This is brilliant.
I highly recommend adding GPS to your project to take care of time and location automatically.
www.adafruit.com/product/746

Hopefully this makes sense. Every 'functional group' is connected in parallel. This made design and troubleshooting easy.

Thanks Ihoujin for your GPS mention - I forgot to mention I have a cheap USB GPS dongle (model VK-172) that is inside the box and connected to the pi via the micro USB connector and a USB OTG cable. The USB model gives location sync but lacks the pulse output to sync time - instead I use a small script at startup to read the time from the USB module and set the time from that.

I'd prefer use a module like you suggested; was implementing this straightforward?

You can configure NTP with GPSD to use the GPS dongle as a source for time without worrying about PPS. All PPS really does is keep it ticking at exactly 1 second once the time is set.
Implementation is fairly straightforward and it allows for attaching an external antenna if necessary. Basic steps include
Wiring to 5V supply
Cross wiring TX and RX to GPIO Pins on the Pi. [Tx to Rx, Rx to Tx]. Page 11 of Documentation
Configure boot/config.txt. Line 1 will enable the MiniUART , and lock to core frequency to 250, disabling turbo because Mini UART and CPU Core frequency are relative. Line 2 defines the pin connected to PPS, example here is pin 4.
Configure /etc/default/gpsd to collectas a device at startup
Finally configure NTP to use time from GPS
/etc/ntp.conf

I just noticed your diagram indicates you connected the fan to 3.3 volts on the Pi. This is unwise. Drawing power from the GPIO pins 1 or 17 for a fan is taking power through the Pi's regulator. The fan's current draw could cause an under current condition or worse case damage the Pi. Pins 2 and 4 are 5 volt taps direct from the USB power supply. You would be better off taking power from there and stepping it down with a simple resistor voltage divider or a voltage regulator.

OMG this is awesome! Thanks so much for posting the pictures and diagrams, I'm already looking around for months to see what would be the best solution. I figured to design it myself, but this is so close to what I wanted I might as well try that.

@Ihoujin Thanks for picking up on that, I'll modify my unit accordingly.
I've started brainstorming 'Mk 2' and will power any fans from a dedicated 5v or 3.3v line - or at least a module that can handle the current with ease.

@supernov - thanks for your kind words. Building my unit was fun and a learning exercise - there's a few things I plan to do a little differently for Mk2. If you have any questions or comments feel free to get in touch.

Yes, welll... I'm a beginner in electronics, but have learned quite a bit about the various components in the past year. I also learned how to solder and read the diagrams. What I always end up finding difficult, is how to properly connect everything to ground. Is it simply, in your case, that all components feed back to the negative wire of the mains?

To ground everything I used a brass 'bus bar', its the first picture (here's more examples www.aquashopkampen.nl/product/busbar-10x8/). This is a common ground connection for the modules and connected to this is negative of the DC input. The DC power is grounded via the DC power supply connection.
This bus bar needs to support the maximum current load of all modules combined so give that some consideration when selecting the part. You could make your own if you have the tools and materials as some bigger modules get quite expensive.
For my project I summed the maximum rated load of all components and then rounded up - in reality it's been overkill, while running my system draws about a quarter of what it's 'designed' to do - but I feel better safe than sorry.

Great work! You've certainly given me some ideas for using my 3D printer for something useful! Thanks for sharing!

Cheers for the feedback. The unit is still going strong and I'm planning out the next revision as I'm finding this a bit bulky. OK for side-by-side mounting but I'm looking to move away from that and wanting to mount the unit on or under the OTA, meaning a longer narrower design would be better (a bit like a QHY Astrobar, or Prima Luce Eagle2).

Some lessons I've learnt and improvements I'm considering:
Move to an arduino dew heater controller, this should be less bulky in the case as using the stripped down LED controllers made things a bit bulky.

The fuse panel module added a lot of bulk to the unit. First appearances make it look a bit snazzy like a RigRunner but really would have been much better if built onto a PCB - or ommited and had a 1 or 2 fuses where it really counts. My next version will be far less busy looking.

Getting a PCB made is pretty cheap these days - many online services are available, even if you end up with 3-5 of the same board it's still cheap. I'm planning to put much of the 'power distribution' side of things to PCB to cut back on the mess of internal wiring. If it wasn't for the wiring and fuse panel the whole unit would be much smaller and lighter.

I now have a 3D printer as well so have a lot more options when it comes to the case design to put it in. I intend to 3D print an inner skeleton which will be sandwiched between two thin aluminium plates. The skeleton will hold the boards and connectors, the aluminium panels will be tough for mounting between rings and dovetails (with plenty of plastic around each through-hole internally).
(I stole some ideas from QHY - have a look at the internals of the QHY Astrobar here: )

Finally, a small blower fan like those used in laptops will allow for a slimmer design.

Have fun!