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.

#!/usr/bin/python
import time
import os
while 1:
  time.sleep(1)
  if os.path.isfile('/tmp/reboot'):
    time.sleep(5)
    os.system("sudo reboot")
  elif os.path.isfile('/tmp/poweroff'):
    time.sleep(5)
    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.

Using a different tool for each job and linking them together with MQTT - part 2

Now we have a working MQTT server, it's time to start making use of it.

As I was planning to use MQTT to broker messages between Web server CGI scripts, Javascript in Chrome web browser and an Arduino the obvious way to do this is push data at the USB serial port of the Arduino.

Normally this port is used for uploading the Arduino sketch to the board and sometimes people end up filling it with debug/status messages once the sketch is running. However there's a long history of it being used as an actual way to control stuff.

So I threw together a piece of 'middleware' that shuffles data between some MQTT topics and the USB serial port.

#!/usr/bin/python
import time
import os
import mosquitto
import serial
# commands to the arduino are as follow...
# R red LEDs
# G green LEDs
# B blue LEDs
# N no LEDs
# O open/go
# P power off
arduinoBootupTime = 5
arduinoIsBooted = 0
debug = True
# Define the various MQTT callback functions as this is an event driven model
def on_connect(mosq, obj, rc):
if debug:
print("rc: "+str(rc))
return
def on_message(mosq, obj, msg):
if debug:
print(msg.topic+" "+str(msg.payload))
arduino.write(str(msg.payload))
return
def on_publish(mosq, obj, mid):
if debug:
print("mid: "+str(mid))
return
def on_subscribe(mosq, obj, mid, granted_qos):
if debug:
print("Subscribed: "+str(mid)+" "+str(granted_qos))
return
def on_log(mosq, obj, level, string):
if debug:
print(string)
return
#
time.sleep(arduinoBootupTime)
# Connect to the local MQTT server to pass stuff to/from a browser
mqttc = mosquitto.Mosquitto()
mqttc.connect("localhost", 1883, 60)
mqttc.on_message = on_message
mqttc.on_connect = on_connect
mqttc.on_publish = on_publish
mqttc.on_subscribe = on_subscribe
# Subscribe to the topic that sends commands TO the Arduino
mqttc.subscribe("arduino/in", 0)
while 1:
if debug:
print "Trying to connect to Arduino\n"
arduino = serial.Serial('/dev/ttyUSB0',115200,timeout=1)
while arduino.isOpen():
if arduinoIsBooted == 0:
if debug:
print(arduino.name) + " connected - Giving " + str(arduinoBootupTime) + "s for it to bootstrap"
time.sleep(arduinoBootupTime)
arduinoIsBooted = 1
mqttc.loop()
arduinoOut = arduino.readline()
if len(arduinoOut) > 0:
if debug:
print "Received " + arduinoOut.rstrip() + " from the Arduino"
if arduinoOut.rstrip() == 'powerDown':
powerOffFile = open('/tmp/poweroff', 'a')
powerOffFile.close()
else:
mqttc.publish("arduino/out", arduinoOut.rstrip())
arduino.close()
time.sleep(10)

This is about as simple as things can be and mostly cribbed from example scripts. It waits a while to give the Arduino time to boot as the USB port attaching to Raspberry Pi will cause it to reset. Then it tries to connect and loops round forever passing messages back and forth.

The meat of this is in two functions, the first of which gets triggered as a callback when an MQTT message arrives...

def on_message(mosq, obj, msg):
arduino.write(str(msg.payload))
return

This simply sends the message straight to the Arduino. Coming the other way is almost an exact reverse except we're using newlines to mark the end of messages so we can read them with 'readline'. This means we need to strip them off before publishing to MQTT. Python has a handy function for this in 'rstrip'.

arduinoOut = arduino.readline()
if len(arduinoOut) > 0:
if arduinoOut.rstrip() == 'powerDown':
powerOffFile = open('/tmp/poweroff', 'a')
powerOffFile.close()
else:
mqttc.publish("arduino/out", arduinoOut.rstrip())

There's another little if/else in here to handle the shutdown sequence. I used the VERY simple method of writing a file to /tmp/ if the Raspberry Pi needs to shut down, which is controlled by the Arduino. I could have had a second script subscribed to the topic for this, but this keeps all the MQTT code in one place. Because the Pi uses memory for /tmp/ instead of saving to its SD card, this safely disappears once the machine is shut down.

That's about it, things coming out of the Arduino USB serial port end up in the topic '/arduino/out/' and stuff in the topic '/arduino/in/' get sent to the serial port. The test for the existence of the Arduino serial port '/dev/ttyUSB0' means it loops round forever trying to reconnect if the Arduino get unplugged or resets.

To make this run at startup I put it into /etc/rc.local which is another quick and dirty 'make it work' solution. It works. This is not a server it's a prop, so I'm being cavalier with scalable/secure ways of doing things.