DMX Demultiplexer

This LeastCost Demux converts up to eight consecutive DMX channels to the corresponding analogue 0-10V signals.

With this circuit you can integrate existing analogue dimmer packs into a new DMX installation.

A DMX Demultiplexer is made of:

1    DMX Transceiver
1-8 low pass filters with levelshifter

DMX-Transceiver (Rev. 3.01)

With this modul DMX data can be received or transmitted.

Because of the complete usage of the RS485 converter, a bidirectional transfer of data is possible (i.e: RDM as described in ANSI E1.20). Till now this feature is only supported by a few expensive devices.

Though the circuit is only made of an AVR and few other parts I recommend a diy etched board.

parts

As you can see, the schematic is quite simple: The heart of the controller is the firmware of the mcu (IC1). The start address is set with SW1. The LEDs indicate the status of the controller. The parts around IC3 are responsable for an exact Vcc of 5.0Vdc. IC2 is the RS485-Transceiver. It allows our controller to communicate via DMX512 with other equipment.

For a dimmer- or switchpack you have to connect the pins of "output" with the "-"pins of the dimmer-/switchmodules because a mcu can drive bigger loads as a current sink. The "+"inputs of all modules must be connected to vcc.

The "spare" port is used for additional pins or to jumper different modes.

AC1&2 have to be connected with a power supply of 9-12V ac or dc. 3-5W should be sufficient.

The following schematic shows you how to connect the transceiver with the DMX bus:

this is the layout:

The board is 48 * 76 mm^2. The resolution of the picture above is 300dpi.

placement:

On the resources site you can find a manual for programming AVRs. The 8MHz crystal has to be selected as clock source by changing the fuse bits.

After changing the fuse bits the transceiver should be flashed with the Demultiplexer Firmware. This program generates high frequency PWM signals from the next eight channels after the start address.

Filter and Shifter

A PWM signal is averaged via R1 and C1 and amplified by IC1A. The OP-Amp doubles the input voltage. C2 prevents the amplification of the PWM ripple after the low pass. I used the quad OP-Amp LM324 in my test circuit. Any other rail to rail OPA should work, too. If the LM324 is used, connect pin4 to +12V and pin11 to GND. The ripple can be decreased by increasing C1 and R1.

Debugging

A change of relevant channels is indicated by a flashing green LED. The red one repeats error codes until the bug is fixed:

Pattern		Error	Solution
	flashing	There is no signal connected to the transceiver.	Connect the transceiver to the DMX bus.
	double flashing	The signal is not accepted as DMX. The transceiver cannot receive all required channels.	Swap D+ and D- at the DMX connector. Transmit more channels or choose a lower start address.