A traffic light that counts to five
Red, yellow and green LEDs that cycle like a traffic light on their own. A 555 timer, a decade counter and four diodes.
Free build sheet
The parts, every wire with its length and bend points, a bend guide, both sides of the board and checkboxes to track your progress. Print it or keep it open at the bench.
Open the build sheetWhat you need
15 parts on a perfboard 16 × 33 holes, plus hookup wire.
| Qty | Part | Where to buy |
|---|---|---|
| 1 | Electrolytic Cap10uF C1 | Buy Electrolytic Cap on Amazon |
| 1 | Ceramic Cap100nF C2 | Buy Ceramic Cap on Amazon |
| 4 | Diode (axial)1N4148 D1, D2, D3, D4 | |
| 1 | Power Input (2-pin) J1 | |
| 1 | LEDRed LED1 | Buy LED on Amazon |
| 1 | LEDYellow LED2 | Buy LED on Amazon |
| 1 | LEDGreen LED3 | Buy LED on Amazon |
| 1 | Resistor (axial)4.7k R1 | Buy Resistor (axial) on Amazon |
| 1 | Resistor (axial)68k R2 | Buy Resistor (axial) on Amazon |
| 1 | Resistor (axial)1.2k R3 | Buy Resistor (axial) on Amazon |
| 1 | DIP-8 U1 | |
| 1 | DIP-16 U2 |
Check every pin against the datasheet of the exact part in your hands before wiring.
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Cut list
27 wires, 742 mm (29.2 in) in total, in the order the build sheet cuts them. Wire numbers match the build sheet. Each length includes 5 mm per end to go through the hole.
| Wire | Colour and net | Length | ||
|---|---|---|---|---|
| 18 | BlackVCC I5 → G5 | I5 → G5 | 15mm | Straight |
| 24 | BlackGND L11 → L8 | L11 → L8 | 18mm | Straight |
| 6 | BlackQ3D L19 → J13 | L19 → J13 | 30mm | 20mm (90°) |
| 12 | BlackTIMING K5 → D6 | K5 → D6 | 30mm | 23mm (90°) |
| 11 | BlueTIMING L6 → M6 | L6 → M6 | 13mm | Straight |
| 17 | BlueVCC H6 → F6 | H6 → F6 | 15mm | Straight |
| 23 | BlueGND K13 → M12 | K13 → M12 | 18mm | 8mm (90°) |
| 5 | BlueCLK E6 → F17 | E6 → F17 | 56mm | 13mm (90°), 41mm (90°) |
| 16 | GreenLEDK I29 → I31 | I29 → I31 | 15mm | Straight |
| 4 | GreenRED E25 → H25 | E25 → H25 | 18mm | Straight |
| 22 | GreenGND N19 → L14 | N19 → L14 | 28mm | 18mm (90°) |
| 10 | GreenYELLOW I28 → J17 | I28 → J17 | 40mm | 33mm (90°) |
| 1 | RedRST C14 → E17 | C14 → E17 | 23mm | 13mm (90°) |
| 19 | RedVCC F5 → B9 | F5 → B9 | 30mm | 15mm (90°) |
| 13 | RedTIMING K7 → E10 | K7 → E10 | 33mm | 15mm (90°), 23mm (90°) |
| 25 | RedGND N8 → H5 | N8 → H5 | 33mm | 13mm (90°) |
| 7 | RedQ0D B20 → F13 | B20 → F13 | 38mm | 23mm (90°) |
| 15 | WhiteLEDK G27 → G29 | G27 → G29 | 15mm | Straight |
| 3 | WhiteDISCH H10 → D10 | H10 → D10 | 20mm | Straight |
| 9 | WhiteGREEN I24 → I30 | I24 → I30 | 25mm | Straight |
| 21 | WhiteVCC C10 → C16 | C10 → C16 | 25mm | Straight |
| 27 | WhiteGND O20 → G17 | O20 → G17 | 38mm | 13mm (90°) |
| 20 | YellowVCC J6 → N10 | J6 → N10 | 24mm | Straight |
| 14 | YellowLEDK N25 → I27 | N25 → I27 | 28mm | 18mm (90°) |
| 26 | YellowGND H3 → C6 | H3 → C6 | 30mm | 18mm (90°) |
| 2 | YellowQ1D E19 → E13 | E19 → E13 | 35mm | 10mm (90°), 25mm (90°) |
| 8 | YellowQ2D J19 → G13 | J19 → G13 | 48mm | 13mm (90°), 28mm (90°) |
About this build
A traffic light sequencer built from two chips. An NE555 astable clocks a CD4017B decade counter at about one step a second.
Four 1N4148 steering diodes OR the counter outputs together, so red holds for two counts, green holds for two, and yellow gets one. After the fifth step the counter's next output wires straight back to its reset, so the light loops on its own with no code anywhere.
Only one LED is ever lit, so a single 1.2k resistor on the shared cathode sets the current for all three. It runs from a 9V supply.
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