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PIC18 Four-Channel DMX Relay Controller

Country/Region china
Company bikerglen limited
Categories Digital Timers
Telephone 11111111111
ICP License Issued by the Chinese Ministry
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    PIC18 Four-Channel DMX Relay Controller

    Driving a 48-Segment RGB LED Bar Graph with a Teensy 3.2

    PIC18 Four-Channel DMX Relay Controller

    Posted on October 3, 2017 by Glen

    A four-channel DMX relay controller based on a PIC18F1320.

    Halloween was right around the corner and I needed a timer with a bunch of relays to trigger some store-bought props and a fog machine periodically. (Mental note: read fog machine specs carefully—not all come with timer remotes.) My first thought was an Arduino and cheap relay board. Second thought was to build something with a micro and some relays. Third thought was that if I’m going to build something, might as well add DMX and package it up into a neat enclosure. Hence, the four channel DMX-controlled relay project was born.

    Design Parameters

    My go to 8-bit micro is a PIC18F1320. I’ve used them on tons of projects, all the development tools are already installed on my computer, and I’m pretty familiar with their idiosyncrasies and peripherals. I’ve also already written both DMX transmitter and DMX receiver code for this micro. The other micros I use a lot are all PIC24s but a 16-bit micro is overkill for this project. PIC18F1320 it is!

    Another constraint on this design was to have something that easily fits in an enclosure. I’ve been using Hammond extruded aluminum enclosures for a while now too. They’re well made and look sharp. One of their smaller enclosures holds a 50mm by 80mm PCB which should be plenty of room for this design. Using the smallest enclosure possible reduces the cost of the board, the enclosure, and the end panels.

    One last constraint that bears mentioning is the relay selection. I’m only switching small low-voltage, low-current loads. The Omron G5V-1 series of relays are physically small and capable of switching up to 1A at 24VDC. This matched the sorts of loads I was expecting to switch and four of them would fit on my PCB.

    Design decisions made: use a PIC18F1320, use a Hammond 1455C801 enclosure, and use Omron G5v-1DC5 relays.

    Schematic

    Here’s the schematic for the DMX relay controller. The controller consists of a PIC 18 microcontroller, a crystal oscillator, a power supply, a programming connector, a DMX interface, some relays and their drivers, and an illuminated pushbutton.

    DMX relay board schematic.

    I had previously built a few PIC18 projects that used pulse width modulation (PWM) to dim four channels of LEDs with 10-bit resolution. Those projects were powered from 24V and ran the PIC18 as fast as it would run (FCY=40MHz) so there would be plenty of time to service the very frequent interrupts while still listening to the serial port for DMX data. I started with that hardware and software as the basis for this new design but swapped out the LED drivers for some relay drivers and relays.

    The PIC18F1320 is capable of running at 40MHz at 5V with a 10MHz crystal oscillator. There’s also a PIC18LF1320 variant that will run at 24MHz at 3.3V with a 6MHz crystal oscillator. Either would work for this design. I went with the 5V version and made this a 5V-only design simply because I’ve had an easier time finding stock, off-the-shelf crystal oscillators in my preferred package for hand soldering that run at 5V than 3.3V.

    For an oscillator, I picked a 10MHz, 5V ECS part that I’ve used on past designs. It’s a little big but it’s also very easy to solder by hand. I’m running the PIC18 oscillator in its HSPLL oscillator mode. With the 10MHz crystal, the PIC18 runs with an FCY of 40MHz—way faster than needed for this project because the relays are only either on or off and creation of PWM outputs in an interrupt service routine is not required.

    For a power supply, I typically use Cui V7805-500 or Cui VXO7805-500 switching DC-DC converters. Both have a very wide input voltage rangefrom about 6 to 30+ volts. The wide input voltage is important if controlling, for example, a strand of five or six blue or green LEDs wired in series. In this example, the LEDs would be powered from +24V and then the +24V would be connected to the input of the regulator to generate +5V for the microcontroller. Using a cheap linear regulator such as the LM7805 with a 24V input would result in at least burned fingers and quite possibly a failure of the regulator without a proper heatsink.

    The Cui regulators can supply 500mA of current while still running cool to the touch. The relays and the rest of the design only consume about 160mA so plenty of margin. As with most regulators, the CUI parts require some capacitors on both the inputs and outputs for stability. I designed the board to hold one of these switching regulators but ultimately decided not to stuff the component and fed the board directly from a 5V power supply connected to the power connector.

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