Here is a circuit 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 circuit for the LED display. The outputs of the interfacing circuit are decoded, inverted, and then connected to LED s through optocouplers. Fig. 1: Interfacing circuit for PC-driven LED display The interfacing circuit 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 circuit, 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 circuit from damage in the event of short circuit in the load. Thus the loads comprising LED blocks LB1 through LB5 are isolated from the interfacing circuit including the PC. Each LED block contains a number of LED s connected in series. The LED s can be arrang...