Wednesday, 19 April 2017

Using a different tool for each job and linking them together with MQTT - part 3

Things you have to think about when putting a Raspberry Pi inside a prop include things you don't normally have to worry about like graceful startup and shutdown. Also, getting enough power out of a battery for a Pi 3 and a 5" display is not totally inconsequential.

I'll deal with the latter first. I luckily had a 5V/3A Turnigy branded battery eliminator (BEC) kicking around in a parts drawer. These are designed for RC vehicles and nicely self-contained in a shrink-wrapped module with inline inductor to smooth things out. I paired this up with a big chunk of 12 AA NiMH cells wired series/parallel to give me ~7.2V which is about right to run the BEC. The more modern solution would be some LiPo cells but then you need to build a balance charger into the device or expose the connectors so you can charge the cells individually. I've got a bunch of stuff that runs off NiMH cells and a nice high current smart charger that automatically selects voltage/current and this also influenced my decision to go this way.

Then there's the switch. People expect things to just turn on/off at the flick or press of a switch but you can't give them easy direct access to the power or they'll switch things off when they aren't ready. The Raspberry Pi really doesn't like that. There are workarounds you can do with mostly read-only filesystems but at the end of the day the Pi really should be shut down cleanly.

Luckily I had some Polulu solid state power switches lurking from a temporarily shelved project. My house may be overflowing with stuff but sometimes it comes in useful. I wired this so the power switch was connected to the 'on only' contact. That gets everything powered up. When you flick the switch to off, the alternate contact takes a pin on the Arduino low and the Arduino initiates a shutdown sequence. You saw the code for that in part 2. Here's the Python script for the shutdown of the Pi, which is just a variant on the old chestnut about fitting a shutdown switch to the case of a Pi.

import time
import os
while 1:
  if os.path.isfile('/tmp/reboot'):
    os.system("sudo reboot")
  elif os.path.isfile('/tmp/poweroff'):
    os.system("sudo poweroff")
Not much to say about this, it just looks for files in '/tmp/' and reboots or shuts down depending on which file it sees. Stick it in /etc/rc.local again and the job's a goodun.

Things I didn't get right are I used a latching switch for on/off and I should have fitted a way to both charge the batteries and power the device externally.

The batteries lasted a couple of hours but then it had to be charged before it could be used again. We had mains power in the end so being able to use a PSU would have been good. Connecting the smart charger with it powered up made the charger shut down, otherwise that would have been an option.

The latching switch prevented me from making the box shut down when inactive. It would try to shut down (the Pi would shut down) but if the switch was physically in the 'on' position the power would stay on until you flicked the switch. I picked this up in time to fix it but couldn't find a nice momentary switch without resorting to mail order. Which would have come too late. It's a lesson learned for next time.

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