Topic 6 of 6

Thermistor and LDR circuits

A sensor changes resistance when its surroundings change. In a powered potential divider, that resistance change produces a changing output voltage.

For an unloaded divider, the output is the supply voltage multiplied by the fraction of the total resistance across the chosen output. Check which terminals are used before predicting whether the voltage rises or falls.

NTC thermistor: Its resistance decreases as temperature increases. NTC means negative temperature coefficient. This behaviour differs from the rising resistance of a hotter ordinary metallic conductor.
Light-dependent resistor: LDR: Its resistance decreases as the light falling on it increases. Its symbol's incoming arrows represent illumination.

These trends do not establish a straight-line or inverse-proportional relationship. Use supplied resistance values or a given characteristic for a calculation.

As an input transducer, the sensor lets a physical input such as temperature or light affect an electrical signal. The sequence is: surroundings change, sensor resistance changes, voltage sharing changes, output voltage changes. The source supplies the circuit's energy; the sensor does not provide a universal on/off voltage by itself.

A sensor changes the voltage share across a named output

Both supplied divider circuits keep the source at 6.0 V and take negligible output current. Read the output between B and C; the sensor is in a different arm in each case.

NTC thermistor below a fixed upper resistor

Supplied 20 deg C condition: NTC 6.0 kohm, output 4.0 V. At 40 deg C: NTC 2.0 kohm, output 2.4 V. The series current changes between conditions.

LDR above a fixed lower resistor

Supplied dim condition: LDR 12.0 kohm, output 1.5 V. Brighter condition: LDR 2.0 kohm, output 4.0 V. Output is across the fixed lower resistor.

Both supplied circuits use a fixed 6.0 V source and negligible output current. The NTC is the lower arm in the first circuit; the LDR is the upper arm in the second. In both, V_BC is measured across the lower arm.

An NTC in the lower arm

A fixed 3.0 kΩ resistor connects A/B. The NTC connects B/C, and output is across the NTC. Its supplied resistance is 6.0 kΩ at 20 °C and 2.0 kΩ at 40 °C.

Temperature to output voltage

Warm the NTC from 20 °C to 40 °C

At 20 °C: total resistance is 3.0 + 6.0 = 9.0 kΩ. Current is 6.0/9000 = 0.000667 A, or 0.667 mA. Output V_BC = 6.0 x 6.0/(3.0 + 6.0) = 4.0 V.

At 40 °C: total resistance is 3.0 + 2.0 = 5.0 kΩ. Current is 6.0/5000 = 0.00120 A, or 1.20 mA. Output V_BC = 6.0 x 2.0/(3.0 + 2.0) = 2.4 V.

Warming reduces the lower arm's resistance and its share of the supply voltage. The output falls even though the series current rises. The fixed upper resistor's p.d. rises from 2.0 V to 3.6 V, keeping the total at 6.0 V.

This changing voltage could be an input to a temperature monitor or alarm. The controller's required input and switching condition would need to be specified; the thermistor alone does not determine them.

An LDR in the upper arm

The LDR now connects A/B, with a fixed 4.0 kΩ resistor at B/C. Output is across the fixed lower resistor. The supplied LDR resistance is 12.0 kΩ in a dim condition and 2.0 kΩ in a brighter condition.

Light to output voltage

Increase the illumination

Dim: total resistance is 12.0 + 4.0 = 16.0 kΩ. Current is 6.0/16 000 = 0.000375 A = 0.375 mA. Output V_BC = 6.0 x 4.0/(12.0 + 4.0) = 1.5 V.

Brighter: total resistance is 2.0 + 4.0 = 6.0 kΩ. Current is 6.0/6000 = 0.0010 A = 1.0 mA. Output V_BC = 6.0 x 4.0/(2.0 + 4.0) = 4.0 V.

More light lowers the upper resistance, giving the fixed lower resistor a larger share of the supply. The output rises. The LDR's own p.d. instead falls from 4.5 V to 2.0 V.

A light-level controller can use this voltage as its input. Moving the sensor to the other arm, or measuring across the other component, can reverse the output trend. Name the resistance that changes and the terminals being measured.

Compare readings under controlled conditions

Use supplied data or a suitable low-voltage arrangement with the output meter across the named terminals. Keep the source voltage fixed. Allow the sensor's response to settle before recording the input condition and output voltage.

For an NTC comparison, measure temperature with a suitable thermometer and keep other conditions consistent. Electrical current can also warm the thermistor, so do not attribute every change to the intended external temperature alone. A voltage response needs calibration before it can be read as a temperature scale.

For an LDR comparison, change the illumination while limiting unwanted heating and keeping other light sources consistent. These supplied resistance values describe the example; they are not a calibration for every LDR or thermistor.

The current can change between conditions. Both arms carry the same current within one unloaded-divider state, but changing a sensor's resistance changes the total resistance and therefore the current. Recalculate it or use the correctly referenced voltage ratio.

Optional check A 6.0 V divider has an NTC thermistor as its upper arm and a fixed 3.0 kilohm resistor as its lower arm. Output is across the fixed lower resistor. Warming reduces the NTC resistance from 6.0 to 2.0 kilohm. What happens to the unloaded output?