New Build Electronics Newb Diagram Help

I have all almost all my pieces together (save for a heater) but am having a bit of trouble wrapping my head around the wiring.

Parts List:
Fridge - Frigidair 4.4cu inch refrigerator. 0.806Amp / 115V / PTC starter relay
Heater - TBD but looking at a 110v / 60W tube heater like this:
2x 10A SSR
Brewpi Spark
Raspberry Pi B+ and 5V 2A Switching Power Supply
3x one wire temperature probes

I have a few questions:

  1. In the fridge hacking guide terminals 3 and 4 of the two SSRs are
    labelled as Ground and Control Respectively. So I am guessing this
    means that these two wires are connected to the Brewpi Spark via the
    Digital Outputs? If this is the case how exactly are these
    Grounded? Since the Spark board is powered via USB from the
    Raspberry B+ and that in turn only has a Live and Neutral power
    supply, how could this be Grounded?

  2. The temp probes also have a Ground wire and doesn’t the same apply
    to them as well? Alternatively should I connect the ground wires on
    the temp probes in the fridge to the green wire from the fridge
    thermostat which is grounded on the metal frame of the fridge? (see
    image below of interior fridge thermostat wiring)

  3. In Image D below you will see the Neutral White wire coming out of
    the PTC relay, which goes into the back of the connector box in
    Image C (not visible in the photo) into the fridge, back out of the
    fridge (still white) to the connector which meets the white Neutral
    wire of the Wall plug wire at the bottom of Image C. I’ve indicated
    this “detour” at the bottom of my proposed wiring diagram with a
    circle around a “?” as I don’t know where this white wire is going
    and for what purpose. Is it reasonable to connect this white wire
    after the “detour” to the the neutral wire coming out of the heating
    element just before returning to the wall plug? Anyone have any
    ideas what it might be?

Image A - Here is existing wiring diagram for my fridge:

Image B - Below you can see the wall Black plug wire (Live Black and Neutral white and Ground green) coming up from the bottom of the image to the connector that leads to the interior fridge thermostat (Live Black, Neutral Red). Here the Red wire (sheathed in white) coming from the thermostat changes to Blue after and leads to the Compressor.

Image C - Here you can see the blue wire from the thermostat going to what I think is the overload protector and then to the compressor, which then comes out of the PTC relay White.

Image D - Inside the fridge is the thermostat and probe. It appears that the Black wire is Live from the Wall plug, the Red wire goes to the overload protector then the compressor and the Green is grounded on the metal frame at the back of the fridge. I plan to remove the thermostat and use these existing 3 wires to hook up 2 interior fridge temperature probes to one of the Brewpi Spark’s one wire RJ12 ports:

Here is my proposed wiring diagram. I realized the only thing I know I have left out of this diagram is the 5V 2amp power supply for the Raspberry Pi, after the main power cable splits to go to the Pi and the SSRs. I have opened up this “wall hog” and am planning to hard wire it between this split and the raspberry pi.

I’ve already modified the interior of the fridge to fit my fermentor from what it was:

To this:

1 and 2: Ground in electronics is not earth in an AC system. They are different things. The 0V reference is often called ground, but this is not the same as the green/yellow earth cable in AC.
You should connect the +/- from the SSR to the +/- on the BrewPi Spark, the green connectors. AC and DC circuits should be entirely separate.

  1. You should rip out the thermostat and place the SSR in its place for the cooler. The SSR should switch the live wire to the compressor. If you want to put the SSRs near the compressor, just take out the thermostat and connect the wires there. Cut the live wire further down the line.

  2. Your drawing looks okay, except it seems that you have connected AC to the pi ;).

Thanks Elco for the speedy reply!

Duly noted about the difference between the use of Ground in the electronics and the Earth in the AC system. I realize I must have mis-interpreted something.

Regarding your comment:

I follow everything up until “Cut the live wire further down the line”. Not sure what you mean by that in reference to inserting the SSR in place of the thermostat.

Also, after I remove the thermostat, is there any reason why I couldn’t use the Live, Neutral and Ground wires that are already running through the insulation to the thermostat housing in the fridge as the 3 wires hooking up the Temperature probes? It would be great to use these wires at they are already there and neatly threaded through the insulation to the exterior in the back.

Regarding #2:
I am trying for the sake of simplicity to design a system so that there is only one AC plug (for the fridge, heater and the Raspberry Pi). To that end, I was thinking I could hard wire or “pigtail” the AC to the disassembled 5V power supply for powering the Pi and also directly to the SSRs. In effect, I would split the AC to run to the SSRs and to a project box (that would contain the 5V power supply, the Pi and the Brewpi Spark). Maybe this isn’t desirable and if anyone has any better accomplish a single AC plug for the whole thing, I’d appreciate hearing them.

I am just saying that the SSR should switch the live wire just like the thermostat. You could do this at the place of the thermostat or somewhere else.

I see no problem with using 1 AC plug to all. Alternatively, you could just use a power strip inside the box.