LED Coffee Table

[Stellarcore]

Ok I'm missing something, 128 LEDs per board, each board has it's own microcontroller with USB. It would be futile to run at

In my original design, there was only one master board with microcontroller and usb populated, all of the others were slave boards with an i2c connection to the master. Also, I looked at that max6952, but it would be way more expensive. Each 6952 can control four 5x7 displays, so to get near1024 LEDs I would still need at least 7 of them. These chips from digikey go for a rediculous $16 each, and you have to buy them in multiples of 20... so I dont really want to consider it.

I think I'll consider the multiplexing thing a bit more.. I did a quick little test with an LED multiplexed with various duty cycles and I think it will work, I'll just have to significantly reduce the current limiting resistor so that the "average" current over an entire cycle is about 20mA. Do you think that running the LEDs at a higher current buy lower duty cycle would damage the LEDs?

 

[justDIY]

I think I'll consider the multiplexing thing a bit more.. I did a quick little test with an LED multiplexed with various duty cycles and I think it will work, I'll just have to significantly reduce the current limiting resistor so that the "average" current over an entire cycle is about 20mA. Do you think that running the LEDs at a higher current buy lower duty cycle would damage the LEDs?

nope, long as your average current is reasonable - most leds handle a generous pulsed current quite well

besides, you're bound to drop a few leds buildin this thing - just make sure it's easy to cut out the bad one and solder in a new one.

 

[Stellarcore]

Ok, well if I am going to scrap the max7313s, I'd like to come up with a better way to handle the IR sensors. Right now I've been just treating the output of the phototransistor as a digital input, and tailoring the pull up resistor to get the sensitivity that I want. But this doesnt work very well as I'm throwing away alot of info and its very picky about different types of glass, etc...

I'd like to find a way to cheaply multiplex the analog sensors to available A/D pins on the PIC. Currently I was planning on 128 IR sensors but I am considering increasing this to 256.

What if I were to multiplex the IR leds the same way, and then feed all of the IR sensors back to a op-amp summer and then I will just assume that whatever signal I get from the opamp is coming from the only device with an active IR led... Wont be perfect, but it should work well enough. What do you think? Can you suggest an adequate (and cheap) opamp?

 

[blueroomelectronics]

You can pump a fair amount of current into an LED when it's muxed. Nice and bright at 50ma at 1/8 duty won't do them any harm. A cheap driver would be simply transistors.
A PIC with SPI would be very fast and you could put several together. SPI can even be daisy chained instead of addressed.

 

[justDIY]

What if I were to multiplex the IR leds the same way, and then feed all of the IR sensors back to a op-amp summer and then I will just assume that whatever signal I get from the opamp is coming from the only device with an active IR led... Wont be perfect, but it should work well enough. What do you think? Can you suggest an adequate (and cheap) opamp?

if you want to stick with reading the IR sensors as digital, you could use the opposite of the 595, a parallel load serial read shift register. this turns 8 digital inputs into a bitstream, and can be daisy chained in the same manner as the '595. I forget what the part number is, but I think it is in the 74HC1xx series.

if analog is your thing, you can get an 16x1 analog mux pretty cheap, the CD4067 (or 74HC4067). 16 of those would give you 256 inputs. the down side is controlling all those mux would be a mess, you'd need a 4 bit address bus plus a 4 bit chip select run to something like a 4514 4 to 16 decoder. bus the address line to all the of the muxes, and then each enable line to one of the 16 outputs on the on the decoder. Then I guess the 16 mux outputs could bus onto a single pic input - don't know what the parasitic capacitance would do to you there, or grab a pic with 16 AD inputs, which is just another mux inside the chip. Reading this, it sounds complicated, but I think it'd be pretty straight forward once you drew it out in Eagle.

 

[ClydeCrashKop]

I just remembered this project and thought it was cool.