DMX – The Complete Project.
This outlines the complete (or almost complete) steps for
using a Raspberry Pi to control my garden lights. The outside lights use the
DMX protocol, so as well as being able to generate different lighting moods;
they can also really annoy the neighbors by turning my garden into a disco.
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| Figure 1: The obligatory picture of the control system |
Figure 1 shows the inside of the control box. (I have a
control box because my wife really, really, doesn’t like wires – so I have to
hide away as much as I can). I’m using a RS485 USB device (around $10, using
the Prolific PL2303 chipset), which you can see crammed in the bottom right
corner, to control the DMX devices. I’m using the PiFace to turn on/off power
to the lights. (I’m also using it to control some LEDs on the front of the
control box, but the cables are not connected in the above picture). The big
yellow thing on the right shows that I live in America, where twisting wires
together is considered a sound connection. I’m using a nano wifi device (the
blue light on the left). Also, I had to use a right-angle USB extension (which
turned out to be left-angle) to attach the RS485 device, as the box wasn’t long
enough. (I could have used a shorter SD card, but I don’t know if they exist)
The above is the basic schematic for the installation. The
Raspberry Pi (Blue) is used to drive the RS485 with the DMX protocol. The green
box represents the RS485 device, that drives the twisted pair cable that
communicates to the lights. At the end of the twisted pair is the terminating
resistor of questionable value (see below). The PiFace (Yellow) is used to turn
on power to the lights. It also used to drive some of the status LEDs on the
front of the control box, but that’s not shown in the schematic. The power
supply is a notebook power supply. These tend to provide a lot of power in a
small device and are somewhat more consumer friendly than the more industrial
power supplies. (Plus, the one i used has a usb port, so I can power the Pi from that.)
Figure 2: One of these lights
Figure 2 shows one of the lights. I should have put some
sort of scale on this, as they’re “Rather Large”. They’re about 12” across. I
wanted to light up my trees, so these are 15W RGB lights. Pretty powerful. The
lights have four wires (Which in my case, were very well labelled). Two 24V and
two DMX, labeled D+ and D-.
Wire turned out to be one of the most expensive parts of the
project. I used 200’ of 16 gauge wire for the 24V DC and 200’ of twisted pair
cable. I was going to make my own twisted pair, which I’d done during
prototyping (using a post and a drill) but I found a reel that was relatively
inexpensive. What I should have done was used Ethernet Cat5e cable – not
because I needed 4 twisted pairs, but because it’s sooooo much cheaper than
just one twisted pair, plus it probably is more durable when buried in the ground,
plus, having underground Cat5e in your garden much cooler than, say, owning a Ferrari.
Figure 3: RS485
Figure 3 shows the RS485 device I used. The DMX protocol
uses RS485 for the hardware/signaling side of the protocol. On top of that is a
relatively simple protocol that uses BREAKs to signal the start of a packet
followed by the data. I had to patch the PL2303 driver to support the DMX
250,000 baud rate and write a utility to generate the DMX packets. [
Link] I was
waiting for my lights, so I ended up buying a $25 RGB DMX light off Amazon that
I could play with. This was quite entertaining, and my daughter was quite
amazed to find out that when you mix Red, Green and Blue light you get White. I
created a web page so that I could control the lights and then added network
support, so that standard DMX software could talk to the Raspberry Pi and
control the lights. So far I’ve tested Vixen Lights 3 on my PC and AuroraDMX on
my Andriod.
Installing the lights took about a day and a half. Most of
the time was spent trenching and burying the cables. I have a
‘Black&Decker’ trencher and edger, so I used that for making the trench. It
draws more of a line than actual trenching. I ended up sitting on a kid’s
camping chair using paint stirring sticks to push the cables into place, but it
worked. The nice thing is that the trencher didn’t get down deep enough to
splice pipes or any other cables. The bad news is I don’t know if I’ll be able
to aerate my lawn again.
I tested out the installation twice before burying the
cable. Once in the house, where I was running into issues and ended up taking
100’ of twisted pair cable and had it going through an assortment of rooms in
the house and once in the yard/garden, where I had it laid in position. DMX
calls out for a 120 ohm terminating resistor on/after the last device. This is
to stop the signal bouncing back. When you have a short run and a few devices,
you’ll not need this, but once you get a long line of cable, it becomes an
issue. When I did the test runs, I needed a larger resistor, around 150 ohms.
Luckily I had these to hand. Once I buried the cables, this no longer worked.
The lights I have go into an automatic mode when they’re not getting any
commands, so it was pretty obvious that something was wrong. Initially I thought that it was a loose cable and had the annoying experience of finding and fixing the loose cable only find out that it wasn’t the real issue. I ended up trying an assortment of
resistors in different configurations until something worked. In my case, two
150 ohm resistors in parallel, so 75 ohms. Next time I think I’ll invest in a
variable resistor/rheostat/potentiometer and use that to find out the correct
resistance, and then connect up the right amount of resistance.
The reason for the 120 ohm resistor is based on the impedance
of the cable. Obviously there’s some dark magic at hand that I don’t quite
understand. maybe cables are like people, they put up less resistance when buried underground.
With the cable trenched, I installed the lights. I dug holes
just slightly larger than the lights and filled it in with dry concrete. (I had
to redo the lights a few weeks later, as while the lights had plenty of
bedding, they didn’t have a whole lot of encasement on the side, so I dug a
wider hole and filled that in with dry concrete. The rain took care of the
rest). I was initially going to use a garden type control box for each
junction, but in the end use some underground spicing devices, some electrical tape
spray and wrapped it up in plastic – this appears to be working, but I’ll have
to see how long they last.
With the lights installed and the testing complete, it was
time to drill a hole in the side of the house and fish the cables through to
the control box. I used the PiFace to control the power to the lights. The positive
wire is connected to the “common” connection on relay#2 and the line to the
lights is connected to the “open” side of the relay. (The side that’s not
connected to common by default, you can see the lose screws in the picture
above). Note that the PiFace spec calls
out 20 volts or 5 amps max, I’m 24v at about 3 amps, so I think I’m OK. (you
can see in the picture above that the relays can cope with more. I assume the
limitation is based in the traces on the PCB) It appears that all the PiFace documentation
points you to perl. I use the gpio
utility, which can be slightly odd (“gpio –p write 200 1” sets the top relay
on, but you need to use “gpio –p read 209” to read the current state of the
relay).
The nitty-gritty inside the Raspberry Pi is difficult to
take a picture of, so here’s a written version. I’ve modified /etc/rc.local to
run the utilities that I need when the system boots up.
/usr/local/sbin/dmx -e &
/root/lights/lights.sh
>/var/log/lights.log &
The /usr/local/sbin/dmx is the utility I created to talk the
DMX protocol to the RS485 device. I’ve documented this in a separate
blog/blogs. [
link]. The ‘-e’ option enables E1.31 unicast network support.
The “lights.sh” is a shell script that is split into three
parts. The first part calculates the amount of seconds until sunset and sleeps
until sunset (garden lights being a more nocturnal activity). The second part
turns the lights on and generates a random set of values for the lights. The
third part shuts the lights down when it gets late.
Figure 4: Unimpressive Web Frontend
The webpage controls the power to the lights as well as the
lights themselves. It’s quite unimpressive as I wanted to use the “slide” type
(for reasons that currently escape me – I think to avoid huge wads of java
script ). There’s some java script, mainly communicating to PHP scripts on the
server that do the actual control of the lights and turning the relay
on/off. Really I should have a front end
where you just select the color of the light you want (well, that’s what my
wife wants).
Figure 5: Vixen pumping up my lights
As well as certain “staged” or set scenes for lights, you
may want to pump up the jam/jelly. Figure 5
shows my take on the middle bridge(?) on “
Hello”. I’m no expert on any DMX
mixing/lighting software. I tend to learn how to use software by banging my
head against it for a while and then giving up and reading the manual. In the
case of DMXcontol and FreeStyler, they both look great but I didn’t make it
over the first hurdle. I then took a look at Vixen.
Vixen appears to be the choice of people who want to overdo
it on Christmas lights. I liked it because it kind of fit in with how I
imagined DMX software would work. It has a timeline and you can plonk different
effects on each light and it has a nice “Preview” that works. It also support
“E1.31 Output Controller”, which is a pretty generic way of talking to lights
over the network, so I updated the DMX utility to understand E1.31 and was able
to make my lights sing and dance. (
Link to setting up Vixen for use with theDMX utility). I’ve banged my head against it for a while, but I’ve not resorted
to reading the manual...yet.
Figure 6: The finished control box.
Power Supply on top.
