I am working with a friend on a “need it done yesterday” project. The finished project will be a Bingo pattern board (for a musical production) measuring 4’x4’ , with 25 compartments (see attachment). Each time a new game is announced, a new pattern will display on the board, with the push of a button, advancing to the next pattern with each push.
The function of the board is working fine. However, it is severely under lite, with the small LEDs he is using. I have only seen photos, so not sure what hardware is being used, but was hoping someone could recommend boards and parts necessary to get the functionality and level of lighting I need.
The finished project needs to be lite as bright as possible, with an even pattern (behind a translucent cover).
What are some possible options I have availble for such a project? Need to be a simple as possible, for the sake of time and skill level. Is there possibly a way to use small AC bulbs and sockets, with patterns controlled by an Arduino board? At least with that solution, brightness would not be an issue.
[These will be so bright, it will physically hurt to look directly into them at even half power. Seriously, they are extremely bright and that’s a pretty good price for them.
A piece of fluorescent light diffusion panel (get from hardware store) will spread the light out nicely so it looks even.
You can drive each one from 12V if you put a 15 ohm, 5 watt resistor in series with it. Don’t use small 1/4W resistors, they’ll just go up in flame!](https://www.sparkfun.com/products/13105)
tamray:
Would the resister be used in place of the FemtoBuck drivers? What item number would you recommend, if that is the case?
Yes. Buck converters are used for efficiency and current stability. They are not always necessary. If using a regulated power supply and carefully chosen resistor values, it is easy to beat the efficiency of a buck supply and build something that costs much less.
In fact, I’d revise my statement earlier. If you drive these LEDs from a 5V supply, then you could use a minimum value of 1.5ohm resistor. I would suggest 5ohms since that will still be very bright and then you can use the cheaper 1/4 watt resistors. I don’t know if sparkfun carries 5 ohm resistors, but they are easy to find. [Here is a source of half watt 2.4 ohm resistors. The LEDs will be very bright driven with a 5V power supply and the larger wattage resistor gives a larger safety margin.
Just remember that the values I suggested will cause each LED to consume 400mA so if all 20 of them are on at the same time, you will need an 8A power supply.](http://www.bgmicro.com/RES1209.aspx)
tamray:
This is the message I got back from my friend: It is not possible to safely drive a 3W LED with a half-watt resistor, even at 5v.
What are your thoughts?
The resistor is reducing the current to the LED to a safe level but still dissipating a safe amount of power. Let’s do the math: the LED has a voltage drop of about 4V and a safe current of 700mA. With a power supply of 5V, we need to drop 1V somewhere. This somewhere is in the resistor. I chose a current of 400mA, so 1/0.4 = 2.5 ohms.
So we have are dropping 1 volt at a current of 0.4A, leading to a power dissipation of 0.4W. A half watt resistor will be safe, although close to it’s power rating. If you are concerned about that, 5ohm resistors are easy to find and will dissipate only 200mA and still be almost as bright. If it were a FET or other active device, I’d be happier with more safety margin, but resistors are pretty rugged, so I don’t worry about them much.
To take things to an extreme: if you used a 4.1V power supply, then the resistor would only have to drop 0.1V and this would result in a power dissipation of 40mW. A 1/8 watt resistor would be quite safe.