First PCB design - Schematic Check

Hi all,

I’ve been working on this project for some time now, and I’ve decided the best way to wrap this up is to have a PCB built to “package” this into a usable item.

Background:
This is for an arduino uno r3 controlled water filter for my saltwater aquarium. Basically, I have 4- 12v solenoid valves that will control water flow. The first will control the water input valve, the second will control the fast flush feature (runs for a few minutes before switching over to clean water output), and the final two will control topping off my two containers when they are low. I also have two float valves in each container (low and high levels) that work with the arduino code to automate the process.

Since this is my first time designing a board, I’d love if anyone can comment on my schematics to ensure I’m not missing anything. Eagle is pretty easy to work with overall, but the component choices are what intimidates me the most. Hopefully I’ve made the right choices.

My first go was not using the optocouplers. I may be able to get by without those, and would be willing to do so if that would be better. I added them in place of flyback diodes, which I didn’t originally realize were necessary when using a solenoid.

I’d be more than happy to post the eagle files if someone would be willing to check them out. Otherwise, the PDF schematic is attached…

RODIControllerSchematic_New.pdf (16.3 KB)

You need series diodes for the LEDs in the opto cct. D2-5.
You should add kickback diodes on all DC solenoids.

Edit:
Breadboard one cct. to make sure it will work as you expect.

edit - missed your comment about 2 switches in each of the 2 containers, though have you got the internal pull up resistors on , if not you need something like a 10K pull up resistor on each line.

While you see a lot of designs with opto couplers on output devices, I do wonder what the changes of a shorted / failed outout transistor or mosfet blowing base via its base to destroy the ardunio ?

Suppose it could happen but never exerienced it ...

As well as the flyback diode you would be wise to fit a fuse on each of your solenoids power lines.

Agree with Larry, forget a pcb for now, prove your design 100% on a bread or vero board etc before even consideriing that, though do not run the solenoids power lines though a breadboard as it will probably be too heavy a current for the breadboards fine contacts.

That is sound advice. I will order some components and wire it up to test before going forward.

I'm really considering just eliminating the optocouplers. I'm really not sure they are necessary in this case, since the solenoids are using low voltage. I can completely understand using them in a high-voltage application to isolate the arduino from the higher voltage.

Is there any reason I couldn't (or shouldn't) eliminate the optocouplers and replace with a flyback diode? I also hadn't considered running a fuse on the solenoid outputs, as they are very low draw (4.8 watt per specs). I suppose I could add 1A blade fuses as a preventative measure.

@LarryD - I'm assuming you meant series resistors from D2-5 to the opto's. I overlooked this, but given my thoughts above I may do away with this altogether.

What is the best way to size a flyback diode?

Ditch the optos and drive the gates of AOI514 from Arduino pin (thru 150 ohm resistor) to the solenoid off & on.
http://www.digikey.com/product-search/en?keywords=aoi514

@CrossRoads - Good find, however other than the rating being 30V instead of 50V of the ones I posted (and the cost is a bit less), is there any difference in the two? There's such an enormous amount of components out there, it's a bit overwhelming to try and find what I think I need.

I believe the original part in my schematic can be driven right off the arduino output as well, and I think adding a flyback diode would be necessary. I just purchased a couple 1A/50V rectifier diodes, along with the COM-10213 MOSFETs. If there's a good reason I shouldn't go with those (other than the fact it may be overkill), I throw them into the bin for a later project. If that's the case and you can help me understand why, I'd appreciate it! ;D

By the way, you can never have "an amount of components". They are discrete things, so you have "a number of components". You can only have an amount of something indivisible, like water. It's the difference between "how many" and "how much".

Sorry, I just had to get that off my chest.

Tango2:
I also hadn't considered running a fuse on the solenoid outputs, as they are very low draw (4.8 watt per specs). I suppose I could add 1A blade fuses as a preventative measure.

4.8W @12v = 400ma per coil, thats not what I would call "low Draw" you will need a decent sized 12v power supply.
If on for any length of time eg 1hour, then you will find they can get very hot.

You may also find with heavy current switch on and the noise created when when they switch off , that you need extra decoupling to the ardunio module.

No expert, but think your flyback diodes need to be a lot higher voltage, 1n4006 or 7 probably better.
I know its only a 12v supply, but the spikes switch off produces is a lot higher.

ricky101:
No expert, but think your flyback diodes need to be a lot higher voltage, 1n4006 or 7 probably better.
I know its only a 12v supply, but the spikes switch off produces is a lot higher.

The whole idea of a kickback diode is to short-circuit the spike.
Reverse voltage is the power supply voltage. 12volt in this case.
Forward voltage is ~0.7volt.
Leo…

So I received all of my components and breadboarded them. Everything appears to be working great.

One other thing I need to incorporate into my shield is the ability to control two dry-contact switches from another device. This circuit originally connected to my float switches, but now the float switches are connecting to my arduino. I need to feed this signal back to the original system.

I could throw a large relay on the board and call it a day, but I'm wondering if there is something else that could be used that is smaller. One thing I found was a SSR. I'm not too familiar with these, but it appears to switch both AC and DC. The part number is G3VM-61E1. Will something like this work in my application or should I stick with a mechanical relay? If the relay route, can someone point me to a small one that would work in my application?

I think a small reed relay should work DIP4 - reed-relay - 5V 10pcs 4.75€.

I have seen a couple of those, and thought it may be a solution. I thought a SSR may work as well, but wasn’t 100% sure. If I could go the solid state route, I would prefer it, just for reliability. Otherwise, I think the relay would work just fine. Thanks for the input - if there are no other options, looks like I’ll place another order.