Nearly every computer has at least one parallel output port. For example, the port to which a printer is attached is a parallel output port. A parallel port has eight electrical contacts which are direclty accessible. The voltage at each of these contacts can be turned on or off by simple program that is entered into the memory of the computer. When a contact is on, 5 volts D.C. is present between the contact and the ground connection of the computer. When the contact is off, zero voltage is present.  
     

Interface Front View
Interface Back View

 

Initial Preparation:
1. Cut the circuit board to the required size on a sheetmetal squaring shear.
2. Clean the copper surface of the circuit board with 2/0 or 3/0 steel wool. Once it is thoroughly clean, do not touch it with your finger tips - the oil from your hands will affect the etching.

  
  Transferring the Circuit Board Pattern:
1. Preheat the press to 325 degrees F.
2. Position the transparency film on the circuit board. Transparent tape may be used to fold it in position.
3. Sandwich the PC board between two layers of kraft paper or paper toweling.
4. Heat the transparency/ circuit board "sandwich" for approximently 5 minutes.
5. Carefully remove the circuit board from the press and allow to cool to room temp.		  
  Etching and Drilling
1. Once cooled, carefully remove the transparency material. All of the detail of the pattern should be adhered to the copper.
2. Fill-in any missing portions of the pattern with a resist pen. A fine-tipped Sanford brand permanent marker works great for this purpose.
3. You will use ferric chloride to etch the copper from the board. Pour about four ounces of the ferric chloride etching solution into a 6" x 8" ZipLoc bag. Seal the bag and place it in a second ZipLoc bag. Seal the second bag. Slowly agitate the solution by rocking the bag back and forth. When all of the copper has been removed in the areas where ther is no resist material, the board is read to remove from the bag. Make sure that is does no "cook" to long.
4. Carefully pour the spent etchant into a plastic bottle for disposal.
5. Blot up any etchant that remains on the board with a rag/ paper towel. Rinse the board in cool water.
6.Drill the holes in the circuit board in accordance with the Component Layout.
7. Solder all components in place following the component layout. Use a heat sink between the soldering iron tip and the diodes, the LED's, and the transistors. A paper clip, needle-nose pliers, or standard heat sink will work.
8. Check to make sure that there are no solder bridges on any connections.
9. Test the interface. Once working properly, you may plug in the power supply.
10. The interface is now complete.