Why your car's dome light fades out slowly
Press a button and three warm white LEDs stay on, then fade out slowly, like a car's dome light. One MOSFET, no code.
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
10 parts on a perfboard 15 × 27 holes, plus hookup wire.
| Qty | Part | Where to buy |
|---|---|---|
| 1 | Electrolytic Cap100uF C1 | Buy Electrolytic Cap on Amazon |
| 1 | Power Input (2-pin) J1 | |
| 3 | LED LED1, LED2, LED3 | Buy LED on Amazon |
| 1 | N-ch MOSFET (TO-92) Q1 | Buy N-ch MOSFET (TO-92) on Amazon |
| 1 | Resistor (axial)1k R1 | Buy Resistor (axial) on Amazon |
| 1 | Resistor (axial)100k R2 | Buy Resistor (axial) on Amazon |
| 1 | Resistor (axial)220 R3 | Buy Resistor (axial) on Amazon |
| 1 | Tactile Button 6mm U1 |
Check every pin against the datasheet of the exact part in your hands before wiring.
As an Amazon Associate I earn from qualifying purchases.
Cut list
8 wires, 215 mm (8.5 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 L14 → L5 | L14 → L5 | 33mm | Straight |
| 5 | BlueGND G17 → J14 | G17 → J14 | 25mm | 13mm (90°) |
| 4 | GreenGND H19 → H16 | H19 → H16 | 18mm | Straight |
| 1 | RedLED_A D25 → F24 | D25 → F24 | 18mm | 10mm (90°) |
| 7 | Red12V E6 → K5 | E6 → K5 | 28mm | 20mm (90°) |
| 3 | WhiteLED_R L20 → L25 | L20 → L25 | 23mm | Straight |
| 2 | YellowLED_B H25 → J24 | H25 → J24 | 18mm | 10mm (90°) |
| 8 | Yellow12V D7 → D24 | D7 → D24 | 53mm | Straight |
About this build
Ever notice your car's dome light fades out instead of snapping off? Real cars do it in software, in the body control module. This board gets the same slow fade from one capacitor and a 2N7000 MOSFET, running on 12V.
Press the door button and a 100uF capacitor charges through a 1k resistor to almost 12V, which holds the MOSFET fully on. Let go and the capacitor drains through a 100k bleed resistor. After about fifteen seconds the gate voltage nears the MOSFET's threshold, and the three warm white LEDs, in series with a 220 ohm resistor, fade out over three or four seconds.
Small MOSFETs don't all order their leads the same way. The BS170 is a common substitute and its leads differ from the 2N7000's, so take the wiring from the build sheet and your part's datasheet. This is a recreation of the effect, not a car's own circuit.
More builds
See all builds
VideoThe Stranger Things light wall, from two chips
Six coloured LEDs light one at a time in a loop, like the Christmas light wall in Stranger Things. A 555 timer and a counter.
13 parts · 28 wires
LightsTimers and logic
VideoA 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.
15 parts · 27 wires
LightsTimers and logic
VideoThe Clapper, recreated: lights that turn on when you clap
Clap once and an LED turns on, clap again and it turns off. A microphone, two transistors and one flip-flop chip.
14 parts · 24 wires
SensorsLights