Trying to figure out mixed voltages

[JackMan]

GOT,
     Just for the record, I figured out what you were doing wrong. The Prop-1 switch has to be in position 2 as I outlined earlier, you had the switch in position 1. Again, do as Jon has directed, I'm not here to give conflicting advice.  8)

[JonnyMac]

But if you only using one power supply and it is connected to V+ and GND, you must have the power switch in P2 in order to get power to the processor.

[JackMan]

I'm quite certain that he had 1 PS plugged into the barrel connector and the switch in position 1 when he tried my suggestion.

[GOT]

You are correct JackMan.  So, Jonny, when and how do you use the second power supply for mixed voltage?

[bsnut]

Got,
I am going to answer "when" part of your question first. The only reason why you use two power supplies would be to seperate the power form the processor and the power running through ULN chip. I have this big question for you and that is. This is coming from an electrician loves this type of hobby and used to work in the amusement industry.

Why do you need two power supplies?

To answer your "how" part of your question and I think we did this in posts before this one. If you are going to use two power supplies with the Prop-1 then the power switch needs to be position 1 to do what you are talking about. Bad things like "Blue Smoke" can happen when two power supplies are providing power to the Prop-1 when the power switch is in position 2. And, I don't think you want that to happen with Holloween only two weeks away.

The best thing that you can do is, use one power supply rated for the amps that you need for your project. You can either can provide power by plugging into the barrel connector and set the switch in position 2 which provides power to V+. Or, you can do as Jon suggested here

But if you only using one power supply and it is connected to V+ and GND, you must have the power switch in P2 in order to get power to the processor.

Remember we, are here to help you get your Holloween project done.

[JonnyMac]

So, Jonny, when and how do you use the second power supply for mixed voltage?

Let's say, for example, that you're running the controller from a 12V supply and you want to run a mix of 12V and 24V devices from the terminals.  The 12V devices would connect normally.  For the 24V devices you would connect the ground from that supply to the GND terminal -- this is required as the outputs switch ground.  Now... and this is important, your 24V devices would connect to the positive side of the 24V supply.  Under no circumstances does the positive side of the 24V supply get connected to the PCB. 

Yes, this is an odd scenario but it does come up.  This is why the ULN is popular: by switching ground (common all supplies) it can control outputs in a mixed voltage system.