How To Build Parallel Port Prototypes

Power Interface

If you need to control other devices than LEDs, you will need to project and build a power interface. The basic idea is to connect a transistor acting as a switch at each data output, and this transistor switching on or off the device you want to control. If you want to control AC circuits – standard light bulbs and home appliances, for example – you will need use a relay. Relay is a switch that turns on whenever current flows through it.

You can see a basic power interface in Figure 9. You will need to repeat this circuit for every bit you want to use, i.e., if you want to use the eight bits provided by the parallel port in order to control up to eight AC circuits you will need to repeat this circuit eight times, one for each data bit.

You will need an external power supply with the same voltage as your relay. So, if you use a 12 V relay, you will need to have a 12 V power supply connected to +Vcc and ground. “AC Power” is the power cord to be connected to an AC outlet on your wall and “AC Outlet” is an AC outlet in your circuit were the light bulbs or home appliances will be connected to.

Figure 9: Power interface.

The diodes work as protections and even though we recommend 1N4148 any other general-purpose diode will work just fine. The same goes to the transistor, we recommend BC547 but any general-purpose transistor will work fine as well.