part · Level 3 · Sense · input
How a thermistor works
Temperature sensor (10 kΩ thermistor) — A resistor that changes with temperature.
What it is
A thermistor is a resistor whose value changes with temperature — the name is thermal resistor. Yours is an NTC type (negative temperature coefficient): warmer means lower resistance. It is a tiny bead of metal-oxide ceramic on two legs, no polarity.
The label "10 kΩ" is its value at 25 °C — a typical room. Hold it between your fingers (about 35–40 °C) and it drops to a few kΩ; dip it in ice water and it climbs above 30 kΩ. The change is big and fast, which is what makes it a sensor.
Like the LDR, it does not produce anything on its own. It sits in a voltage divider and shifts the middle voltage as the temperature moves.
How it works
The curve. An NTC thermistor's resistance follows an exponential law set by its B value (often 3950 K on hobby parts):
R = R25 × e^(B × (1/T − 1/298.15)), with T in kelvin (°C + 273.15)
Run it for B = 3950 and R25 = 10 kΩ:
- 0 °C: 10 k × e^(3950 × (1/273.15 − 1/298.15)) ≈ 10 k × e^1.21 ≈ 33 kΩ
- 25 °C: 10 kΩ by definition
- 40 °C (warm hand): 10 k × e^(−0.64) ≈ 5.3 kΩ
- 60 °C (hot mug): 10 k × e^(−1.39) ≈ 2.5 kΩ
Real beads vary — a warm hand can read anywhere from about 3.5 to 5.5 kΩ depending on the part's exact B value and how well it is touching your skin — so measure yours before you trust a threshold.
The divider. Thermistor on top, 1 kΩ on the bottom, 4.5 V pack: V_mid = 4.5 × 1 ÷ (R_therm + 1).
- Room (10 kΩ): 4.5 ÷ 11 ≈ 0.41 V — below a transistor's 0.7 V wake-up line, so the buzzer is off.
- Warm hand (≈ 4 kΩ): 4.5 ÷ 5 ≈ 0.9 V — over the line, buzzer on. That is the Fever Beeper.
The bottom is deliberately small (1 kΩ, not 10 kΩ). With 10 kΩ on the bottom the room reading would already be 4.5 × 10 ÷ 20 = 2.25 V and the alarm could never be off.
Self-heating. The divider's own current warms the bead slightly: at 0.41 mA through 10 kΩ that is 0.41² × 10 000 ≈ 1.7 mW, a tenth of a degree or so. Fine here; it matters for precision thermometers.
The numbers
| Resistance at 25 °C | 10 kΩ — the "R25" rating |
|---|---|
| B value (typical) | 3950 K — check the packet; sets the curve |
| At 0 °C | ≈ 33 kΩ |
| At 40 °C (hand-warm) | ≈ 3.5–5.5 kΩ — 5.3 kΩ for B = 3950; measure yours |
| Divider mid at room (1 kΩ bottom) | ≈ 0.41 V |
| Polarity | none |
See it working
The Fever Beeper circuit, simulated live. Open it on the Workbench to change anything.
simulating…
Mistakes everyone makes
- Using 10 kΩ as the divider's bottom resistor. The middle point sits at 2.25 V at room temperature and the transistor is already on. Fix: 1 kΩ on the bottom for a warm-detect alarm.
- Bead not touching the warm thing. Air is a poor conductor of heat; the reading barely moves. Fix: press the bead itself against the skin or mug, not the legs.
- Confusing NTC with PTC. A PTC thermistor rises with heat and would invert every result. Fix: hobby "10k thermistors" are NTC unless labelled otherwise.
- Soldering-iron heat during a build. The bead stores heat for a minute and reads high. Fix: wait, or use the breadboard.
Questions people ask
- What does 10k mean on a thermistor?
- It is the resistance at 25 °C. The value falls as the bead warms — roughly 5 kΩ at 40 °C and 2.5 kΩ at 60 °C for a B = 3950 part — and rises in the cold, to about 33 kΩ at 0 °C.
- Does a thermistor have polarity?
- No. It is a resistor; either leg can face +.
- How do I turn a thermistor into a temperature switch?
- Thermistor from + to a middle point, 1 kΩ from there to −. Feed the middle point through 1 kΩ into a 2N2222 base. At room temperature the middle sits at about 0.41 V (transistor off); hand-warm it rises past 0.7 V and the transistor switches on a buzzer or LED.
Projects that use this part
- 🌡️ Warm Hand Alarm — Cup your hand over the sensor and the LED lights up — a thermometer that turns into a switch. L3 · 30 min
- 🤒 Fever Beeper — The Warm Hand Alarm's brain wired to a buzzer instead of a lamp — silent in a normal room, beeping the second a warm palm covers the sensor. L3 · 25 min
- 🎺 Warm Hand Chorus — Fever Beeper with a red LED added in parallel with the buzzer — one warm-hand sensor drives a chime and a lamp at the same time. L3 · 30 min
verified against: Adafruit — Thermistor · SparkFun — Voltage Dividers · SparkFun — Transistors