Parallel Interface Relay Box

December 19, 2005

This is something I did on a small scale a couple years ago and thought I'd take it a step further. I originally had a full blow circuit drawn out with an 8 bit latch and several D flip-flop ICs giving me 40 outputs from the base 8 from the parallel (excluding controls). After a quick trip to Radio Shack and looking around online, the price was just not worth it. Some of the relays I'm using are around $5 a pop. The circuit is comprised of hooking the following independent circuit fragment to parallel pins 2,3,4,5,6,7,8, and 9.


Parallel pin 18, ground, is common with the ground of the circuit. It is worth noting that the outputs of the relays are inverse with the parallel outputs. So, when a parallel output is on, it is really off. The relays are then used to switch the 120V AC on and off for each of the outputs. A single power supply is used to power the circuit and the 120V AC outputs.

You can find the software controlling this on GitHub. It includes a plugin for XMMS that will control the lights to any song you play in the XMMS media player.

As a side note, I've seen different responses on computer boot up related to the parallel outputs. One of the most drastic i've seen is when I have the relay box on while i'm booting my laptop, a bunch of garbage output is generated for several seconds and the result is undefined- some outputs are on and some are off. For this reason, it's important to clear the outputs with software used to control the box.





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