DMX is a method of computerised control of stage and disco lights. The data is produced by the lighting controller, then each light is daisy-chained into the data line. At the far end of the data line, a resistor is needed to terminate the signal, essentially stopping the data packets reflecting from the end of the cable run and interfering with the new data being sent. However, even with this precaution taken, having purchased some new budget LED stage lights (made in China), the DMX data was suffering from odd interference.
Whilst programming the lighting for a local band’s staging, one of the more expensive lighting fixtures connected would suddenly ignore the control signals and begin to behave erratically. This did not happen if the new LED lights were removed from the rig; but if any of the 4 were connected, the problem would arise. From power-on, the time until the problem occured was not consistent (from a few minutes, up to a few hours). To stop the unwanted behaviour, one had to turn the fixture behaving erattically, off and then on again. (Yes, the standard IT support desk fix!)
Obviously the Chinese lights were causing the DMX signal to be misunderstood by the more expensive light. Both types of fixture would have no problem if the other was not in the lighting rig, but I needed them all to work together.
It was found that the cause was in the mains power arrangements for the misbehaving unit. In the setup being used to program the lights, this fixture was connected to an extension reel which was still partially coiled up. Uncoiling this seemed to prevent the issue. (Presumably the coil was acting as a choke, stopping the rogue signals that make their way into this light from reaching earth, eventually causing the interference issue).
However, the lighting rig is a portable one and needs to work reliably no matter what the stage or power arrangements are, so I needed a more definite solution…
The answer seemed to be to run the Chinese LED lights on a DMX line of their own, so that they cannot cause this issue with any other of the lights, no matter how the lights are connected to the mains supply. However, as the lighting console I use only has one DMX output, this would need to be split into at least 2 separate DMX lines.
DMX splitters/isolators/repeaters of the type I needed are available commercially, but not at a price I was willing to pay. As a result, I found the following article (Published by Elektor Electronics in 2003) which contained a circuit design, board layout and instructions to build my own 3-Channel DMX Splitter.
The unit receives the DMX data and decodes it into TTL. Then it is passed through an opto-isolator for each of the 3 output circuits, to ensure electrical isolation. Each output circuit then turns the TTL levels back into DMX. The power supply uses separate transformer windings for each of the 4 circuits, to electrically isolate the supply lines too.
The design was followed exactly, and mounted into a die-cast metal box. The PCB was printed and etched ourselves. All other parts were sourced from CPC. Photos of the finished item can be seen below.
This unit completely solved the issues described for much less than would have cost to buy a commercial equivalent. It has also made connecting the lights on more complicated stages much easier, as I now have three different DMX lines which can each head in a different direction from the splitter.