16 LED- need help with circuit

[thenewguy]

I'm trying to build a 16 LED array (5mm white pdf link) and need help designing a simple circuit. I'm willing to attempt a few things but I am very much a newbie and still have issues figuring out schematics. I've scoured the internet and have found 16 LED arrays but most of them blink, scroll, etc and are way too complex for my abilities and needs.

My battery options must be alkaline for convenience and due to the slim form factor I would like to use 3AA, 4AA, or possibly a 9v. Once I get this figured out I plan to make a few of these.

Here's what I'm thinking I might be able to do…least to most desirable

Option 1: On/Off, 16 Led's in parallel –resistor on each led, based on info from this site http://led.linear1.org/led.wiz My concern is how efficient this circuit will be for life expectancy assuming the resistors will be burning off power. My current calculation have this lasting 2-3hrs with decent light always getting dimmer.

Option 2: Low/High/Off, I would prefer all 16 LED to dim to a low setting since this product will be used as a backlight and I would like an even distribution of light.
Option 2B could be powering on only 4 LED's, then 16 LEDs for the high if necessary for simplicity. I've played around with a breadboard wiring 12 LED's and 4 LED's with a 2-way switch. ON1 lit the 4 LED's ON2 lit all 16 due to a diode connecting the 12string to the 4string and preventing ON1 from lighting the 12. It seems simple and works in testing but might not be the best way of doing things. Again I'm not sure what kind of life I could expect running just 4 LED's.

Option 3: Dimming/Off, This seems like a big stretch for me. I've tried figuring out MCU's, IC's, and PWM but I'm completely lost. Most of the drivers I've been finding allow you to control each led which I believe is overkill for what I need but again I don't have the knowledge to be sure. A couple chips that looked promising are TLC5940, CAT9532, CAT9552 and LT3486. The prices seem to range from $2-$5 which is feasible but I don't know what other components will be need for the function I want. Figuring out the schematics on the datasheets were a bit perplexing to me too. I've also read about charlieplexing with a smaller chip but then my head exploded and I went to bed. Overall, programming anything is going to be over my head. Does a chip exist that does this function out of the box without programming? I.E chip + POT + 16LED's +a couple resistors here and there = me happy?

My current toolbox for this project consists of 18 or so AMC7135 rated for 350mA 2.7v-6v left over from a single high powered led project that works. Could I use that in lieu of the 16 resistors in option 1? About 120 of the LED's mentioned above, a bunch of IN914-Type and zener 5.1v Diodes, and various resistors and switches. I'm willing to buy some more parts to get this working right.

Your help is appreciated.

 

[HarryN]

Lets run some math on this setup first.

16 LEDs x 20 ma = approx 350ma, so in the range of many drivers and within the range of most alkaline battery setups.

Since you want to keep this as simple as possible, one possibily is to just use a variable resistance pot with a knob.

3 cells in series, all of the LEDs in parallel, and all of the power running through a single variable resistance pot. If you can find something in the 1 - 10 ohms range it should do what you are after, or be very close. Turn the knob, dimming + on/off. You will never get perfect light uniformity this way but it is not bad.

There are also some controllers designed specifically for cell phone use that will run multiple small LEDs like this, but that is outside of my expertise.

 

[thenewguy]

really? awesome, simplicity at its best. I would have thought I needed resistors on each if I ran 16 LEDs in parallel and 3 AA's in series.

easy enough to try. I ran the 16 LEDs in parallel with brand new (3) AA's without resistors, everything went fine and leds were very bright and pretty uniform to the naked eye. But i was told never to do that and that it should have burnt the leds out, what's the deal?

 

[usLEDsupply]

if you want' to run them using a resistor i would check out this http://ledcalc.com just put your info in and it will show you how to connect

if you are running off batteries and looking for longer run times at least while dimmed i would use a driver and dim via PWM

 

[HarryN]

really? awesome, simplicity at its best. I would have thought I needed resistors on each if I ran 16 LEDs in parallel and 3 AA's in series.

easy enough to try. I ran the 16 LEDs in parallel with brand new (3) AA's without resistors, everything went fine and leds were very bright and pretty uniform to the naked eye. But i was told never to do that and that it should have burnt the leds out, what's the deal?

There is "correct" and then there is good enough. What is the risk of a burned out LED in your setup if it goes to a direct closed circuit? Worst case, you are talking about discharging 3 alkalines through a 1 ohm resistance, which is in the range of not that terrible of an event. If it bothers you, put a 1 amp fuse in line with the main line to stop higher current than this for overall safety. A diode or very small resistance in line with each LED can also help force balancing.

If a 25 cent LED fails - is that a big risk?

Frankly, this balancing problem is one reason I don't design with arrays of small LEDs running near their max drive rating. A more robust approach is to use LEDs designed for say 100ma each, and drive them at 20ma. That way if there is a slight imbalance, it does not matter.

I assume this is for personal use under your won control, not a commercial product or something you are giving to kids to play with anyway - right?

 

[Rogue_monkey]

16 or 18 or even 24. the design and schematic are all the same. open the pdf on the page and it will show you how to wire it. Or better yet just buy the 18, or 24 led board if it fits your needs. its a really easy product to wire and use. look around on his site he has a few other products that might fit your bill.

wire in 3 leds in series. using the appropriate resistor. keep wires this sets in parallel to you get the desired amount of leds. there are lots of calculators on line to help you fig out the resistor youll need.
in general vf 3v, 20ma current to each led. use te batteries highest expected voltage and your golden.


disclaimer. this is not my website nor do i get anything from promoting it. I have used a lot of his products in my fix lighting solar system at home. He sells some great stuff so I promote it own my own.

Cheers