Quick answer: An LED chaser uses a 555 astable timer to generate clock pulses and a 4017 counter to select successive LEDs. The counter has ten decoded outputs, with one HIGH at a time, so the illuminated position moves along the sequence. (Texas Instruments timer datasheet, Nexperia counter datasheet)
This guide covers a low-voltage, battery-powered or regulated-DC chaser: choosing the supply, setting the speed and calculating LED resistors. The counter specifications below apply to Nexperia's HEF4017B; do not transfer those electrical ratings to every part sold as a 4017.
How the timer and counter make a running light
The timer sets the pace; the counter selects the position. The HEF4017B is a five-stage Johnson decade counter with outputs Q0 through Q9. “Decade” describes its ten decoded positions, not ten internal counter stages. (Nexperia HEF4017B datasheet)
According to Nexperia, a rising edge at CP0 advances the counter when CP1 is LOW. Driving master reset, MR, HIGH returns it to Q0 HIGH and Q1–Q9 LOW regardless of the clock. Thus a clock pulse advances the selected position; reset returns it to the beginning. (Nexperia clock and reset behavior)
In the astable timer, the capacitor charges through RA plus RB and discharges through RB alone. Its voltage moves between approximately one-third and two-thirds of the supply, producing the repeating clock waveform. (Texas Instruments NE555 astable operation)
What supply voltage should you use?
Choose a voltage within both chips' operating ranges. A counter's minimum supply does not establish the minimum for the complete chaser.
| Part or combination | Supply range | Meaning for this circuit |
|---|---|---|
| Nexperia HEF4017B | 3–15 V | According to Nexperia, this is its recommended operating range. (Source) |
| TI NE555 bipolar timer | 4.5–16 V | A bare 3 V source is below this timer's minimum. (Source) |
| Shared supply, derived from those ranges | 4.5–15 V | Lower bound = max(3, 4.5); upper bound = min(15, 16). (Nexperia, TI) |
The overlap establishes supply compatibility, not LED drive capability. Size the LED branches separately, and use the voltage available at the circuit when calculating their resistance.
How do you calculate the chase speed?
Each counter step takes one complete clock period. For the timer, TI gives these approximate astable equations, with resistance in ohms and capacitance in farads. (Texas Instruments timing equations)
- HIGH time: tH ≈ 0.693 × (RA + RB) × C.
- LOW time: tL ≈ 0.693 × RB × C.
- Period: T ≈ 0.693 × (RA + 2 × RB) × C.
- Frequency: f ≈ 1.44 ÷ ((RA + 2 × RB) × C).
Worked timing example with assumed inputs
Assumed inputs: RA = 1 kΩ = 1,000 Ω, RB = 47 kΩ = 47,000 Ω, C = 10 µF = 0.000010 F, and N = 10 LED positions. Substituting into TI's equations gives:
- tH ≈ 0.693 × (1,000 + 47,000) × 0.000010 = 0.693 × 48,000 × 0.000010 = 0.33264 s.
- tL ≈ 0.693 × 47,000 × 0.000010 = 0.32571 s.
- T ≈ 0.33264 + 0.32571 = 0.65835 s.
- f ≈ 1.44 ÷ ((1,000 + 2 × 47,000) × 0.000010) = 1.44 ÷ 0.95 ≈ 1.52 Hz.
- Full chase time = N × T ≈ 10 × 0.65835 = 6.5835 s, or about 6.58 s.
The selected LED stays on for about 0.66 s, the complete clock period, rather than only the timer's HIGH time. These are calculated values, not measurements; the slightly rounded coefficients also mean the frequency equation and 1/T are not exactly identical. (TI equations, Nexperia edge-triggered counting)
Increasing RB or C lengthens the period. With the other assumed inputs unchanged, doubling C gives T ≈ 2 × 0.65835 = 1.31670 s, so the chase runs at half the previous step rate. This follows directly from TI's period equation. (Source)
Can the counter drive LEDs directly?
Use low-current LED branches for direct drive, or transistor buffers for higher current. According to Nexperia, at a 5 V supply and 25 °C the HEF4017B guarantees at least 1.4 mA of HIGH-level source current at an output voltage of 2.5 V. That is a specified operating point, not a universal current ceiling or a promise that the output remains at the supply voltage. (Nexperia static characteristics)
The site's red-LED preset uses 2.0 V and 20 mA as calculator planning figures, not universal LED ratings. That preset current is not justified for direct counter drive by the operating point above. A buffered design lets the counter select a transistor-controlled LED branch; its resistor calculation must include the buffer's voltage loss.
According to Wikipedia's LED overview, LEDs require current regulation, commonly a series resistor for indicator LEDs. Give each independently driven branch its own calculated resistor. (LED current regulation)
Worked resistor example with assumed inputs
Assumed inputs: supply = 9 V, one red LED with forward voltage = 2.0 V, target current = 10 mA, and an ideal buffer voltage loss = 0 V. The forward voltage adopts the calculator's planning figure; the ideal buffer assumption makes this a calculation example rather than a finished transistor circuit.
The LED resistor calculator uses R = (supply voltage − total LED forward voltage) ÷ current:
- Current = 10 ÷ 1,000 = 0.010 A.
- Resistor voltage = 9 − 2.0 − 0 = 7 V.
- Required resistance = 7 ÷ 0.010 = 700 Ω.
- The calculator's nearest E24 choice is 680 Ω: its difference is 700 − 680 = 20 Ω, versus 750 − 700 = 50 Ω for the adjacent higher choice.
- Actual calculated current = 7 ÷ 680 = 0.0102941 A ≈ 10.29 mA.
- Resistor power = I² × R = (7 ÷ 680)² × 680 = 49 ÷ 680 ≈ 0.0721 W.
- The calculator's planning margin is twice the dissipation: 2 × (49 ÷ 680) ≈ 0.1441 W. Its next sufficient listed rating is 0.25 W.
The nearest resistor raises current above the assumed target. If the target is a ceiling, the higher choice gives I = 7 ÷ 750 = 0.009333 A ≈ 9.33 mA. Recalculate with the actual buffer loss before selecting parts.
How do you shorten the sequence and check the circuit?
Use master reset to end a shortened sequence. For an assumed six-position chaser, use Q0–Q5 and connect the first unused output, Q6, to MR. This is a common technique that follows from the reset behavior described by Nexperia (the datasheet does not spell out this recipe): HIGH at MR restores Q0 and clears the other decoded outputs. (Nexperia reset behavior)
Check the functional blocks in this order:
- Confirm the supply fits the overlapping ranges above.
- Confirm the clock reaches CP0 and CP1 is LOW for rising-edge counting, according to Nexperia. (Clock conditions)
- Check MR: holding it HIGH keeps the selected position at Q0, according to Nexperia. (Reset conditions)
- Give unused inputs a defined connection; according to Nexperia, they must connect to VDD, VSS or another input. (Input requirements)
- Check each LED branch against its calculated current and resistor power.
For a remote DC supply, the voltage drop calculator calculates cable loss using Vdrop = I × R and includes both outgoing and return conductors. Use the wire gauge calculator to compare conductors against the whole circuit's supply current and your selected voltage-drop allowance.
FAQ
Do I need a microcontroller?
No. The timer's astable clock and the counter's ten decoded positions provide the basic repeating sequence. (TI timer, Nexperia counter)
Will this work from a 3 V battery?
Not with the specified bipolar NE555: its minimum supply is 4.5 V. The counter's lower minimum does not remove that restriction. (Texas Instruments supply limits)
Why is only the first LED staying on?
Check reset first. According to Nexperia, HIGH at MR holds Q0 HIGH regardless of clock activity; that is one concrete explanation for a stationary first position. (Nexperia reset behavior)
Does changing the supply set the chase speed?
Use the timing resistors or capacitor to set the speed. TI describes the astable charge and discharge times, and therefore frequency and duty cycle, as independent of supply voltage. (Texas Instruments astable operation)
Does a shorter sequence make each LED step faster?
No: shortening the sequence changes the number of positions, not the clock period. With the earlier assumed T = 0.65835 s and N = 6, full chase time = N × T = 6 × 0.65835 = 3.95010 s, about 3.95 s.
Jack Shi
Founder & editor, LEDaskJack Shi builds and writes LEDask, an independent LED-lighting tools project operated by clooms. He designs the calculators, checks their formulas and reference values against published engineering data, and writes the guides across the site.
