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I'm a long-term software developer who hasn't touched a soldering iron since Uni. I've been mulling an idea for a hardware project for some time, and have cajoled a raspberry pi into managing the following devices:

  • Touch screen
  • e-Ink Display
  • 2 x Stepper Motors [Very infrequent use - <5 mins/day]
  • Thermal receipt printer [20cm print ~hourly]
  • GPS/LTE module [On for extended periods (hours)]
  • Arduino Zero [Not actually connected yet, but will be "always on"]

I have a bare-bones implementation running, and -now I know it's possible- I'm turning my attention to packaging it all up.

At present, I've got half a dozen DC power supplies connected to the various components.

I've started to put together a rough layout (for scale, the box is 28x18x13cm internally)

CAD

Some of the devices offer power saving modes which I intend to use when possible, but it'd be nice to be able to use it all simultaneously if required.

The pi 3 B+ itself requires 5V and up to ~2.5A. That's powering the touch screen, e-ink display and stepper motor driver board.

The motors themselves require a 12V supply (350mA/ea)

The thermal receipt printer requires 5-9V at 1.5A.

The GPS/4G module requires 3.3V/1A.

So that's....

  • 12v/700mA [Motors]
  • 5V/2.5A [Pi/Screens/Motor Driver/a few sensors]
  • 5V/1.5A [Printer]
  • 3.3V/1A [Comms]
  • 3.3V/1A [Arduino]

I know some of those are max draw (eg the Pi) but I'm actually quite close in that case (largely thanks to the touch screen). Eg if try to run the comms board from the Pi's 3.3v rail, it's enough to trip the pi's power.

So, my primary question is... How can I (cleanly) run all of this from a single wall socket? Due to my lack of experience, I've got a preference for keeping everything in the case low voltage (or at least keeping mains voltage fully enclosed/isolated).

Ease of integration is a consideration considering my soldering "skills" and scarcity of equipment.

I'm hoping that the subsequent step will be to use a large power pack, and have found this https://www.amazon.com/NOVOO-22500mAh-Universal-Compatible-Smartphones/dp/B07JKP3PWY but haven't really had time to look into it yet (presumably converting up to 240v and back is wasteful). I mention it in case the idea of a power pack changes the answer significantly.

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  • \$\begingroup\$ If you want to run this on batteries someday, you should start moving away from the pi now, as it's lack of power saving modes means it is really not suitable for that. \$\endgroup\$
    – Chris Stratton
    Commented Jul 19, 2019 at 23:48
  • \$\begingroup\$ @ChrisStratton Thanks for the advice. I'm pretty deeply committed to the Pi now (I've built an extensible remote command-and-control system which supports downloading new "skills"). Moving that to something lower-level will be a long job. However... I'd like this to last a solid 24 hours, so perhaps I can hack it around a bit and the use arduino to toggle the Pi's power supply. If I can get the arduino to talk to the LTE directly, I can "phone home" infrequently and emulate a "Wake on LAN" [For context, this is a box being given to a friend, so no privacy/scale concerns]. \$\endgroup\$
    – Basic
    Commented Jul 20, 2019 at 0:35
  • \$\begingroup\$ You don't have to drastically change your software, but you need to move to a platform with a heritage to mobile rather than set top boxes, and some ability to doze when nothing is scheduled. \$\endgroup\$
    – Chris Stratton
    Commented Jul 20, 2019 at 0:41

2 Answers 2

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Well the cleanest option is to have it use a single 12VDC coming in from a wall AC/DC converter. Then you would have voltage regulators to get the specific voltages you need within the unit. Easiest way to do that is to have a custom PCB made, enabling the regulators to be done on board. If that isn't an option, you should be able to get pre made voltage regulator modules from amazon/adafruit/sparkfun.

I agree with @ChrisStratton that for battery usage, a PI isn't really usable.

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  • \$\begingroup\$ That's helpful, thanks. Given my inexperience/ignorance, a custom board isn't something I can solo (my first thought was to chain diodes to drop the voltage, which I'm guessing is not wise). So... I'll need to find someone to lend a hand. Presumably any rent-an-ElecEng could design it for me, but are there services which will etch/add components to a custom design? Or is the only available option going to be me trying to solder dozens of components without melting something? \$\endgroup\$
    – Basic
    Commented Jul 20, 2019 at 0:27
  • \$\begingroup\$ @Basic Yeah so you can find plenty of companies to design a board for you. But these are super easy to use, and will easily be able to drop the voltage to what you need, and are adjustable to whatever voltage is required amazon.com/dp/B07PDH5HM3/ref=cm_sw_r_cp_apa_i_c2MmDbGZHTBKY \$\endgroup\$
    – Kyle Hunter
    Commented Jul 20, 2019 at 1:26
  • \$\begingroup\$ Thank you, that's exactly what I needed. (Well, the concept... I might try to find something a little more lightweight) \$\endgroup\$
    – Basic
    Commented Jul 20, 2019 at 4:07
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Get a 12v5amp wall wart. Use 2 or more dc-dc buck converters to get the 5v and 3.3v supplies. Plenty off-the-shelf modules available these days at low cost. Use lot's of capacitors.

[Added another idea] Alternatively, get a power supply used in desktop PC's. They are cheap yet robust enough due to volume production, and provide 12v, 5v, 3.3v simultaneously at substantial current capability.

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