Wiring nightlight with LED strip and Wall Wart

Discussion in 'The Projects Forum' started by harpy, Mar 1, 2011.

  1. harpy

    Thread Starter New Member

    Mar 1, 2011
    4
    0
    Greetings, I'm new to the forum, as well as circuitry wiring. Was hoping I could get some advice from folks on this forum.

    I've been studying a different thread titled 'Simple Photocell LED Night light' in preparation for a similar project: an edge-lit acrylic, light sensitive night light, powered by a wall wart power supply.

    What I have: A Power supply that puts out about 14.5 VDC and an LED strip that comes ready for 12V power.

    What I would like to do: Wire up a photocell, an appropriate resistor (for indoor light differences), transistor, and a resistor to bring 14.5V down to 12V (if necessary).

    This picture shows my power supply and led strip, as well as my scribbled schematic based off of a schematic posted on the aformentioned thread.

    [​IMG]



    I finally started understanding some of the wiring basics that have to do with photocells controlling power to an LED. What I want to know is whether or not a schematic similar to the one in my notebook could be applied to an led strip that is already set for 12 volts.

    What would I have to do differently in order to get this set up to work properly?

    I'm a bit ignorant with this stuff, but eager to learn. I appreciate any advice you can offer!
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    It may work as is, the logic is sound, However...

    With most project like this a variable resistor is thrown in. This is because little things add up, and a simple adjustment will bring them back to doing exactly what you want.

    Something like this...

    [​IMG]

    If you reread your link you will find other people giving more or less the same recommendations. Have fun!
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,103
    3,038
    A couple things. First, the transistor will turn on anytime its base voltage is more than about 0.6 volts above its grounded output. With a 14.5v supply, that means the base voltage will be proportionally closer to ground than with a low voltage supply. All that means is that the resistor values may change.

    Secondly, your power supply might be too much for your LEDs, but maybe not. You need to determine if it's regulated at 14.5v, or if that's a rating at a higher current. It MIGHT, for instance, be 16v when not loaded, and that would be a problem. There will be a drop of ~0.6v across the transistor when it's "on", so if your LED strip is OK at 14v, you may not need any resistor to limit current. But I'd start with a resistor that is too large and work the resistance down while checking the current passing through the strip. Current flow is a MUCH more important measure for an LED than is voltage.
     
  4. harpy

    Thread Starter New Member

    Mar 1, 2011
    4
    0
    The power supply says 12V, but actually measures 14.5V without any load on it. The LED strip measures a current load of 160 mA, and does not seem to cause a voltage drop. The LEDs work nice and bright with the 14.5 V power supply, but I'd rather bring the voltage down so as not to overload the strip over time.

    I plugged the info above into a resistor calculator and was told to use a .4 watt, 18 ohm resistor. I didn't have one of those resistors on hand, but when I tried a 1/2 watt, 33 ohm resistor, it only dropped the voltage down to 13.5 v (which, to me, might be acceptable). I suppose the suggested resistor will bring it down to 12v, right?

    I've never seen or used a variable resistor. Is this similar to a pot, only that it uses an adjustable slider instead of a rotary knob?

    Thanks for the feedback! I'm getting there... slowly.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    2,536
  6. harpy

    Thread Starter New Member

    Mar 1, 2011
    4
    0
    Okay, so I've wired up the circuit based on some of the schematics provided here and the previously mentioned reference thread, and the opposite of what I would to happen is taking place!

    The LEDs are on with full light, but turn off with darkness. I've rewired based on the schematic and tested a few different times, but I can't seem to figure out what I'm doing wrong.

    For what it's worth, I'm using a 1w 16 ohm resistor to drop voltage from 14.5V to 12V, and I'm using a variable resistance cermet potentiometer listed as '500 3/4 15 turn' to adjust for light differences... I bought these based on the advice of the EXTREMELY knowledgable folks at our local electronics store.

    But still, something still doesn't add up! Any idea what could be wrong here?
     
  7. harpy

    Thread Starter New Member

    Mar 1, 2011
    4
    0
    Actually, scratch that last post. I realized that the schematic I've been working with was made to do exactly what it's doing. Bmorse wrote and posted it here: http://forum.allaboutcircuits.com/showthread.php?t=27186&page=3

    Using a previous schematic posted by the same user, I think I can get my circuit to work ...fingers crossed!
     
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