dc42 - Thank you!  I will look into that chip you suggested.  I have been messing about with a mega board I have, trying resistor networks, and such, but I was looking for something clean.  I wasn't sure what the pin protection diode's ratings were for, or if I could get away with a current that small.   I'm learning quite a lot, and I'd like to learn more about how the clamping diode functions.  Any links/sites you can recommend that will explain the function and schematic layout of such a feature?

MarkT - I have been trying dividers, but was scared of burning out my mega's chip with too much current, and then I also had concerns since the signal back from the device will be over a fair range (20-27 volts) so I didn't want to be outside the safe input voltage ranges in the arduino at any time.

Thanks guys!  Any other suggestions, feel free to keep them coming!

I also just came up with problem number 2...  the TI CD4051 multi chip i am mucking about with doesn't appear to like 24V signals...  At least not what I can understand from the datasheets...
Anyone know of another good option?  I don't mind running the chip's Vdd from the seperate 12v source, but it has to be able to switch signals up to 27volts, so lets say 30 to be safe.

I am building a circuit that is basically testing for continuity across a very low ohm circuit.  I'm using a multiplex chip to sample multiple lines for a split second and then rotate to the next one, it will update a remote arduino that will then display the status via LEDs lighting or not.  I can not draw more than 10mA from the circuit without consequences.  I'd be happiest with 5mA.

the 24VDC source will be batteries, and I will run a 7812 (or equivelant) to power the mega as well as a couple other small devices in the enclosure.  Total resistance in the device circuit (not counting anything added for this continuity test) will be in the 2-12ohm range.

The "continuity circuit" is operating on 20-26v (batteries under varying condition), but the arduino (5v MEGA) needs a 3-5V input.    My first thought was to use a 5V regulator chip to punch it down to 5v exactly, but I don't think I can get one that will safely stay within my 10mA limit total draw can I?   Next I considered an optoisolator, but i'm not sure if that works either - the ones I looked at (4n25 i think?) looked like the draw was higher, but I'm not 100% sure I understand exactly how they work.

I'd also strongly like to minimize the chances of a failing chip causing the draw to go over the 10mA mark, so things that reduce the risk of shorting to gnd make me happiest.


Sorry about the really bad mspaint jpeg, but thought it might help illustrate what i'm trying to do.
Thanks in advance for the help!

Apologies - I did read the first half of that post and thought the rest was getting too far down the process to apply to me.

Bad newb.   

the error I don't think I can copy & paste - its friggin huge.
But as always, as soon as I posted a question I think I figured part of it out.  I was just frustrated after 30 minutes of messing about that I hadn't gotten anywhere.  I moved where i was putting the code outside of the setup/loop functions and it started working better for me.

I did restart the IDE after installing the library.
I'm not 100% sure what the errors where, I started a fresh sketch to attempt to understand how to use the library and now I'm making a bit of progress.

I'll be happy to plug away at it a bit more now and I'll post back when I hit the next wall.  Thanks for the super fast reply!

I'm trying to make a lighting controller that will fade lights in and out to simulate daytime.
I googled and came up with the TimeLord library, which seems popular for people doing this task.

Problem is, I can not figure out how to get the library to work in my program. 
I have tried using the <include> statement, as well as copy & paste the code from the .cpp file into my sketch.

I don't get any warnings when i try to compile, but the compiler spits back errors saying there is multiple declarations of various things from the library's code.

I'm pretty new to all this and tried googling to get answers, but I can't figure it out. 
Can someone tell me what i'm doing wrong? or maybe give me a snippet of code to pull something like a sunrise time?

Thanks in advance for the help.

If it matters, I'm using an ArduinoMega2650 and the 1.0.1 IDE

Got it working - thanks.

Looking at it, I'd like to make some changes like add a 5V regulator (ie 7805) onto the board.  I'm thinking a passive mod, just add the 3 pad holes so I can solder it in on the first board, and then jumper 3 or 4 boards together to control all the channels.. Just gotta figure out how to use Eagle now...

I was poking around onSemi and figured out they have sample units for shipping, so I'm going to order a  10pc sample set of the CAT4101 chip (~$15) so I can try them out. 
I like the idea of a constant current driver being in charge instead of just a transistor.  Overall, I think i'm going to have to put out about the same money either way, so might as well do it right.

Thanks for the help so far guys - I'll post up details/updates/etc as i work through it all in case it helps someone else in the future.

Poking around I noticed that there's already an Eagle schematic for this:
http://code.google.com/p/hpled/downloads/detail?name=CAT4101_triple1_0.zip&can=2&q=
For $10 at iTead or Seeedstudio you could have 10 of these boards made up. There's only 12 components per board and you could even convert those resistors and capacitors to through-hole if you wanted to.

I can't open that file - i installed a copy of Eagle and it says its not a valid file?  I also registered with onsemi to look at ordering the chips, but they're out of stock...  boo...
Anyway - am I doing something wrong with Eagle?  will the freeware version work for it?

Yes, I realize this now - Its been a while since i did electronics calculations.  Touched on it a bit in my Electrician training, but not really any designing stuff.

Thanks johnwasser.  I knew when I was looking at the FETs and High Current BJTs that I was missing a key part of what i needed to be interpreting in the charts.

Fantastic info Chagrin, thank you.  The saturation stuff is what i needed to know about...

I will look into it some more and make my decision.

Thanks!

@johnwasser - Do you have any suggestions on a part?  I'd be dealing with either 24V or 36V power supply and a max current of only a few amps per Group.  another friend of mine suggested a MOSFET as well but I didn't get any more details than that.  How much of a difference are we talking about with the internal Vdrop across a FET vs Transistor?  Sorry, I'm really quite new to FETs and using them - any links or part suggestions would be greatly appreciated.

@chagrin - the CAT4101 chips seem like they're kind of pricey compared to a simple FET or Transistor - Aside from the human error/calculation issues you mentioned is there another reason to go that route?  I found them on digikey at about $3.36/ea, so seems like a pricey route to go since i'd have to get boards made and such.  Now pricey is relative, since we're still only talking small money.  But is there a solid reason to go one way or the other?  ie, better dimming in low levels or more stable voltages at the LEDs?

I'm not terribly optimistic about my SMD soldering skills, but I'm not totally against trying - I enjoy conquering new skills.  I just want to make an informed decision about which route to go.

I'm going to dig through those board sketches and such later tonight, for now I gotta go help a friend with a broken car...

Thanks for the input so far!

Thanks for the reply - the question i have relates more to dimming using PWM signals as opposed to how to properly power them.

I've done a pile of calculations for a pile of configurations (using ledcalculator.com of course) and have a good idea of how i'd like to connect them, but the method of dimming becomes the issue that may determine how many strings of how many LEDs I end up using.

Any input on the best/simplest/cost effective way for me to achieve the dimming functions?

Ok Disclaimer - Total noob to micro controllers, but i have some basic understanding of electronics theory so I'm not attacking something that is way over my head.  And I have some basic programming knowledge from a while back, plus I've been looking for a project to get me back into it for a while now.

I am aiming to control 60-90 LEDs broken into 3 groups.

Group 1: 20-30x 1W (~3.2v @ 350mA) Cool White LEDs
Group 2: 20-30x 1W (~3.2v @ 350mA) Warm White LEDs
Group 3: 20-30x 3W (~3.6v @ 700mA) Blue LEDs

I have an Arduino Dumilanove that I borrowed from a friend to prototype with, and I've ordered 20 of each LED that I intend to use so I can do some testing.


My goal is to have dimming control from 0-100% for each group using PWM from the Arduino.
I have been looking at constant current drivers on ebay that support PWM (ie http://www.ebay.ca/itm/350539542188?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649) and using a few of them in parallel, perhaps on a 36V power supply.  But reading on here many people are suggesting using a CAT4101 chip for constant current driven PWM LED dimming?  I looked at a spec sheet and it looks like it'll work for me since it peaks at 1A, I'd have to break up the groups into a few strings in each since its < 25V (same as the drivers i've found online @ < 36V ).  Also, it appears to be a SMD chip, and I'm looking to use through-hole components since i don't really have the means to do SMD just yet.

The other option that I looked at, was using a large transistor to do the dimming - i used a 2n222a and a couple small LEDs to do some testing on a breadboard and it seems to work - any reason it wouldn't scale to a high power led if i was to use a larger transistor (ie http://www.digikey.com - Part 497-7169-5-ND - I can't figure out how to link directly to it, sorry.)??  I've done some current calculations and resistance calcs so I realize i'd have to break the groups up into strings and use some sort of 2W resistor or something to current limit, depending on my source voltage and string size... etc etc.

This is an aquarium lighting project, and i've searched and googled and i think i just need a push in one direction or the other as to which way to go - what do you all think?  I'm going into mental melt down mode with the searching, i've got 3 windows open on my 24" lcd and i'm still scrolling left/right in the tabs lol.

The end goal is to use a RTC unit to control a sunrise/sunset/lunar kind of automated light cycle, and tie it all in with a temperature and PH monitor for the tank, all interfaced with a LCD and possibly keypad for programming/setting time/etc.  maybe even get onto an Ethernet shield one day... but now I'm getting ahead of myself.


Oh Follow up Q - Can you set the PWM frequency on the arduino outputs?  I saw someone talking about you could set it at 3khz or 16khz or something...  or did i misunderstand something?