CD4017 10-LED Knight Rider Chaser
A 555 clock and a CD4017 decade counter give you a marquee chaser of ten LEDs in sequence.
Difficulty: Through-hole. Estimated build time: about 40 minutes. Estimated parts cost: about US$4.50. 9-line bill of materials. Compare supplier offers when available. A classic showcase of CMOS digital logic: a 555 timer running at a few hertz feeds clock pulses into a CD4017 decade counter, and the CD4017's ten outputs Q0–Q9 light up sequentially, one at a time, in a marching pattern.
Build a slow 555 clock
Wire a 555 astable with R1 = 10 kΩ, R2 = 100 kΩ, C = 1 µF. Frequency ≈ 1.44 / ((10k + 200k) × 1µF) ≈ 6.8 Hz — a comfortable chase speed. Reset (pin 4) tied to Vcc, control (pin 5) bypassed with 10 nF.
Place the CD4017
Insert the CD4017 across the breadboard divider. Vcc is pin 16, GND is pin 8. Critically: tie pin 13 (clock enable, active-low) to GND, otherwise the counter ignores its clock. Tie pin 15 (reset) to GND for a full 10-step chase.
Connect the clock
Wire 555 pin 3 (output) to CD4017 pin 14 (clock). The CD4017 advances on each rising edge of pin 14. If the chase looks erratic, place a 100 nF cap across the CD4017's V+/GND for supply decoupling.
Wire the ten LEDs
The CD4017 outputs in physical order are Q0=pin 3, Q1=pin 2, Q2=pin 4, Q3=pin 7, Q4=pin 10, Q5=pin 1, Q6=pin 5, Q7=pin 6, Q8=pin 9, Q9=pin 11 — yes, the layout is non-sequential, blame the 1968 silicon. Connect each through a 470 Ω resistor to its own LED (anode to chip output, cathode to GND).
Speed knob and length trim (optional)
Replace the 555's R2 with a 100 kΩ potentiometer to add a 'speed' knob. To make a shorter chase (e.g. 5 steps), tie the corresponding output back to pin 15 (reset) — the counter will reset the instant that output goes high, so it never reaches step 5. Try connecting Q6 to reset for a six-step chaser.