Now we get to have some fun and start looking at actual equipment and electronics. Since we’re going to use the industry standard DMX control protocol, we should be able to leverage that in deciding what to use to control the lights and other things in the holiday display.
As I noted in an earlier post, the one area I’d rather get ‘off the shelf’ and avoid doing myself is the 120VAC dimming and switching. Sure, it can be done, but utilizing some off the shelf equipment for the line dimming and switching will not only make things safer, it’ll also allow more time to be spent on the control side of things. One of the big advantages of doing it yourself, however, is cost. Will off the shelf components be economical? Well, they can be if they come off your own shelf.
Some things in the display will require a simple on/off control. LED light strings don’t dim very well and they draw very little current. I might try putting together some other things using high brightness LEDs that blink/move, etc. So the power draw will be minimal and they will just need an on/off switch. Other loads might have motors or won’t look good if they’re dimmed, which sounds like a perfect job for a relay. I priced some DMX controllable relay packs, but they were expensive since they controlled big loads. I did, however, find a possible inexpensive do it yourself relay pack. Turns out I also have a potential low current relay pack sitting on my workbench.
I’ve always been interested in Home Automation and a while back I designed a circuit board with relays that could be controlled with X-10 commands. People used these to control irrigation systems via X-10 commands. Utilizing a small processor to do the heavy lifting, it was a simple design. Here’s one version with 8 relays on it.
While I was writing up my earlier post about the overall show and what I wanted to do with it, I realized I could modify these boards for DMX control fairly easily. All I have to do is put an RS-485 transceiver chip inside the housing of a DMX connector and run the digital side of the transceiver to the RJ-11 jack on the board and the hardware will work ‘as is’. All I need to do is modify the firmware to handle DMX instead of X-10 bit streams and probably boost the crystal speed up a bit since DMX operates at 250kbps.
I had created boards with 4, 8, and 16 relays on them so that should give me some flexibility. The only thing about these boards is they were designed for low voltage control. The relays they use are small and can only handle about 1 Amp of current at 120VAC when both sides are connected in parallel - about 120 Watts. The traces on the board can handle a few amps, so I’m going to see if I can find heftier relays with the same footprint.
Another option for relay control is a mix of home grown and commercial stuff. A few companies sell 8-Relay devices that are controlled by a separate set of switches. Check out the Elation SC8 system. The part of the system with the plugs has 8 relays inside that are controlled by a simple 9-conductor cable that connects to the controller.
The best part is you can buy the relay part of this system for $30-$40 to ‘expand’ an existing controller. So all I need is some type of RS-485 controller that can flip some digital I/O lines to control the relays. The perfect mix of pre-wired/commercial 120VAC control with some custom DMX control. Given the size of the relay part, I bet the board could easily be located inside the existing chassis with room to spare. Now, there are commercial DMX relay packs out there, but they can cost almost $250. So I’m hoping to take another board I designed which can receive RS-485 data and control an 8-bit data port. Depending how the relay pack is designed, I may need to add a driver circuit to fire the relay coils, but beyond that, it’ll be mostly software. Like the above relay boards, I can update the code to receive DMX and control the relays.
With any luck I can fit this board, two DMX connectors, and any additional driver circuitry into the existing relay chassis for an ultra compact 8-Relay DMX pack to control heavy duty lighting loads for a lot less than it would cost to buy (I’m shooting for about $50 per pack)
Hopefully that’ll help keep the costs down a bit. I’ll be spending enough on lights and such for the display anyway. If I can save money doing some stuff myself on the control side and using existing things I already have, all the better. I’ll be sure to keep you posted on the development of the firmware for these boards and how the final controllers turn out!
Further Reading:
I’ve been toying with the idea of doing a bit of automated Christmas lighting. I’m really interested in your DMX setup and I already have a 485 controller that I can use LabVIEW to control.
Just curious if you are still updating the blog! Been a fascinating read!
I haven’t updated in a while - life got in the way of my little project here. But I still plan to work on it - ironically once the weather gets colder near the holidays. So I may be pushed into 2008, but I definitely still want to work on this and see where RS-485 and DMX will take us!