Simple LED driving circuits

Simple LED driving circuits

You can make simple circuit for driving a small led through PC parallel port. The only components needed are one LED and one 470 ohm resistors. You simply connect the diode and resistor in series. The resistors is needed to limit the current taken from parallel port to a value which light up acceptably normal LEDs and is still safe value (not overloading the parallel port chip). In practical case the output current will be few milliampres for the LED, which will cause a typical LED to somewhat light up visibly, but not get the full brigtness.

LED and resistor

Then you connect the circuit to the parallel port so that one end of the circuit goes to one data pin (that one you with to use for controlling that LED) and another one goes to any of the ground pins. Be sure to fit the circuit so that the LED positive lead (the longer one) goes to the datapin. If you put the led in the wrong way, it will not light in any condition. You can connect one circuit to each of the parallel port data pins. In this way you get eight software controllable LEDs.

LEDs wired to parallel port

The software controlling is easy. When you send out 1 to the datapin where the LED is connected, that LED will light. When you send 0 to that same pin, the LED will no longer light.

Here are two photos of circuit above I have built:

LEDs wired to parallel port circuit photo 1
LEDs wired to parallel port circuit photo 2

Pn those circuits I have wired the ground wire only to one ground pin (it works also well, you can use any of the ground pins).

Technical specifications of parallel port pins

The IBM specifications says accoding the following: The data output pins (pins 2-9) sink 24 mA, source 15 mA, and their high-level output is min. 2.4 V. The low state for both is max. 0.5 V. Pins 1, 14, 16, and 17 (the control outputs) have open collector drivers pulled to 5 V through 4.7 kiloohm resistors (sink 20 mA, source 0.55 mA, high-level output 5.0 V minus pullup). Non-IBM parallel ports probably deviate from this standard.

Warning: Be careful with grounding. You can break parallel ports by connecting devices to them when PC is powered on. It is not a good idea to short the pins to ground or +5V, this can damage the port. It might be a good thing to use a parallel port not integrated on the motherboard for things like this. (You can usually get a second parallel port for your machine with a cheap standard `multi-I/O’ card)

Control program for DOS (and Win9x)

The following program is an example how to control parallel port LPT1 data pins from your software. This example directly controls the parallel port registers, so it does not work under some multitasking operating system which does not allow that. It works nicely under MSDOS. You can look the Borland Pascal 7.0 code (should compile also with earlier versions also) and then download the compiled programLPTOUT.EXE. This has worked nicely for me in DOS systems and Windows 95/98 systems. On recent testings this program has worked unreliably on some Windoes 2000 systems.

Program lpt1_output;

Uses Dos;


Programming I/O


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