Step 1 : It's Use & Parts


Use this circuit to isolate, & control up to 8 mechanical relays from your PC's parallel port.

The parts needed are:

A short lenght of 9 conductor wire.
1 - male DB25 parallel connector
1 - pcb perfboard
1 - 4.7k SIP (resistor pak)
1 - ULN2803A IC
8 - relays (total current draw not to exceed 500 ma.)
1 - 12 volt Wall wart power supply

Optional parts:

IC socket (18 pin)
13 - dual pcb wire barrier connectors.

All the parts can be ordered through www.jameco.com or other parts suppiers.

Step 2 : The Layout and Wiring



Here is the layout and wiring of the perfboard looking down from the top of the board through it as if you had x-ray vision. Take note pin 10 (upper pin of the SIP is not used. I started by dry positioning each component for fit then flipped the board over and soldered the SIP and IC socket followed by the pcb wire connectors. I then added the connecting wires between the components. As you can see the majority of the wiring is straight across the components.

Now prepare the PC parallel cable by wiring one wire each from the 9 conductor cable to the following pins of the DB25 connector: 2, 3, 4, 5, 6, 7, 8, 9, and the last conductor to pins 18-25 (ground). Keep track of the color of the wire you use on each pin.

You could purchase a ready made cable here and cut off the printer end of it but you would have a lot of unused wires. Or you could re-due the basic board to include the appropriate connector, maybe DB25 to DB9 and again purchase a cable. I chose the make my own for cost savings and once made up you don't need to change it out.

Step 3 : The Hookup



Turning your attention to the un-used end of the 9 conductor wire, connect the wire from pin 2 of the DB25 to the pcb binding post #2 and so on down the line with the remaining 7 wires. You are now left with one wire - the ground that connected to pins 18-25 of the DB25. Connect this wire to the minus(-) pcb binding post. Also connect your power supply minus to this post. The power supply positive connects to the plus(+) pcb binding post.

The relays connect to pcb binding posts A thru H. One relay coil wire to the binding post left side of A, and one to the right side of A. Continue to connect each relay coil in this manner at B,C,D,E F,G, and H. Mount the relays in a manner of your own chosing.

Now all that's left to do is write / obtain some software to control the ULN2803A board. There are many examples of code bits on the web and several of the folks on this site have posted information on it.

Good luck with your project ! ! !