Materials needed to make interactive LED table.

Sounds like fun! are you looking to detect the proximity vertically, horizontally, or both?  I.E. do you want to detect somebody walking up to the table, or how close, say, their hand is to the table top?

@jbix detect their proximity vertically. i had in mind ding something like a analog read of the ir reciever and do an analog write of that value to the color led, thats a simple version of what. i think for the programming side im pretty secure im an undergrad software major, my main questions is about the electrical side of things. for instance what kind of resistors should i be using is im wiring the leds in parrell, and the driver chips needed

More details about the design?  How many Emitters/detectors are you planning on using?  Have you selected a microcontroller yet?  Have you selected your proximity sensors?  You mention wiring the LED's, so I'm guessing you're using separate IR Emitters/Detectors?

Regarding resistor selection, when you wire devices in parallel, they all receive the same source voltage, and their currents are added up.  You'll need to know the operating current (forward current) of your LED's to select your resistor(s).  You'll also need to know your source voltage, and the forward voltage of the LED's you will be using.  To select the current-limiting resistor for the LED, take your source voltage, subtract the LED forward voltage from it. divide that value by the forward current of the LED (note, if the current is 30mA, you divide by .03, not 30).

For example, if your source voltage is 5V, your LED forward voltage is 1.2V and the forward current is 30mA:

5V - 1.2V = 3.8V

3.8V / 30mA = 126.667 Ohms (Select the nearest higher value available)

There are plenty of calculators available to do the math for you as well.

i was planning on building a 8ft by 2ft table to house the unit, i want to use an arduino uno for the microcontroller. i want to fill the table with an LED display. grouping the LEDs in 4s putting emitters around those groups with a single detector in the center of the group. and repeat this pattern until the table is filled with this array. the IR emitter/detector will be used as proximity sensors in this case. thanks for that example it helps alot

Have you done any math on how many sets of LED's you'll be using?  Each cluster will be pretty small depending on the spacing, you could be looking at hundreds of clusters.

Note that with the volume of LEDs in parallel and with most of the power / voltage being dissipated in the resistors which equates to 114mW per resistor, that will be allot of wasted power. You may want to consider grouping LEDs in series from a higher supply or providing a 1.5 or 2V supply for the LEDS and reduce to power

The table may get quite warm / hot (4 LEDS per 2" = 192 * 48 = 9,216 LEDs  = 9216*114mW = 1KW of dissipation from resistors and that's only 4 * 30mA led in a 2" square

btw, this adds up to over 250Amps if all LEDs are on at the same time (That's 6Amps on 240V supply or 11 on 110V)

More if you use RGB LEDs with 20mA per LED * 3 ie 500Amps and 2KW dissipated in the resistors, (that's 11Amps on 240V or 22 on 110V supply)

Something to contemplate as you build bigger and bigger arrays

May want to ensure not all the LEDS are ever on at the same time

I Haven't done the math just yet. I really just have a concept down. I wanted getan idea of what kind of things I'd need to get. As far as spacing goes I was going to use peg board (hard cork with holes in it) to place the leds and what not since they'd fit snugly.  With the peg board I'll be able to evenly space the leds in such a way that the clusters could be larger than just a group of four

OOkay that makes sense. I thought parrell wiring was a good idea since all leds would then get the same voltage. After reading the differences between parrell and series I saw that with series the voltage would drop down and it's possible all the leds wouldn't recieve enough power to glow and if one were to go out then the entire circuit would fail. But in parrelell wiring multiple bulbs could die and the circuit would still hold true. Which would you prefer?

Great point peter.  That's a lot of heat.

TreJon,

When you are picking your LED's, make sure to get a good look at the data sheet and check the forward voltage/current.  I'd recommend, like peter says, to use a power supply high enough to run all the LED's in a cluster in series (depeinding on the size of your clusters).  So if you stick with 4 LED clusters, figuring about 2V per LED (could be more or less depending on LED selection), 2V * 4 = 8V.  A 9V power supply could work nicely for that, but you'll need to select one capable of delivering the necessary current.  With that set-up you'd be looking at about 20-30mW per cluster in this example.  Note, this is just an example of the way Peter mentions optimizing the layout/power supply.  You can adjust the power supply selection and/or how many LED's you have in series to minimize your losses/heat generation.

I'm guessing you are using standard peg board with 1" spacing?  Assuming every hole has an LED that's 24 x 96 = 2,304 LED's, plus IR emitters & detectors.  That's ~263W of heat dissipation using Peter's math of 114mW per resistor (one per LED).  But if you put 4 LED's in series with one resistor like the example above, you cut that down by 4 times (~66W), just to illustrate the point Peter makes. That's also assuming all LED's on at the same time, which I agree with peter in avoiding.

Realistically, you need to do some of the circuit design before going too far.  It wouldn't hurt to narrow down your LED selection so you can design the circuit and do the math.  Then you can figure what resistors you'll need and what sort of demands you'll be making from the power supply.  Also, you might want to start looking at control.  In the above configuration, that's 2304 / 4 = 576 LED clusters (you can adjust the cluster size and change this).  If you use transistors/FETs to drive the LED's clusters, you'll need to make sure you have enough of them.  You'll also need the means to drive the transistors.  I don't think you're going to find a microcontroller with anywhere near that many I/O pins.  You'll need to use multiplexing, I/O expansion, LED Drivers or some combination of them to achieve that task.

TreJon House wrote:

 

OOkay that makes sense. I thought parrell wiring was a good idea since all leds would then get the same voltage. After reading the differences between parrell and series I saw that with series the voltage would drop down and it's possible all the leds wouldn't recieve enough power to glow and if one were to go out then the entire circuit would fail. But in parrelell wiring multiple bulbs could die and the circuit would still hold true. Which would you prefer?

The best solution is going to lie in some combination of series/parallel.  Lets say all LED's in a cluster are in series, and all clusters are in parallel.  If one LED blows, it will prevent that cluster from working.  All other parallel clusters should remain unaffected.