Sensor circuits: the LDR and the thermistor sim
The student measures how an LDR's resistance falls with light and a thermistor's with temperature, plots their own graphs, then builds a night light and a fire alarm from a potential divider and finds which way round each sensor must go.
D.C. circuits: sensors and potential dividers · covers the O-Level, IGCSE, GCSE topic
What the sim shows
Part 1: an LDR's resistance falls as the light on it gets brighter, from about 39 kΩ at 2 lux (dark room) to 10 kΩ at 10 lux and about 200 Ω at 1000 lux, modelled as R = 10 kΩ × (E ÷ 10 lux)^−0.85, typical of a GL55-series LDR. The lamp gives E = 2 lux of room light + 1000 lux × dimmer × (20 cm ÷ d)², the inverse-square law. An NTC thermistor's resistance falls as it gets hotter: 33.6 kΩ at 0 °C, 10.0 kΩ at 25 °C, 3.59 kΩ at 50 °C and 0.70 kΩ at 100 °C, from R = 10 kΩ × exp[3950 K × (1/T − 1/298.15 K)], T in kelvin. Part 2: in a potential divider the 6.0 V is shared in proportion to the resistances, V_out = V_in × R₂ ÷ (R₁ + R₂), where R₂ is the bottom part, which the output is taken across. The switch turns on above 3.0 V, half the supply, so it switches when the sensor's resistance equals the variable resistor's. Night light: LDR at the bottom (in the dark its resistance is high, so it takes most of the 6.0 V). Fire alarm: thermistor at the top (when hot its resistance is low, so the bottom resistor takes most of the voltage). Turning the variable resistor changes the light level or temperature at which it switches. Time never runs on its own: each heating notch, stir and "wait" is one step, and until it is stirred and settled the thermometer and the sealed thermistor read behind the water, because they lag it (first-order lag, 90–120 s still, 15–20 s stirred). Limits: the switch box is idealised; the meters and switch draw no current; the hot plate settles the bath within ±1.0 °C of its setting and never above 100 °C; the water is one temperature once stirred and settled; the thermistor's self-heating and the LDR's slow response are ignored; the ohmmeter scatters ±1 %, the voltmeter ±0.02 V, the light meter ±1 %, and the thermometer reads to 0.5 °C, so your own graph differs a little from the model.
- LDR: its resistance falls as the light gets brighter
- NTC thermistor: its resistance falls as it gets hotter
- Red mark on the voltmeter: the switch turns on above 3.0 V
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