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# 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                                 |
    ---------------------------------------------+

Below is the key to the diagram above:

    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.

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System Requirement |  | Examples of Relays