive trigger that turns on when it is darkened.
     
  3. aespino88

    Thread Starter Member

    Mar 31, 2010
    41
    0
    i don't know yet, i was thinking takin apart an LED flashlight and using the 3 AAAs.
    what is an optocoupler??
    is there a photocell cell that goes from 0% in dim light to 100% when covered?
     
  4. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    The usual problem with this type of setup is the problem of feedback. i.e. the light from the LEDs confuse the sensor once they are turned ON.

    The sensor then thinks the glass had been removed and turns OFF the LEDs. Then it sensed the glass above and turns ON the LEDs again....

    Make sure the sensor cannot see the LEDs or LEDs do not shine onto the sensor.
     
  5. soda

    Active Member

    Dec 7, 2008
    174
    13
    Hi,
    Why don't you change the LDR with a ntc thermistor and design the diagram to work on temperature
     
  6. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
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    You could also set up a grid of electronic eyes. You could use laser pointers like used in the fast horses thread. That way, you would know what "block" had the beer on it and light an LED from underneath.

    And is it still called inspiration when you get the idea while drinking? ;)
     
  7. aespino88

    Thread Starter Member

    Mar 31, 2010
    41
    0
    thank you for pointing that out mr. chung i didn't even think about that

    soda, that sounds like a good idea thanks alot but i would like for it to light for other things than just a cold beer.

    I read that thread about the laser timer but i'm confused i don't understand what its talkin about. i really like the idea of using the laser sensor than the photocell. what exactly do i do? how does that work?

    i get some of my best ideas with a good buzz lol

    i know this doesn't relate to circuits but i've been trying to add a signature but i can find where i can edit it, i can just choose to show it but not actually type 1.
     
  8. retched

    AAC Fanatic!

    Dec 5, 2009
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    312
    For your signature, click on User CP in at the top of the thread menu.

    On the left side of the User CP screen, in the menu, you will see "Edit Signature" in the Settings & Options area of the menu.
     
  9. BMorse

    Senior Member

    Sep 26, 2009
    2,675
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    You could use a similar circuit like this one to connect the photocell to, it has an adjustable threshold, the output can be used to drive a transistor to light the LED's....

    [​IMG]

    B. Morse
     
  10. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
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    If you use the laser pointer as a beam, shine the laser pointer across the table into the photoresistor, it will register as nothing is there. When the beer can (Or whatever) breaks the beam, the circuit will say "There is something here"

    By making a grid pattern, you can figure where the item is located and light a corresponding led from below the table.
     
  11. BMorse

    Senior Member

    Sep 26, 2009
    2,675
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    or you can use the LED's as light sensors themselves and then use them to also light up the area.....
    B. Morse
     
  12. aespino88

    Thread Starter Member

    Mar 31, 2010
    41
    0
    it all sounds good but its kinda confusing. i found this night light with a photocell and i thought i could use the same circuit (cept for a different resistor) and make like a tiny tube with foil or something so it could keep the LED light from gettin to the LDR but at the same time getting the light from above. what do you guys think???

    oh and i don't have that edit signature in my settings and options
     
  13. Wendy

    Moderator

    Mar 24, 2008
    20,765
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    That's funny, Raytheon gave us some swag that was coasters microswitches that turned on a bunch of SMT LEDs from a watch cell. Not real practical, but fun to watch. The microswitch was one the bottom of the plastic body of the coaster, it was weight activated.

    What part of Texas are you? We might be neighbors.
     
  14. aespino88

    Thread Starter Member

    Mar 31, 2010
    41
    0
    thats a good idea too but would it mess if something was to spill on it?
    what about those motion sensor lights like the kind that gas stations have in the bathrooms for the towel dispensers. could put one in the middle of some LEDs and when something covers it it would light up.

    i live a little south of Dallas. You?
     
  15. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    Garland. You sound like you're in DeSoto.

    The coaster is water proof where it counts. A thin puddle isn't too much of a danger I would think. The switch idea has the benefit of uber simple, a battery, the switch, a resistor, and LEDs and you're in power.

    The one I had was slightly more complex. It had SMT circuits that would flash different colors.

    If you are new to electronics I know where most of the really good parts houses in the Dallas area are. One of my favorite stores is in Carrolton, about as far from me as it is from you I suspect.

    Look for tactial switches.
     
  16. aespino88

    Thread Starter Member

    Mar 31, 2010
    41
    0
    close, but wrong highway. try 30 miles south on 45.

    i really want to use some sort of sensor that would be simple to wire and would work with just about any type of power output. i would like to use a photocell but the feedback problem ruins it. so i would like to use a sensor like i described earlier. what could i use that when blocked it would let electricity flow and light the LEDs??? i like the laser beam but it seems a bit complicated. i would like something i could do small and simple.
     
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