I've made a bit of progress on my LED table project, I thought I would share.

I've purchased some IR diodes and phototransistors from Digikey that I have been playing with. I just received them yesterday, so I'm still trying to work out the best configuration to use with the photo-transistor. The best configuration I've found so far is to connect the emitter to ground and then use a 10k pull-up resistor on the collector. In this configuration, the output voltage swings nicely between ~2volts and ~4 volts. To calibrate the sensors, I place one of the IR LEDs right next to the phototransistor and ever so slightly angle them together such that the photo transistor is just on the threshold where it can be detected by the PIC. Once its setup like this, if I put my finger or any object within about 2 inches above the sensor, enough IR light is reflected to throw it under the voltage threshold (~3V).

As for the IR leds, they seem to work well enough, but they are a bit more power hungry than I had hoped, they are rated at 50mA each. I've been powering one through a 100 ohm 1/4 watt metal film resistor which pulls ~32mA. It seems to be getting quite warm, which has me concerned a bit. I may have to figure something out for this, because the finished table will have 128 of these IR LEDs, each LED/Resistor pair consuming approx 160mW means that I'll be consuming ~20W continuously just from the IR diodes. One possible solution to this could be to supply several voltage sources to the LEDs and switch them off so that only a few of the LEDs are operating during idle time, switching the rest of them on when the first sensor is triggered).

I've also been working on my eagle schematic and board layout while waiting for my parts. I've decided to elminate the mosfets for each individual LED because I think the MCP23016 should be able to handle the current. I plan on using molex power connectors to string power to each of the PCBs and IDE cables to string the interrupt and i2c lines. The PCB still needs a bit more work, I would like to make the power lines a bit thicker (especially the lines that drive the LEDs). Please have a look at the following links and let me know what you guys think:

http://www.stellarcore.com/schematic.png
http://www.stellarcore.com/pcb.png

 

It just occurred to me that this is very similar to the MIT Disco Dance Floor project:

http://web.mit.edu/storborg/ddf/

You might have a look at their design to see if it gives you any ideas.

 

The disco dance floor is simply a display I think. It would be really cool if there was little touch screens on each square that way you could light up where people were standing and whatnot.

 

There are touch sensors on the MIT dance floor. They came up with a very simple mechanism, but I can't find the link at the moment. I think the 5th photo on this page shows the sensor (thing running off to the right):

http://web.mit.edu/storborg/ddf/photos.html

(Edit: I did find out how they did it: Saran wrap and single strand wire.)

 

Robot builder 101

Wow, have fun with it! What is your power source?

If they are getting warm, they will turn black due to too much overheating. Just use 1/2 watt and you should be fine. If not, use a couple 100ohm 1/4 watt in series or parallel. Forgot what formation will shed heat better.

 

Stellacore - another thing that has come to mind is that I think you'll want to use constant current led drivers instead of fixed resistors. If you ever want (or need) to change the current drive to the leds, a c.c. led driver is the only way to go with this many leds.

You can still vary brightness with PWM - many c.c. led drivers have the ability to vary both current and duty cycle.

 

I'm looking into that dance floor setup. It seems quite simmilar to my coffee table, and I think it wouldn't be too difficult to extend my table design to work on a dance floor as well (the only exception is that my design isnt RGB).

One thing that I noticed about their design is that they are using an I/O expander from Maxim that is designed specifically to drive LEDs. It even has an PWM to vary intensity. I've sampled a few of these and will see how they perform in comparisson to my MCP23016s.

As for my power source, I think I'll probably end up with a computer power supply. Maybe a small compact PSU that fits in those small form factor PCs.

Right now I'm in the process of shopping for LEDs. I've borrowed a bunch of 13000 mcd white LEDs from a friend for testing purposes so I should be able to get a very rough breadboard prototype going in the next few days. I'm looking around on ebay. There is one supplier who I've bought from in the past and I'm hoping he may give me a deal if I buy more than 500 (as well as a bunch of other basic components I'm in need of).

While perusing through the disco dance floor website I had a bit of a cool idea. I've been trying to think of some interesting patterns that I could run on my table that would show off the digital capacity of the table (the original LED coffee table I got the idea from is analog), and I think I found a neat application: Conways game of Life. I'd like to have a mode where the table plays the game of life, and whenever an IR sensor goes from low to high, (or high to low, doesnt really matter), it will insert a glider in the location of the sensor. Could be really neat, but might be tricky to implement.

Other neat ideas could be a simple table sized IR pong game, a signboard, or just "random" twinkling. Let me know if you guys have any neat ideas for different table "modes".

 

I would try and put the power source inside the table, to minmize space

 

I've built a somewhat simplified prototype of one of my boards. It uses 5 of the MCP23016 expanders to monitor 16 of my IR sensors and control 64 white LEDs. All of the expanders are on a single I2C bus, but soon I'll be testing the control of multiple busses with one PIC (shouldnt cause too much of a problem I think, I just have to write a wrapper for my i2c methods that switches between buses).

Right now, I'm using a glass baking dish to simulate the glass that I'll be using for the tables surface. I seem to have found a happy medium of sensitivity with the IR detectors; they are not set off by the ambient light in the room, only by items placed on the surface. However, some items are more responsive than others. For instance, a black roll of electrical tape absorbs most of the IR light so it doesnt trigger the detectors, but anything white or shiny does. The human hand seems to trigger the sensors as well, but not as well as I'd like.

Also, good news on the power consumption issue. The LEDs that I've got seem to be relatively bright regardless of the amount of current. If I put the full 25-30mA through them, they are visibly brighter, but not by much. Right now I'm actually running them at around 5mA and they are surprisingly bright. This means that each slave board draws a max of about 1.5 amps (the IR LEDs on the other hand need approx 30mA each). This means that my entire table should draw approximately 12 amps at 5 volts giving approximately 60 Watts when fully illuminated. The same as an average lamp.

I've taken a really crappy video of my prototype which can be seen here: http://www.stellarcore.com/prototype.mpg (~8mb). I used the video mode on my 3 megapixel digital camera to film this.

My next step will be to add some mosfets so that I can PWM the brightness of the LEDs and also to experiment with multiple busses. I've also been starting to play around with using the 64 LEDs as a scrolling message board. I will also test out my ability to control the I/O expanders on multiple i2c buses. Another issue that I need to fix is to add in some debouncing so that a sensor has to be triggered for a millisecond or two before being considered active/deactive (you'll notice some flickering in my video). Finally, I need to refine my software a bit so that the sensors are read primarily by interrupts, but a global poll as well every once in a while. Once I'm satisfied with the way my prototype is working, I'll go ahead and order my PCBs. I may also need to come up with some kind of PCB or free-form prototype for the IR sensors which consist of the LED, the IR transistor and two resistors (the LED and transistor have to be carefully aimed/positioned so that they work correctly.)

 

hmmmm the vid doesn't seem to work for me?

__________________
The 3 laws of science....
1. If it smells bad, It's chemistry.
2. If it's mushy, It's biology.
3. If it doesn't work, It's physics.
www.laserpointerforums.com A forum for disscusion of lasers and laser shows!
www.laserchat.org IRC chat for laser and electronic related stuff.

 

Hiya Stellacore,
Eh mate after reading this thread I reckon thats one ambitious project your on and yet in the last paragragh your talking about ordering PCB's ?. Eh mate go grab a laminator some press'n'peel and some copper clad board and make your own boards. The reason I mention the laminator press'n'peel can be a pain to iron on but by using a laminator it's breeze even for 2 sided boards. All you need to do is print your pcb file on a laser printer fix it to the board and your away.

By the way I mentioned this to the war dept and the response I got was if you want to make something like this why do you need a farm.

Cheers Bryan

__________________
" The only way to avoid human error is to avoid the use of humans"

 

There are two reasons for that:

1. The eye has a logarithmic response to light intensity so you need to increase the intensity by a factor of four (I'm not certain of the actural figures this is just an educated guess) for the perceived brightness to double.
2. White LEDs are more efficient at lower die temperatures which means that you don't gain much by pushing them to their absolute maximum ratings unless the ambient temperature is very low.

__________________
I also post at the following sites:
http://www.stop-microsoft.org http://www.heated-debates.com
Screen name: Aloone_Jonez
And http://www.silicontronics.com, same screen name as here.

 

Hrm... I'll see if I can get it into youtube sometime today.

I've been thinking about making my own PCBs for some projects recently, but I dont think this is one of them that I could do well. There are a lot of traces, and most of them are 8mil width. I made a few PCBs in high school and it was very difficult to do anything but really thick traces. I would be partial to try photo lithography for making the boards, but its quite expensive to buy the photosensitive copper.

Besides, I've been looking around and I've found a couple chinese pcb houses that do boards for pretty cheap, it would end up costing me like $10 per board and I'd end up with the full two sided boards with 8mil traces, professionally drilled, solder masked, and silk screened.


Not sure I follow.. war dept? farm?

 

I've uploaded the video to youtube:
http://www.youtube.com/watch?v=hF396jTNMkg

 

A Touch-Screen that gets so many finger prints on it that you can't see anything.

__________________
Uncle $crooge