10-LED Chaser Circuit with 555 Timer and CD4017 Decade Counter
An LED chaser lights a row of LEDs one after another, giving the effect of a moving light. Festival decorations and shop signs use the same principle. This project combines two classic chips: an NE555 timer that produces a steady stream of clock pulses, and a CD4017 CMOS decade counter that moves a single high output along its ten outputs with every pulse.
Beyond the visual effect, the project teaches how a clock source and a counter work together, the core idea behind every sequential digital system. You will also see how to change the speed, shorten the sequence and drive brighter loads.
Components Required
| Part | Value / Type | Quantity |
|---|---|---|
| Timer IC U1 | NE555 | 1 |
| Counter IC U2 | CD4017B | 1 |
| IC sockets | 8-pin and 16-pin | 1 each |
| RA | 1 kilo-ohm | 1 |
| RB (fixed) | 10 kilo-ohm | 1 |
| VR1 (speed) | 100 kilo-ohm potentiometer | 1 |
| C1 (timing) | 10 microfarad electrolytic | 1 |
| C2 (pin 5) | 10 nanofarad | 1 |
| C3 (decoupling) | 100 nanofarad ceramic + 100 microfarad electrolytic | 1 each |
| R_reset | 10 kilo-ohm (pin 15 to GND) | 1 |
| LED resistors | 1 kilo-ohm | 10 |
| LEDs | 5 mm, any colour | 10 |
| Power | 9 V battery or 9 to 12 V DC adapter | 1 |
Circuit Explained Stage by Stage
Stage 1: 555 astable clock
The 555 is wired as an astable multivibrator: RA from +V to pin 7, RB plus VR1 from pin 7 to the joined pins 6 and 2, and C1 from pins 6 and 2 to ground. C1 charges through RA and RB until it reaches two thirds of the supply, then discharges through RB into pin 7 until it falls to one third. The output on pin 3 is a square wave whose frequency is set by VR1.
f = 1.44 / ((RA + 2 x (RB + VR1)) x C1) VR1 = 0: f = 1.44 / (21 k x 10 uF) = about 6.9 Hz VR1 = 100 k: f = 1.44 / (221 k x 10 uF) = about 0.65 Hz
So the light steps along roughly seven times a second at the fastest setting and about once every 1.5 seconds at the slowest.
Stage 2: CD4017 decade counter
The CD4017 has ten decoded outputs, Q0 to Q9. Only one is high at a time. On each rising edge at the clock input (pin 14) the high output moves to the next one, and after Q9 it returns to Q0. Pin 13 is the clock inhibit (or enable) input; it must be low for counting. Pin 15 is reset; a high level forces Q0 high, so it is held low through a 10 kilo-ohm resistor. Pin 12 is the carry out, useful for cascading counters.
| CD4017 pin | Function | Connection |
|---|---|---|
| 3, 2, 4, 7, 10 | Q0, Q1, Q2, Q3, Q4 | LED 1 to LED 5 via 1 kilo-ohm |
| 1, 5, 6, 9, 11 | Q5, Q6, Q7, Q8, Q9 | LED 6 to LED 10 via 1 kilo-ohm |
| 8 | VSS | GND |
| 12 | Carry out | Not connected |
| 13 | Clock inhibit | GND |
| 14 | Clock | 555 pin 3 |
| 15 | Reset | 10 kilo-ohm to GND |
| 16 | VDD | +V |
Stage 3: LED outputs
Each output drives an LED through a 1 kilo-ohm resistor to ground. CMOS outputs have limited drive and their output voltage drops as current rises, so a few milliamps per LED is the realistic range. Since only one LED is on at a time, the total current is small and a 9 V battery runs the circuit for many hours.
+V --RA--+-- 555 pin 7
|
RB + VR1
|
+-- 555 pins 2,6 --C1-- GND
555 pin 3 ---------> 4017 pin 14 (CLK)
4017 Qn ---1k---|>|--- GND (x10)
4017 pins 13, 8 to GND; pin 15 --10k-- GND; pin 16 to +VHow It Works
At power-up the counter starts at a random or zero state and quickly settles into the sequence. Each clock pulse from the 555 advances the counter by one, so the lit LED walks from LED 1 to LED 10 and wraps around. Turning VR1 changes the 555 frequency and therefore the chasing speed.
Shortening the sequence
To use fewer than ten LEDs, connect the output after your last LED to reset (pin 15). For a six-LED chaser, connect Q6 (pin 5) to pin 15 and leave the 10 kilo-ohm pull-down in place, since the output easily overrides it. The moment Q6 goes high the counter resets to Q0, so Q6 is never visible.
Brighter loads
For high-brightness LEDs or LED strips, drive a BC547 or a logic-level MOSFET from each output through a base or gate resistor, and let the transistor carry the LED current.
Assembly and Testing
- Insert both sockets. Wire power and ground to 555 pins 8 and 1 and CD4017 pins 16 and 8, with decoupling capacitors close to each chip.
- Build the 555 astable and test it with one LED and 1 kilo-ohm resistor on pin 3. It should blink, and VR1 should change the rate.
- Fit the CD4017. Tie pin 13 to ground and pin 15 to ground through 10 kilo-ohm. Connect 555 pin 3 to pin 14.
- Add the ten LEDs in the physical order Q0 to Q9 using the pin table, since the output pins are not in numeric order around the chip.
- Power up and check the light moves smoothly along the row.
Troubleshooting
- Only the first LED stays on: reset (pin 15) is high or the clock is missing. Check the pull-down and measure pin 14 with an LED probe.
- Nothing counts, random LEDs on: pin 13 is floating. CMOS inputs must never be left open; tie it to ground.
- LEDs skip or jump in order: wiring follows pin numbers instead of output order. Re-check against the table.
- Counter skips several steps at once: noisy clock edges or poor decoupling. Add the 100 nanofarad capacitor right at pin 16.
- LEDs very dim: the supply may be low, or the resistors too large. Do not go below about 470 ohm at 9 V; use transistor drivers instead.
Summary
The 555 supplies the clock, the CD4017 turns each clock pulse into a step along ten outputs, and the LEDs make the sequence visible. You calculated the clock range from the astable formula, learnt the CD4017 pinout including clock inhibit and reset, and saw how to shorten the count and drive heavier loads.