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Design LED-lighting patterns without controller

05 Aug 2015  | Jeff Tregre

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This design idea describes a simple LED-lighting-effects circuit comprising only five chips and costing only a few dollars. When you first observe the circuit in action, you will think that it uses a PIC (peripheral-interface-controller) chip requiring you to program hundreds of lines of code to generate the lighting effects.

The circuit comprises seven functional blocks (figure 1). IC1 is an LM556, which has two 555 timers in one package. The first timer produces the main clock frequency of approximately 0.105 Hz. It toggles high to low approximately every 10 seconds. The polarity of the clock's signal changes the frequency of the VCO (voltage-controlled oscillator) that makes up the other half of IC1 from low to high. Resistor R2 and capacitor C2 set the clock frequency. Changing either component changes the frequency.


Figure 1: Two 555 timers create the clock pulses that drive blue and red LEDs.


The output from the first 555 timer feeds the control voltage input on the second 555 timer, letting it function as a VCO whose output frequency ranges from approximately 10 Hz when the first 555 timer output is high to approximately 33 Hz when the output is low. Components R4 and C3 set the VCO's frequency, and R6 and C4 control the smooth transition of the VCO from 10 to 33 Hz.

LED3, C5, and R8 act as a self-start circuit. Without it, you would need to add a pushbutton switch to toggle the data input of IC3A from low to high during start-up. IC2, a CD4070 quad exclusive-OR gate, acts as a pseudorandom-data generator. This circuit gives the illusion that bits of data span the bar graph.


Figure 2: The bar graphs have both red and blue LEDs; turning on both yields violet.


IC3 and IC4 are CD4015 four-stage shift registers cascaded together. The data bits span the bar-graph displays in sequence from Output 1 to Output 16. IC5, an LM555 timer, produces Clock 2's frequency of approximately 0.144 Hz. The inverse of this frequency toggles high to low approximately every 7 seconds, feeding the gates of N-MOSFET Q1 and P-MOSFET Q2, which act as the red/blue/violet LED-display driver. Clock 2 toggles high, enabling Q1 and giving the blue LEDs a source to ground.

When Clock 2 toggles low, Q2 turns on, giving the red LEDs a path to ground. C6 and C7, together with R9 and R10, respectively, act as a slow discharge circuit on the gates of the MOSFETs, keeping them on for approximately 2 seconds longer than Clock 2's pulse. The delay lets both the blue and the red LEDs be on at the same time for approximately 2 seconds and produces the color violet. This circuit uses N- and P-channel MOSFETs from STMicroelectronics, but any general-purpose MOSFET should work. Just make sure that each one can handle at least 0.5A.

The four-segment red/blue-LED bar-graph displays are unique. Each bar-graph display comprises one red and one blue LED in the same bar (figure 2). Each LED has its own anode and cathode connections, thereby keeping this circuit simple without the need to add extra transistor drivers for each LED. You'd have to add them if their anodes, cathodes, or both were connected. This circuit requires four bar-graph displays. If you install any of the LED bar graphs backward, you will see the second color displayed, so that, if you were expecting red, you would get blue, and vice versa.


About the author
Jeff Tregre is with BuildingUltimateModels.com in Dallas, TX.


This article is a Design Idea selected for re-publication by the editors. It was first published on March 18, 2010 in EDN.com.




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