Led array q

[beans18]

Hello, I own a small coffee shop. I recently bought out the store next door to expand my sitting area. As a result I will have a lot more wall space to decorate .
I have this old wooden frame that is divided into 3 squares wide x 6 long. My plan is to use white LEDs and diffuse it with different color tissue paper and put glass over the frame, and run a few sequences with the Leds. (I know it sounds hokey, but for some reason my customers love things that are bright and blinky. maybe it's all the caffeine?) Each individual square is 6"sq, so I plan on using 9 LEDs in a 3x3 array (total of 162 leds). Each set of 9 LEDs will be treated as one LED and hooked up to a 74HC595, and ULN2803 and programmed with a pictcontroller with a 9v DC wallwart. From my calculations peak V will be 3.24V, so I am not worried about that. I am unsure of what resistor value to use. Since the 74HC595 does multiplexing, only one square will be on at a time. If I calculate resistance values based on that - 2.8 forward voltage, 9v dc, 20ma, 9 leds into a LED calculator, it tells me to use three 33ohm resistors in a 3x3 array. Is that right? Or should I be using a resistor for each individual LED?

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[audioguru]

The voltage of each LED is different. When they are connected in parallel then the one with the lowest voltage will get all the current and burn out, then the one with the next lowest voltage will burn out.
But you might be lucky to find LEDs that were all made at the same time. It is your gamble.

I would connect 3 LEDs in series and in series with a current-limiting resistor and use a 12V supply. 3 of these strings in each square.

The wall-wart won't be 9V unless it is regulated. It might be 12V or even 18V.
The ULN2803 has an output loss of about 1V.

 

[beans18]

hmm alright thanks for the advice. so what you are saying is that i should do this times 3?

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If i were to do that, could I connect the 3 strings per square to the same output pin on the 74HC595?

Or I could do a 1x9 array in parallel with a resistor on each led. 470ohm 1/2w no?

 

[ericgibbs]

hmm alright thanks for the advice. so what you are saying is that i should do this times 3?

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If i were to do that, could I connect the 3 strings per square to the same output pin on the 74HC595?

Or I could do a 1x9 array in parallel with a resistor on each led. 470ohm 1/2w no?

hi,
The diagram looks OK.
The 74HC595 has a maximum working voltage of +6V and also it cannot sink all the current from 3 LED strings.
You need to connect the ULN 2803 between the outputs of HC595 and the ULN2803.

The ULN can accept the +12V and the current from 3 LED strings.

You could go the parallel route but its VERY inefficient.

Does this help?

 

[beans18]

hi,
The diagram looks OK.
The 74HC595 has a maximum working voltage of +6V and also it cannot sink all the current from 3 LED strings.
You need to connect the ULN 2803 between the outputs of HC595 and the ULN2803.

The ULN can accept the +12V and the current from 3 LED strings.

You could go the parallel route but its VERY inefficient.

Does this help?

are you saying I need to use 2 ULN2803s?

And I could not connect the 3 LED strings to one output pin, because the voltage would be too high.

 

[ericgibbs]

are you saying I need to use 2 ULN2803s?

hi,

There are 8 output pins on the 74HC595 and there are 8 drivers in the ULN.

So you could connect 8 strings of the series connected LED's, one to each ULN output.

The ULN is capable of driving a number of strings on each output pin. each pin can sink upto 500mA.
So if an array draws 20mA, you could have 500/20= 25 arrays on one pin, but it would be a boring light show.

Is this what you mean?

 

[beans18]

hi,
You need to connect the ULN 2803 between the outputs of HC595 and the ULN2803.

i think its a typo?

 

[ericgibbs]

i think its a typo?

hi,


Does the last post make sense?

 

[beans18]

yes. let me draw a schematic to make sure i fully understand. Thanks for all your help

 

[beans18]

I threw together a rough schematic. I plan on putting each 9 LED section on its own pcb, then using terminal blocks or some other type of connector to connect it to the main board. I've also been looking at the A6821 from Allegro Micro but see that it can only take in 5v. I'm still deciding between AVR or PIC to control it.

I'm sure I'm missing a few capacitors and resistors somewhere. I haven't done electronics in about 5 years so I'm still a bit rusty, but slowly its coming back to me.

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[philba]

That looks fairly good. Some suggestions:

- put a 1K resistor in series with the inputs to your '595s and put a 100K resistor to ground on each of those inputs. This will protect the '595s. I've blown more than a couple of those chips but, once I added the resistors, I've not blown once since.

- you are showing Vcc powering both the 595s and the uln2803. I think you want to use 12V for the 2803s and 5V for the 595s.

- You will need to factor in the voltage drop of the 2803. check the datasheet for Vsat or something similarly named. subtract that from the 12V for your LED resistor calcs.

- you will definitely want bypass caps for the chips.

- why are you bothering with 3 pin connectors? I'd just use 2 pins - one for the control line and one for ground.

 

[beans18]

ok. updated the schematic with your suggestions. i figure about .1uf for the bypass caps should be alright? for the 2803, the Vsat is 1, so I reconfigured the LED resistors off of that. 150ohms instead of 220.

any more suggestions?

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[ericgibbs]

ok. updated the schematic with your suggestions. i figure about .1uf for the bypass caps should be alright? for the 2803, the Vsat is 1, so I reconfigured the LED resistors off of that. 150ohms instead of 220.

any more suggestions?

hi,
Looking at your original box layout it could be made more 'interesting'

You have a box with square compartments, have you considered some circular type compartments? or other shapes?

A short lengths of plastic pipe, about 3inch dia, cut to the depth of the box, each with an LED array and a different transparent face colour.

Hope this is clear, if not I'll do a sketch

 

[ericgibbs]

hi,
Looked thru drawing.

The ULN devices 'sink' current not 'source' current, so I have edited drawing.

The LED arrays are connected to +12V via a series resistor and the other end of the array goes to the ULN output pins.

Its not required that the ULN pin10 be connected to +12V for this application..
[thats for back emf control with relays, etc]

The HC devices requires +5V supply to pins 16, add some decoupling capacitors.

 

[beans18]

alright, thanks for all your help. I really appreciate it. will be breadboarding it next weekend. thanks