Guys, I may be new to Arduino, but not simple programming, mechanics, electronics, etc. Gaining an understanding of the micro controller, electronic circuits, and the mechanical devices that can be controlled with them , and all the programming possibilities, makes for a steep learning curve. This can be simplified by completely isolating the boards outputs from the devices they control and their respective power supplies. One less thing for the nubee to deal with would speed up the process. Opto isolators are probably the best bet overall. MIDI devices have proven this. There is no reason we cant easily control larger stepper motors, or any other high current devices, without blowing back the boards. Problem is that there are few resources for sizing and selection of these devices. The various output capabilities of the board will interact with different isolators in various ways. These board specific variations should be common knowledge among arduino folk if we want to utilize the versatility of this great device. Ive seen various ways to deal with this, but not an arduino specific knowledge base. Charts Would be great. Anything would help make this a more accessible process. the more folks we see using this board, the more we all get out of it. Wish I could do it all myself, but that is what forums are for right? Thanks
One of our members already produces that sort of thing:-
I had checked out his site prior to posting. Great product but it still cant handle the current needed to power the kind of steppers used in serious cnc machinery. Wouldn't be able to handle high impedance injectors either. We got the processing power and software to do this but maybe a break out board of some description would work. I just paid six hundred dollars for the controllers and power source for my plasma table. That's ridiculous! I think the arduino could handle it if we isolate it. Maybe I am missing something. Thanks for the reply though!! I rely constantly on folks who know more than I.
Great product but it still cant handle the current needed to power the kind of steppers
I thought you were talking about isolation? Motor drivers are an entirely separate issue (and one with many more variables, as you note)! If you're asking for a list/matrix of opto-isolated motor drivers that are compatible with arduino, that's yet a third question...
You're very right sir. What i am getting at is, regardless of application or driver design, when we are using our low voltage outputs to interact with more powerful systems, isolation techniques should be common knowledge. At the clock speeds that the controller is capable of, the opto isolator seems the best bet. being able to control a high current circuit with a low voltage signal without worrying about frying the board has got to be handy. Low power drivers could easily be "scaled up" ,if you will, to control larger motors and power sources. Or any other device. An oversimplified example would be to use the arduino to to turn on a headlight hooked to a car battery. The opto isolator serves as the switch that closes the loop. The board activates the isolator, light comes on, the board is never affected by the current through the light's circuit. Even if there's a short on the high current side, the board is safe. Now it is just a matter of knowing which one to use for the application.
Hey thanks for the reply. Hope I'm not sounding too much like a retard, but it seems like an easier fix than worrying about board,driver, system design. If everything has its own power source that leaves the controller alone, there would be less to worry about.
I dunno. You seem to be using "optoisolator" interchangeably with "solid state relay." The usual opto-isolator has a relatively low-power transistor on its output side, so connecting it to control arbitrary loads is relatively complex. Solid state relays have their own sets of issues (many are good only for AC, for example.)
Optoisolators are also slow, large, not bi-directional, not tri-state, and expensive by arduino standards, and frequently not relevant to the sorts of projects people use Arduino for. I spent a bit of time once looking for a simple circuit or chip to upgrade the current output of an Arduino (or other microcontroller) pin to 200mA or so, without losing the other features of the Arduino pins. I didn't find anything, and I wasn't even looking for isolation. Try to make things bulletproof and very general, and you rapidly arrive at your $600 sorts of solution 
There was this opto-isolated H-bridge posted recently:
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1265418204
All very good points. Thanks for the reply, I think ill chew on this one for a minute. The link to the board was very helpful. A good starting point if nothing else.
I'd like to go against the general trend in this discussion and say that opto-isolators are more of a pain then they are worth for low-voltage circuits (< 50V)
Firstly having two separate power supplies is a complication - perhaps you need two power switches/connectors whereas running the arduino from a voltage regulator (on board or off) from the motor supply is dead easy - and modern voltage regulators are cheap and reliable. It does then matter that protection diodes are installed, but that is always the case with driving motors from silicon.
Secondly you can have separate supplies quite easily and get everything to interface just by having a common ground - if your drive transistor is in the usual common-emitter or common-source configuration this works fine and the transistor acts to isolate the controller from the high voltages/currents nicely. Its not difficult to make sure the drive transistor/MOSFET is plenty beefy enough to be bomb-proof (100V 120A mosfets cost £1 or so))
Of course for driving high voltage circuitry safety comes first and an opto isolator is just the job. But for high-speed switching they will introduce significant switching delays which can lead to thermal problems.
I only use opto-isolators when it's important or desirable to not have the grounds connected between the Arduino and the external circuit being switched. If there is a need to switch a AC circuit then solid state relays (which include a optoisolator internally) is the best solution, however at a higher price.
If I have a need to switch a DC load that uses higher voltage (relative to the Arduino +5vdc) or high current (over 40ma) that can share a common ground connection with the Arduino, then I've found that logic level N-channel MOSFET transistors to be as simple and cheap a solution as anything else I've seen.
Lefty
Firstly having two separate power supplies is a complication
That is the whole point of an opto isolator.
Secondly you can have separate supplies quite easily and get everything to interface just by having a common ground
Again the whole point is isolation. Not just for safety but for isolation from things like noisy motors that would conduct the interference through the common ground.
A couple of optos to isolate Tx and Rx lines from the usb to serial is what I'm playing with now before I connect up my higher (24v) voltage steppers and other stuff. I just want to save my PC if anything goes wrong! I'm not that concerned about destroying my arduino mini.
Point taken about earth loops and noise though correct 0v wire/trace routing should mimimise the problem. I'll use the optos only if I need them. Maybe some one could do an optically isolated USB to Serial board. The readymade isolated USB cables are very expensive.
WOW! All the input is great. Ive been looking at the way a Megasquirt EFI System handles the injector drivers. The hardware is open source and it can handle the 12v auto power. I just think its too darn expensive for The processing power that you get. The common ground problem is something I hadn't considered. It is inescapable in an automotive app i guess. But I Still think that the isolators are plenty fast enough for stepper motor setups. I know they may seem expensive in Arduino standards But That's The Point. Expand the capabilities and more folks are empowered to be even greater makers!