NTC thermistor: the resistor that feels heat

Heat this tiny bead and its resistance DROPS 🔥⬇️

That black bead is an NTC thermistor, the cheapest temperature sensor in electronics.

1️⃣ HOTTER = LESS Ω: inside is a metal-oxide material. Heat frees more electrons, so current flows more easily. A 10 kΩ NTC: 10 kΩ at 25 °C, about 2.5 kΩ at 60 °C, about 0.7 kΩ at 100 °C. NTC = Negative Temperature Coefficient.

2️⃣ ARDUINO CAN READ IT: add one 10 kΩ resistor to make a voltage divider. 5V → 10 kΩ → A0 → NTC → GND. At 25 °C A0 sees 2.50 V (analogRead ≈ 512). At 60 °C it drops to 1.00 V (≈ 204). Hotter = lower number.

3️⃣ IT GUARDS YOUR PHONE: phone and laptop batteries have an NTC on a "T" pin. When the battery gets too hot (around 45 °C), charging slows or pauses. They also hide in chargers, 3D printers, ACs, fridges and thermometers.

BONUS: a PTC does the opposite. Heat → MORE resistance. That's how self-resetting fuses work.