Hardware · build

A candle that lights itself with one transistor

Marekon GitHub

Blow on it and it goes out. Heat the thermistor and it glows. The whole fake flame runs on one MOSFET, one thermistor and an LED.

Build sheet

Core: 1 MOSFET, 1 NTC thermistor
Output: LED
Trigger: Heat or breath
Variant: Badge SAO version

Can I build this?Intermediate · AI 2/5 · $5–15

Difficulty: Intermediate
AI-buildability: 2/5
Time: A weekend
Cost: $5–15

Parts

NTC thermistor MOSFET LED resistor

Tools

soldering ironmultimeterhot air or heat gun (optional)

Starter prompt

Based on "A candle that lights itself with one transistor" on Afterware: https://afterware.xyz/p/a-candle-that-lights-itself-with-one-transistor

Help me design a tiny analog circuit where an NTC thermistor heats up and turns on an N-channel MOSFET that lights an LED, with a series resistor that keeps the thermistor warm. Give me a parts list with resistor values, a breadboard layout, and tips for tuning the thermal feedback so the LED stays on after warming and turns off when cooled. Then suggest how to add a flicker effect with a flicker LED or a PWM signal from a small microcontroller.

Estimates based on the original write-up. How AI-buildability is scored. Building with an AI assistant? Connect it to Afterware (free MCP server).

Story

The entire candle is one transistor, one NTC thermistor, an LED and a couple of resistors. Warm the thermistor and its resistance drops until the MOSFET gate goes high enough to light the LED. Blow on it and the thermistor cools, the MOSFET shuts off, and the flame dies, just like the real thing.

The trick is thermal feedback. A resistor in series with the LED gets warm enough to keep the thermistor conductive, which keeps the MOSFET on. [Marek] says the design is finicky and sensitive to how heat moves between parts, and it took fine tuning to behave well. An advanced version adds a voltage regulator for steadier operation across battery voltages.

There's also an SAO version for an ersatz candle on a Hackaday conference badge. The one thing it lacks is flicker. The write-up suggests a flickering LED could add that without reworking the circuit.

Resources · 2

Candle project repo SAO version repo