In order to include X-10, I just need to include a routine to send out X-10 commands via the main controller. I’ll also have to include some type of throttling for the X-10 commands since they can’t react as fast as DMX. Perhaps only let the states flip or a dim sequence be sent every X seconds. Otherwise it would impact any other X-10 use in the house. We’ll see.

Regardless of the details, I may be able to utilize X-10 in a way that makes sense, even if just for a small part of the over all show.

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!

To get things started, I want to talk about the electrical side of things.

To reduce the length of the extension cords, I want to have a modular control setup where I can have ‘nodes’ around the house to connect things to. These nodes will each be connected to one of probably four 20A branch circuits that will be hard wired outside and control circuitry inside will control the group of lights connected to it. I expect the house will have a variety of string lights, lit figures, maybe one of those tacky Happy Holidays signs, etc. We have a detached garage that is bound to have some lights on it as well, but it has a 20A circuit for the outside outlets which should be enough. I also expect to have a variety of things in the front yard as well as lights on bushes that are a fair distance from the house. I will probably put a number of those short heavily lit trees throughout the yard. This means whatever ‘nodes’ there are in the system will need to be ‘networked’ together.

I don’t expect any inductive loads (i.e. motors), but it’s a possibility, especially if I do a display for Halloween. I want to be able to dim the light strings as well as turn them on and off. I want to have the ability to flash the lights and possibly even have them twinkle. This requires a fairly fast switching method – a solid state switching method.

In order to switch the lights that quickly, you need a triac based control board. Triacs allow for very fast switching and dimming. The popular boards from places like Light-O-Rama and X-10 Lamp Modules use triacs. However these devices can only be used on resistive (incandescent lighting) loads. They are not designed to handle inductive loads. You can, however, get dimming systems that are designed to handle inductive loads utilizing zero-crossing circuitry (the triac is fired in sync with the zero cross and is fired long enough to handle the current lag of inductive loads) Many dimming modules used for stage lighting are designed to handle inductive loads.

To handle any type of inductive load (low voltage lighting, fixtures with motors), people often use relays. Relays are most often mechanical devices. A low voltage energizes a magnet coil which pulls a mechanical switch closed, completing the circuit. There are also solid state relays which use high current semiconductors to switch the loads on an off. Given their nature, relays can switch very high currents for their size and also can switch inductive loads as well as resistive loads. However, they are on/off. You can’t dim with a relay. X-10 Appliance Modules use a heavy duty relay, thus the loud ‘click’ when you turn them on and off.

One key difference between the two types of setups is boards with relays tend to be cheaper than dimmable boards with triacs, but not always.

Given the type of show I have in mind, I’ll have a number of loads that I want dimmed as well as loads that just turn on and off. Instead of using just one type of control method, I expect I’ll use both triac dimmers and relays.

I’ve have a healthy respect for electricity which can be the riskiest part of putting together a big holiday display. GFCI outlets and proper overcurrent protection are important. However, when putting together a DIY show, there are a lot of wires carrying 120V running around inside and outside the enclosures increasing the possibility of getting shocked. Point to point wiring is a common practice in DIY projects, but can be dicey when dealing with AC current. Another risk is a short causing 120VAC to feed back into the low voltage control side – a very dangerous situation. So one of my design requirements is to ensure that all AC switching is done via circuit board triac/relay based systems or opto-isolated module packs (see photo right). This also makes construction easier.

So what requirements are there so far?

• Try to use off-the-shelf, optoisolated modules or triac/relay based circuit board for 120VAC switching.
• Ability to handle resistive and inductive loads.
• Ability to dim some circuits (triac based). Relay control acceptable for some things.
• Fast response and switching time. Ability to switch in sub second cycles (twinkle effects, etc).
• Modular control system with networked ‘nodes’.
• GFCI protected circuits. 4-6 dedicated 20A branch circuits.