The 2007 Color Changing Christmas Light Project: DMX Offset Machine
The tricky bit about the official DMX spec is that only 32 devices should be installed in a single daisy chain. If your lighting rig is larger than 32 devices, you need a controller with multiple outputs or something called an 'optosplitter.'
There's a lot of math behind the concept, but it boils down to this: more than 32 devices in one chain could 'load' the data line excessively and compromise the high-speed signal.
An properly designed optosplitter has an input optically and galvanically isolated from the upstream transmitter. It also has one or more outputs optically and galvanically isolated from each other, and from the receiver.
It's a fairly simple design, but because the market for them is so tiny, they're somewhat expensive to buy commercially. However, if you're designing a lighting rig to tour around the world, I'd heartily recommend anything sold by Doug Fleenor Design.
That being said, I needed a sub-$1,000 way of controlling more than 32 pixels from as single DMX generator.
The result is this simple design.
It contains standard XLR-5 input and throughput jacks, which are properly isolated from the rest of the circuit.
It also contains a DMX driver chip and RJ45 jack in the proper pinout for driving strings of pixels. Finally, a set of heavy-duty screw terminals are used to supply power to a pixel string.
So in theory, 32 of these offset machines can each drive 32 pixels, all from a single DMX output on your control system.
Of course, the channel count would be too high, but that's not the point.
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More interesting, I think, is the inclusion of a PIC 16F88 processor, a programming header, and a 9-channel DIP switch.
The processor can be totally bypassed if you'd like. The system will work perfectly without it.
However, the isolated DMX input is connected to the processor's Rx pin and the pixel-drive ouput is connected to the processor's Tx pin.
Thus, you could:
* Omit your control computer / console completely. Store a light show in the PICs memory and let it run forever. Trigger it using serial input to the processor's Rx pin.* Store specific effects in the processor, then trigger them with an external DMX input.
Your imagination is the only limit.
Grab bare circuit boards and fully assembled systems in the online store.
The photos below show an assembled unit. However, the photo was taken before the LEDs, RS-485 drivers and optocoupler were installed.
[click a thumbnail to enlarge]
Bill of Materials:
BYPASS 1x2 .1" pin header (optional)
C1 .1 uF CAP-2,5 C-2,5
C4 100 uF 25V ELC-2,5 ES-2,5
C5 10 uF 15V ELC-2,5 ES-2,5
C6 22 pF CAP-2,5 C-2,5
C7 22 pF CAP-2,5 C-2,5
C8 .1 uF 10V CAP-2,5 C-2,5
D1 1N4001 DIODE-7,5 D-7,5
DC_IN Screw Terminal, .2" pin spacing
DMX 1x3 .1" pin header (optional)
DMX_IN Neutrik NC5MAH XLR connector
DMX_THROUGH Neutrik NC5FAH XLR conector
IC1 7805 regulator, TO-220 package
OK1 6N137 + 8 pin DIP socket
PGM 2x5 .1" pin header (optional)
PIXEL 1x3 .1" pin header (optional)
PIXEL_DRIVE RJ-45 jack
PWR 3mm LED
Q1 20 MHz crystal
R1 300 ohm
R3 1K ohm
R5 100 1/2 w
R7 300 ohm
R9 300 ohm
R23 4.7K ohm
RN1 4.7K ohm resistor network
S1 9 position DIP switch
SER-DEBUG 1x2 .1" pin header (optional)
STATUS 3mm LED
U$2 5V-5V isolated DC-DC converter, 4 pin SIP, Digikey P/N 102-1360-ND
U1 PIC 16F88 DIP-18 package + socket
U2 MAX485/SN75176 DIP-8 package + socket
U3 MAX485/SN75176 DIP-8 package + socket
