How I make LED shoplight/billboard lights

zscaler Sep 25, 2017

  1. zscaler

    zscaler TrainBoard Member

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    Next Project: How to make shop lights and learning about LED voltages:
    LEDs usually run on 3v DC. When I power lights, I like to use 9v DC. Now you can put a resistor connected to one side of the LED (I like 470k ohm) or... You run the LEDs in series. Even though the LED is rated at 3v, 9v going through 3 LEDs in series works.
    So I have made billboard lights the same way and they never burn out. Or like on my gas station project, one LED outside and the two inside in series. When I run streetlights, I will put them in groups of 3. And it works. And is simple.
    So here we go:
    Get some circuit board and cut it to the thickness of the short side of the LED. Here I have 2.5 x 3.8mm LEDs. I marked where I wanted the lights to be on the circuit board and cut through the copper with a dremel cutting disk. Then tin the copper with solder, just not too much. You could just tin one side of the copper, then solder the LEDs in the correct polarity order. Lets just say + on the left. As you solder each side of LED, test with 3v (watch batteries work pretty good for this) for a good solder joint. Do NOT overheat the LED. They melt. Just get the solder to flow and it will connect to the LED. You could do 4 LEDs on 9v if you want it dimmer or put a resistor in line. I like to use magnet wire for connecting because it is easier to hide. These LEDs I just got are very bright. I put a nine volt battery in one picture for size comparision. I could of made it a little shorter, but soldering is easier. If you use the even smaller LEDs, you can have a thinner light.
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  2. zdrada69

    zdrada69 TrainBoard Member

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    That is really clever idea. Thanks for sharing.
     
  3. markm

    markm TrainBoard Supporter

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    Dave,
    I do have to point out that LEDs are current devices and not voltage devices. The light intensity depends on the current flowing through the LED and the forward voltage across the diode varies depending on the current by about +- 20%. The voltage also depends on the color of the LED, ranging from about 1.7V for IR & red LEDs to 3.5V for white. Example datasheet: http://www.lc-led.com/products/m500rgb4d-a.html

    Your method works because on forward voltage across the white LEDs worst case is greater than 3 volts and the LEDs are under driven. Were you to connect 3 red LEDs to 9V you will very quickly burn them out.

    A more sophisticated (and pricier) alternative to the current setting resistor is a constant current diode. After a turn-on voltage has been reached, the current remains constant regardless of the applied voltage. Useful for LEDs being driven by a DC throttle.

    Mark
     
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  4. zscaler

    zscaler TrainBoard Member

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    Added a 47k ohm resistor to one end and it dimmed it by half.
     
  5. markm

    markm TrainBoard Supporter

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    Dave,
    If you haven't seen it already, check out a similar discussion on the AZL site: http://www.azlforum.com/thread/1044/lighting-layout
    where the issues have been discussed in more detail.

    If you want more light, put a second 470k resistor in parallel with the first. The key thing is to control the current going through the LEDs to prevent thermal runaway. The situation is fairly rare, but I figure a resistor is a lot cheaper than fire insurance.

    Mark
     
  6. SJ Z-man

    SJ Z-man TrainBoard Member

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    Dave,

    Get yourself some SMD solder past. Like ChipQuik. And an 11 watt solder with the small point tip. Makes it much easier.
     

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