Best way to wire a bushel of LEDs?

[Peter_wadley]

Hey,

Im making a little LED based project.. basically I have a bunch LEDs in the form of letters.. each letter contains 8-10 LEDs..


I plan to use a PIC to control the letters..

I have over 100 LEDs in total.. Im just wondering what the best way to wire them would be.. each letter will have common anode

I really dont want my PCB to contain 100+ resistors.. but I also dont want to loss brightness.

Any ideas?

So that will be 10 LEDs to one pic output (Transistor used for power amplification)

Thanks

 

[Ambient]

What is the main source voltage? 12V? I would avoid running the LED's off of a voltage regulator, as you will end up with a lot of wasted power. Running the LED's in series is best, as you can use 1 resister per series, and you will need significantly less current.

We need to know:
LED voltage rating/current rating
available voltage sources

 

[Gayan Soyza]

If it red LEDs I wire them in two strings parallel each containing 5 LEDs per string (series) for a 12V supply.
A string will driven by a ULN or UDN line drivers.

If I have more letters I’m extending my columns using Johnson counters.

 

[justDIY]

10 leds per letter ... if they're red yellow or low voltage green, find a 24v power supply and wire all 10 in series with one resistor, and maybe a rectifier diode just to provide some reverse voltage protection. if they're high power green/blue/white, you'll need 36volts ... luckily both can be found pretty cheap on ebay or surplus websites.

the advantage to straight series connections is each led receives the same current, and will shine the same brightness.

 

[Peter_wadley]

OK,

Some more info for you guys

They are standard red LEDs 1.7v - 2.0v drop?

Current.. dependent on resistor value used..

The problem I have is some letters contain 4 leds while some can have up to 11!

I am going to grab a 24 volt supply..

2.0v x 11 LEDs = 22v - Good

2.0v x 4 = 8v .. leaving 16v for the resistor.. will this cause over heating?

How will I make sure each letter, given that they each have different number of LEDs, will be of the same brightness?

Just use V = IR?.. or I = V/R, Rather..?

Thanks for the help

 

[justDIY]

OK,
How will I make sure each letter, given that they each have different number of LEDs, will be of the same brightness?

if each letter has different number of leds, they're going to be different brightness, there's no way to solve that short of adding more leds to the letters that don't have enough leds.

the rest of your message is correct ... the excess voltage in your 4 led scenario will get burned off by the resistor ... you can help spread that heat out a little using multiple resistors in a series parallel configuration, but in the end, you get the same amount of heat.

you could look at using LDO regulators for the small letters, or a switch mode buck converter, improving the efficiency a lot over a plain resistor.

 

[Peter_wadley]

OK

I have the power supply.. its is 24v DC rated for 1.67A

So.. The wiring...

There are 10 Letters.. so I will use a 10 PIC outputs..

Should Each output turn on an NPN transistor? Then... each of these transistor will source a letter... by attaching first to a resistor... then each led one by one...

Now I was thinking... to have the same brightness (well, close enough) could I just say.. add 7 DIODES in series on the PCB for the letter containing 4 LEDs.. just make each letter have 11 diode drops.. erm Oh wait diodes only drop .7 volts right?

This is something of concern.. I have 1/4 watt resistors... will I need .. power resistors for this setup? 11 leds.. with 24v source..

Calculations:

Each LEDs drops 1.8v and requires 25ma..

so the resistor will be subject to ... 24v - (1.8v X 11) = 4.2V

in wattage... 4.2V X 25ma = .1 Watt ..

Therefore I can use 1/4 watt resistors?

thanks for your help again

 

[weegee]

I do many projects with leds and my favorite weapon of choice just now is a couple of Max7219 chips, which can be found really cheap, the chip will use just 3 lines on the pic, and each 1 can handle 64 leds, and they are cascadable without adding any additional pic data lines. the benefit is that you only need 1 resistor for each group of 64 leds, and they will all be the same brightness. by using a small pot for the resistor on each max7219 means that you only have 1 adjustment to make to match the brightness of each batch of 64 leds, and it doesnt matter if you have 1 or 64 leds connected, they will all be the same brightness.

the added benefit comes from the ability to change the brightness in code (16 steps), and can individually address each led if neccessary. (allows flashing, running light patterns ect)

these chips are very easy to wire with just a connection to the pic, and a couple of decoupling capacitors, and 1 resistor to set led brightness.

code wise, its very simple too, especially as there is a built in test mode, and by entering testmode all leds are turned on at full brightness, so basically you are send just a single 16bit number to the chip, and on comes the display.

I probably made this sound much more complex than it really is, but believe me its really really simple. it runs in 5v.

 

[justDIY]

Peter if you can wire your leds as a matrix, then that changes everything.

I was basing my recommendations that you have discrete modules, each one drawing a letter out of leds?

Your individual leds will all be the same brightness, but I mean, a letter made with only 4 leds is not going to be as bright as one made with 11

putting a bunch of rectifier or switching diodes in series would eat up some of the voltage, but it would be cumbersome. just grab some 5-10watt sand resistors cheap from a surplus store and let 'em bake ... it won't hurt anything.

 

[Peter_wadley]

hmmm

Could I not just wire each letter in series.. then using ohms law ensure that each letter is getting the same amount of current?

For the letter containing four LEDs:

Let each LED use 2 volts.

Resistor Voltage:

24V - (4 LEDS X 2V) = 16v

Find needed resistance for current of 25ma:

R = V / I

R = 16V / .025A

R = 640 ohm

Wattage of 16v X .025A = .4W ... therefore half watt resistor would be needed.

For letter containing 11 LEDs:

Resistor Voltage:

24V - (11 LEDS X 2V) = 2v

Find needed resistance for current of 25ma:

R = V / I

R = 2V / .025A

R = 80ohm

Wattage of 2V X .025A = .05 therefore 1/4 watt resistor can be used.

Does this sound logical?

Thanks

 

[Hero999]

Be warned that the LEDs' voltage drop will vary considerably, you might have 10 LEDs with a voltage drop of 1.8V or 10 with a voltage drop of 2.2V. What's normally listed on the datasheet is either the typical or maximum voltage drop.

This means that if you use the maximum voltage drop in your calculation more current will flow than you expect which could destroy the LEDs. It's best to use the maximum voltage drop when you're estimating how many LEDs you can power from the miniumum supply voltage and and the minimum voltage drop when the supply voltage is at its highest. This will give you a good estimate of the maximum and minimum current draw, battery life, brightness and resistor power dissipation.

 

[justDIY]

using a constant current driver eliminates many of Hero's concerns - as long as you have more than enough forward voltage, the driver takes care of the rest.

 

[Peter_wadley]

I am unsure what is meant by 'constant current driver'

Could you please elaborate

Thanks

Also, The max current for standard RED LEDs is usually around 25ma, Correct?

 

[Hero999]

He means a current regulator that keeps the current the same regardless of the foward voltage drop across the LEDs.

 

[atferrari]

The 7219 is a good chip!

 

[justDIY]

Peter, if you can wire your leds as a big matrix then you can spell out the letters you need in the matrix, as well as change them down the road. you can then use the maxim matrix driver chip.

but if you're wanting to wire up discrete letter modules, the matrix driver chip will get expensive, you'll need one for each letter, unless you want wires running between the two.

 

[Hero999]

The 7219 is a good chip!

I've never heard of it and I've done a quick Google and couldn't fine it.

Could you please post a datasheet or a link to a place where you can buy them?

 

[futz]

I've never heard of it and I've done a quick Google and couldn't fine it.

Could you please post a datasheet or a link to a place where you can buy them?

**broken link removed** makes them (and sells them).
Digikey sells them. For sure lots of other suppliers sell them too.

 

[Hero999]

I see, you mean the MAX7219, I wish people would privide propper part numbers, 7219 doesn't mean anything to me.

 

[weegee]

...and my favorite weapon of choice just now is a couple of Max7219 chips, which can be found really cheap, the chip...

i included the full chip name in my earlier post.

as a side note, (and not one i'd reccomend), but if you use these chips and leave the data in, and load lines, and clock floating, they tend to drift into test mode, which is ALL leds on full brightness.