Advertisement
Online Test Banks
Score higher
See Online Test Banks
eLearning
Learning anything is easy
Browse Online Courses
Mobile Apps
Learning on the go
Explore Mobile Apps
Dummies Store
Shop for books and more
Start Shopping

Digital Electronics: How to Design a Parallel-Port Circuit

Each of the eight data output pins on a parallel port provides 5 V DC to your electronic circuit. That amount of voltage can source about 10 mA or 12 mA — enough to drive an LED directly. Alternatively, you can connect the data output to the base of a switching transistor, which allows you to control circuits that require more current.

The first circuit drives an LED directly. A current-limiting resistor is required to prevent the LED from pulling too much current and damaging the LED, the parallel port itself, or both.

The second circuit shows how to use a transistor to switch a circuit by using the output from a parallel-port data pin. As you can see, the data pin is connected through a resistor to the transistor’s base. When the data pin goes high, the transistor is turned on, allowing current to flow through the collector-emitter circuit.

Two ways to connect an LED to a parallel-port output pin.
Two ways to connect an LED to a parallel-port output pin.

Using a transistor driver this way provides two benefits:

  • The transistor can switch more than the 10–12 mA that the parallel port’s data pin can source directly.

  • The collector-emitter circuit can operate at a different voltage from the one that the data pin’s +5 V HIGH signal provides.

  • Add a Comment
  • Print
  • Share
blog comments powered by Disqus
Advertisement
Advertisement

Inside Dummies.com

Dummies.com Sweepstakes

Win an iPad Mini. Enter to win now!