74HC4017 Decade Counter LED Runner

Ten LEDs light up one after another in a tidy parade, driven by a CMOS decade counter — the classic chip used in everything from disco bar graphs to the first LED dice projects.

Difficulty: Breadboard. Estimated build time: about 30 minutes. Estimated parts cost: about US$4.60. 8-line bill of materials. Compare supplier offers when available. A 10-LED running-light strip clocked by a 74HC4017 decade counter.

Build the 555 clock

Wire up an NE555 astable: pin 8 to +5 V, pin 1 to GND, pin 4 to +5 V. Put R1 = 10 kOhm between pin 7 and Vcc, R2 = 47 kOhm between pin 7 and pin 6, and C = 10 uF from pin 6/2 (tied) to GND. Pin 3 is the output clock; with these values it ticks at about 2 Hz, slow enough to see each LED clearly. A 10 nF capacitor from pin 5 to GND filters the control voltage.

Wire the decade counter

Connect the 74HC4017: pin 16 to +5 V, pin 8 to GND, pin 13 (CE) to GND so the chip is always enabled, pin 15 (Reset) to GND so it counts the full ten steps. Take the 555 output (pin 3) and feed it into pin 14 (CLK) of the 4017. The chip's outputs Q0..Q9 appear on pins 3, 2, 4, 7, 10, 1, 5, 6, 9, 11 in counting order — yes, the pinout is the famously scrambled CD4017 order, double-check against the datasheet.

Add the LEDs

On each of Q0..Q9, connect a 330 Ohm series resistor in line with an LED and tie the LED cathode to ground. Power up: you should see exactly one LED on at a time, sweeping cleanly from left to right with each clock tick. Try moving the CD4017 reset pin from GND to Q5 (pin 1) — the counter will reset every five steps for a half-length chase. That's how you build an LED dice (reset on Q6) or a 4-LED battery indicator (reset on Q4).