So while I've worked with
LED products and I understand the principles and theories behind LEDs, I'm now realizing that I really don't know much about working with individual LEDs. I've worked on large-scale
LED projects before, but even those usually used
LED rope/tape or other products that already had LEDs integrated into them. I'm working on something now where I essentially need to
wire up individual LEDs, and I'm really not sure even where to begin.
So here's the scenario. I need to put tiny, dim
LED indicators into the
deck along the
downstage edge of a show
deck. There will be a total of 19 LEDs in the
deck, each spaced 2' apart, with each
LED being one of 5 different colors. These will be sunk into the
deck slightly to hide them from the audience while being visible to the actor, so small size is important. They will be used both as a marker for where the pit is, but the colors will also indicate location on the
stage - they
line up with the dance numbers along the DS
edge - so they need to be very dim, but at least bright enough to be seen by the actors under
stage lighting. They also need to be switchable from the lighting
console, so they'll eventually go into a 2P&G
connector and cannot run off DC.
My initial thought was to
wire groups of low-voltage LEDs in series to make a full 120V - which is what I would do if I was doing this with low-voltage birdies or similar. However, I'm having a hell of a time even locating individual LEDs for
purchase. I haven't found anything at my local stores, and even McMaster-Carr seems to be failing me. What I really want is tiny individual LEDs with leads that I can
wire together, but I can only find pre-built items containing LEDs. Is this normal, and it's not actually possible to buy just the
LED itself? Am I thinking about this too much like a
conventional wiring project? Also, for those of you who have done this before, what kind of
voltage/wattage combination would get me light that's the right
intensity for my purposes?
While this does have an end result, I'm also looking to try to learn more about this with this project, so feel free to provide auxiliary information if relevant. Thanks!!
Ok, let's get started: I will try and go through this as best I can, but I am sure there will be many others adding to or correcting me.
LEDs must be driven from DC, and be
current limited. Now how you take the AC from a
dimmer or wall
circuit to use in an
LED can be accomplished in several ways. If it from a constant
power source and does not need to dim in a smooth manner, then you can use a stepdown tranformer and a halfwave bridge, of one
diode or a full wave bridge from four diodes. This same thing can be accomplished with a "Wallwart supply" or a regulated loptop supply. The
current limiting is accomplished with a
resistor of the appropriate value.
Single color LEDs have two leads. The
anode and the
Cathode. These are terms meaning positive and negative and leftover from the old tube era. (you know those glass thingies that proceed solid state electronics.) Most all LEDs are manufactured with a small collar at the bottom. One side of this collar will have a
flat spot. this is on the same side with the negative or
cathode lead. The positive or
anode lead will be longer than the
cathode. Of course once you cut the leads, you will have to depend on the
cathode marker on the collar.
LEDs have a forward
voltage and
current. This can differ from manufacture to manufacture and also from color to color. Having said that, most all LEDs except white and blue will have a forward
current of 20 miliamps and a forward
voltage of 1.6 to 2 volts. So for our first experiment, let us take a 9 volt battery and
power one green
LED. We know that the battery will have 9 volts, and we want to use two volts of that to
power the
LED. We need to get rid of the other 7 volts. We do this across the
current limiting
resistor. So we want to
drop 7 volts at 20 mils accross the
resistor. So using
Ohms law, we divide the 7 volts by the .020 amps and get 350. Resistors come in all kinds of values if you buy 1% resistors. When you
purchase 5% or 10% resistors, your choices are limited to fewer values. In the 5% values which are available at Radio Shack, there is no 350
Ohm resistor. There are 330s and 470s. If we use the 470, the
LED will be less bright but will last much longer. If we use the 330 the
LED will be brighter, but will exceed it's published spec and will become less bright over time. When it comes to the brightness of LEDs, the difference that will occur by a slight change in
resistor values is hardly noticeable. So use the 470
resistor. You could easily put the
resistor and
LED in a 9 volt battery
connector and seal with
hot glue. This can now make a pretty good marker for
escape stairs. Just gafftape it to the side of one of the steps.
LEDs can be ganged into muliple units either in series of parallel. They both have their advantages. I built an
LED star curtain with a little over 200 White LEDS. They were the first of the inexpensive White LEDs and really didn't have a consistent
color temperature, but when it comes to stars, this is a good advantage. The
PWM dimmer (discussed later) could handle a maximum of 32 volts. As 24 volt
power supplies are easy to come by, I chose to make each string a series of LEDs of 24 volts. These White LEDs had a forward
voltage of 3 volts and a forward
current of 30 mils. If I put my LEDs in series, the
current through the string stays the same at 30 mils. This means that I can use a much smaller
power supply. At 3 volts each, I could theoretically put 8 LEDs in series and use up the entire 24 volts. This would be a disaster as there would be no
current limiting and there would be either a bright flash or a series of small firecrackers. Therefore, we will limit each string to 7 LEDs and a
current limiting
resistor. In my case, I used a four
channel PWM dimmer and wanted the stars to be scattered around to make full advantage of the 4 channels. Because of this, not all strings were 7 LEDs. There were also six, five and four
LED strings. With each string, there had to be and appropriate
current limiting
resistor. Using 5% resistors it worked out to 100
ohm for 7 LEDs, 220
ohm for 6 LEDs, 330
ohm for 5 LEDs and 470
ohm for 4 LEDs. These values were nearest to required and made some of the LEDs slightlly brighter than others, but this is desirable in a star field.
Now LEDs dim properly by changing the amount of
current they draw and keep the
voltage constant. Therefore if you want smooth dimming, you need to use Pulse Width Modulation circuitry. There are several companies that sell such devices, and they can range from fully packaged to
circuit boards and from very expensive to quite affordable. They also come in 0-10 contol or
DMX. After many devices built and a lot of searching, the best bargain is Northlight
DMX. He has four and eight
channel boards, but you will need to package it your self and supply an external DC supply. His
manual explains it very well and gives a few examples of
hookup. The
manual can be downloaded without a
purchase.
For LEDs and
power supplies to make all of this work, the least expensive are mpja.com and electronic goldmine. There is one other that I have used a few times. I believe it is called all electronics. If you want the high quality stuff, there is mouser and digikey.
LEDs brightness is measured in MCDs, which I think stands for millicandles, or milicandelar. Not important which. The higher the number the brighter. For
stage markers, you really don"t want bright LEDs. Most LEDs are now made for maximum brightness, but at mpja.com they have some real cheapies that are quite dim. When I purchased them, they were $2 for 100.
LEDs come in several shapes and sizes, but the most popular are round and come in 3mm, 5mm and 10mm. The 5mm is by far the most popular. I have some that are 1mm, but you better be really good at soldering and have some strong magnification to work with those.
Well I am tired of rambling and need to go apply 19,000 watts to a "Full Monty" sign. I hope this has been helplful. Further questions can be addressed to
[email protected] But it will be close to a week before I can give any kind of lengthy answer.
Tom Johnson
Florida's Most Honored Community Theatre
