3. Electronic Circuit

The data line of the parallel port can provide between 0 and 5 volts. Therefore, we can use this line to control a relay and let that relay control an electrical device.

Use pins 2 through 9 to get +5V

Use pins 18 through 25 as the Ground

 ____________________________________________________________ / \ \ 13 12 11 10 9 8 7 6 5 4 3 2 1 / \ / \ 25 24 23 22 21 20 19 18 17 16 15 14 / \______________________________________________________/ | Vcc for Relay | | / -------+--------| |---------| P data 4.7K B / c | 1N4002 |_| |------- -----------/\/\/\/\---------+-----| NPN ___ | relay |home device | \ e __\_/__ |-| |------- LED X \ |--------| |---------| P Ground | | | ----------------------------+-------+--------+ Relay Ground | ---------------------------------------------+

Key to the Above Diagram

 P data parallel port data line(pin 2 to pin 9, use any) P Ground parallel port ground (pin 18 to pin 25, use any) Vcc depends on the type of your relay. I'm using 12VDC Relay Ground the ground for 12VDC 4.7k 4.7K resistor NPN C2235 NPN transistor 1N4002 1N4002 diode LED light emitting diode relay a coil relay

+5 volts from parallel port will turn on the relay. Once the relay is on, the device connected to that relay will be on. When the parallel port's data line goes back to 0, it will turn off the relay, and device is off.

I wrote a simple program that can control 8 relays. The program is available at the following location and will need g++ in order to be able to use this program. http://home.att.net/~ruili/goldencat/paralle.tar.gz

A complete circuit can control up to eight relays, that means that a complete circuit can control eight different devices. You will need a DB25 connector.Using a parallel switch cable, connect this relay board to your computer. I put the relay board in a box and use a parallel switch cable to connect it to my Linux box:

 Panel: LED, Switch Back: DB25 connector

The following sites contain some photos of the circuit:

• http://home.att.net/~ruili/goldencat/p1.jpg

• http://home.att.net/~ruili/goldencat/p2.jpg

• http://home.att.net/~ruili/goldencat/p3.jpg

The following URL shows a complete electronic circuit (control 8 devices): http://home.att.net/~ruili/goldencat/01.png