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Topic: Arduino 17x18 Controlled Single White LED Ceiling using 74hc595 (Read 1 time) previous topic - next topic

Magic_V

Dec 09, 2013, 05:01 pm Last Edit: Dec 09, 2013, 05:07 pm by Magic_V Reason: 1
Hello everyone,

My situation:
Me and some of my roommates are currently working on a lighting project in my college house. It regards a party ceiling that is already operational but is currently using AC powered christmas lighting. We wanted to make it DC with micro controlled LED's for a while already and this christmas holiday we will have the time to work on it. If necessary I can provide a picture of what the ceiling looks like, but for now basically imagine a 4 m2 white wooden square with a 17x18 LED's matrix with led's coming through holes drilled in the wood.
I'm planning to control the LED's from  the Arduino using 5 x 74hc595 8bit shift registers, giving me effectively the  ability to control 40 outputs using 4 outputs from the Arduino. The LED's themselves will be set up in a matrix using transistors at all the rows and all the columns of the matrix. The collector side of the transistors on the rows will have 12V (external 12V-12A power supply, converted PSU from a computer) and the emitter side of the transistors on the columns will be connected to ground. The gate connectors of all the transistors will be connected to the shift registers (with some resistor in-between there) and thus be controllable.

My problem:
The question I've been asking myself is what would be the best way to get these 12V over the LED's (and if 12V is actually a good voltage). I first thought of connecting the LED's in series per 3 in a row, and then hooking them up in 6 parallel circuit's along the row. But this would only work if I make only that parallel section switchable I guess.  I mean, let's say I would want 1 LED to light up; in that case there would be 12V over the LED, cause the other LED's are not connected that time to give me a sufficient voltage drop. Another option I guess is to put an appropriate resistor across every LED, but this seems kind of crazy for going from 12V to 3.5V...

Does anyone have an idea about how to fix the holes in this plan?  :)

fungus

The transistors on the rows they need to be PNP, on the columns NPN.

Assuming you're going to multiplex by rows (ie. power up a row, use column transistors to control the individual LEDs on that row) you need a resistor on every column between the led and the transistor for that column. This resistor needs to drop 8.5V at whatever current you want the LEDs to work at.
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

fungus

PS:

A ratio of 16:1 in your multiplexing won't be very bright.

12V isn't ideal, your resistors will be working hard and will get hot. Use 1W or 2W resistors.

If it's a computer PSU then doesn't it have a 5V output?

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

Magic_V

Ah thanks for the feedback!
I think the 5V out on the PSU would be a good option yeah! It would definitely make everything more balanced than when using the 12V.
About the brightness, would it really be not bright enough when using 0.02A LED's? Are there ways to improve the performance?
Thanks for the help!

fungus


About the brightness, would it really be not bright enough when using 0.02A LED's? Are there ways to improve the performance?
Thanks for the help!


That depends on lots of things - ambient light, what your requirements are...

If it's not bright enough you could:
a) Split it into two halves.
b) Overdrive the LEDs a bit - many LEDs can take more current if they're being multiplexed. If you have a datasheet it will tell you the limits.

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

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