A railroad crossing light, from two transistors
Two red LEDs that take turns flashing like a railroad crossing signal, about fifty times a minute. Two transistors, no chip.
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
11 parts on a perfboard 15 × 27 holes, plus hookup wire.
| Qty | Part | Where to buy |
|---|---|---|
| 2 | Electrolytic Cap33uF C1, C2 | Buy Electrolytic Cap on Amazon |
| 1 | Power Input (2-pin) J1 | |
| 2 | LEDred LED1, LED2 | Buy LED on Amazon |
| 2 | NPN Transistor (TO-92)2N3904 Q1, Q2 | Buy NPN Transistor (TO-92) on Amazon |
| 2 | Resistor (axial)33k R1, R2 | Buy Resistor (axial) on Amazon |
| 2 | Resistor (axial)470 R3, R4 | Buy Resistor (axial) on Amazon |
Check every pin against the datasheet of the exact part in your hands before wiring.
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Cut list
14 wires, 329 mm (13.0 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 | ||
|---|---|---|---|---|
| 6 | BlackGND J19 → F19 | J19 → F19 | 20mm | Straight |
| 12 | BlackBASE1 G21 → K25 | G21 → K25 | 24mm | Straight |
| 11 | BlueBASE1 E16 → E21 | E16 → E21 | 23mm | Straight |
| 5 | BlueGND I5 → I20 | I5 → I20 | 48mm | Straight |
| 4 | GreenCOL2 J23 → L23 | J23 → L23 | 15mm | Straight |
| 10 | Green5V E10 → H5 | E10 → H5 | 30mm | 18mm (90°) |
| 7 | Red5V D11 → D9 | D11 → D9 | 15mm | Straight |
| 13 | RedBASE2 K16 → K21 | K16 → K21 | 23mm | Straight |
| 1 | RedCOL1 D16 → D23 | D16 → D23 | 28mm | Straight |
| 9 | White5V L11 → L9 | L11 → L9 | 15mm | Straight |
| 3 | WhiteCOL2 L16 → K22 | L16 → K22 | 28mm | 20mm (90°) |
| 2 | YellowCOL1 E24 → E22 | E24 → E22 | 15mm | Straight |
| 8 | Yellow5V F11 → J11 | F11 → J11 | 20mm | Straight |
| 14 | YellowBASE2 I21 → E25 | I21 → E25 | 24mm | Straight |
About this build
The alternating red flash of a railroad crossing, with no chip and no code. Two 2N3904 NPN transistors cross couple through two 33uF caps, so the two red LEDs take turns. When one transistor switches on, its cap holds the other one off until that cap recharges through a 33k base resistor, and then they swap.
It comes out to about fifty flashes a minute, inside the 35 to 65 per minute range the US MUTCD sets for crossing signals. Real crossings use their own railroad signal equipment, not this circuit.
Run it from 5V only. At 9V the off transistor's base gets pushed past its reverse voltage rating every cycle. Each LED gets a 470 ohm resistor.
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