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Using tricolor LEDs to design a flashing array

05 Jun 2015  | Jeff Tregre

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You can create a matrix of RGB (red/green/blue) LEDs using a simple and inexpensive circuit comprising the control logic and driver circuit in figure 1 and some LEDs (figure 2). The center RGB LED is the first to come on, after which each sequential LED in the 8×8-LED matrix follows. This process gives the appearance that the display is alive and moving outward. This sequence repeats, producing a rainbow effect of colors.


Figure 1: Three 555 timers generate clock signals, and CD4017 counters provide the drive signals for the transistors.


You can adjust the frequency of each clock by changing the values of R17, R19, and R23. Use different frequencies for each clock, which will display eight colors from the 65 tricolored LEDs, because using the same frequencies for all the clocks causes your display to appear white. The cost of building this circuit should be $25 to $30. You can purchase 100 5-mm RGB LEDs from eBay for a total of about $18. Be sure to use common-cathode LEDs.


Figure 2: The LED in the center lights first, and the light then moves outward until the circuit products an 8×8-LED display.


This simple circuit comprises three clocks and three counters, one for each of the three LED colors. Setting each clock frequency to a different rate causes each color of each LED to appear to be random. All resistors are 0.25W, except for R3, R8, and R13, which are 0.5W; R4, R9, and R14, which are 1W; and R5, R10, and R15, which are 1.5W resistors. These high-wattage resistors and the 12 NPN transistors are necessary because all LEDs in this matrix, except the center one, connect in parallel. Start by bending all of the ground leads flat and connecting them together. When wiring the LEDs, begin in the center and work outward. You can then mount the LED board onto the top of the PCB (printed-circuit board). Figures 3-7 show the circuit in action.


Figure 3: Front side of boards showing 65 LEDs.


Figure 4: Front side of LED board showing more wiring.


Figure 5: Control board with timers, counters, and discrete components.


Figure 6: Control board mounted to LED board.


Figure 7: Side view of control board mounted to LED board.


To add the finishing touches to your project, use a small picture frame and install waxed paper onto the inside of the glass. Mount the LED board ¼ to 1 in. away. The magnifying lens of the LEDs will produce a beautiful effect when they shine through the waxed paper.


About the author
Jeff Tregre contributed this article.


This article is a Design Idea selected for re-publication by the editors. It was first published on June 24, 2010 in EDN.com.




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