PC Driven LED Display Wiring diagram Schematic

Here is a schema to generate sequentially running light effects using a simple program written in C. The output of the program is taken from the LPT port of a PC and then fed to the interfacing schema for the LED display. The outputs of the interfacing schema are decoded, inverted, and then connected to LEDs through optocouplers.

 Fig. 1: Interfacing schema for PC-driven LED display

The interfacing schema along with the 25-pin parallel port is shown in Fig. 1. IC1 (74LS138) is a high-speed 1-of-8 decoder/demultiplexer. In the schema, only five outputs (pins 10 through 14) of IC1 are used. These outputs are inverted using  NOT gate IC2 (7404). Optocouplers IC3 through IC7 (MCT2E) are used to prevent the schema from damage in the event of short schema in the load.  Thus the loads comprising LED blocks LB1 through LB5 are isolated from the interfacing schema including the PC. Each LED block contains a number of LEDs connected in series.

The LEDs can be arranged, for instance, to display a sequential running light and fountain pot as shown in Figs 2 and 3. The colour of LED blocks can be green and red alternately.

Fig. 2: LED block arrangement for sequentially running light   

 Fig. 3: LED block arrangement for fountain pot

The program can be compiled and run through Turbo C compiler. In the program, the function outport(p,x) is used, where ‘p’ is the address of the controller port and ‘x’ is the value sent to it. Here, controller port  LPT1 is used, whose base address is 378H. If LPT2 is to be used, the base address must be 278H. When the delay time delay() is increased, the running speed of LEDs decreases and vice versa.

In the schema, 100-ohm fixed resistors R6 through R10 can be replaced with 100-ohm or 200-ohm presets for decreasing and increasing the intensity of LEDs.

The program for LED display is as follows:

This schema costs around Rs 100.

Sourced By: EFY Electronics Author:  R. Karthick