Input Sensors
Thermistors
- A thermistor is a non-ohmic conductor and sensory resistor whose resistance varies with temperature
- Most thermistors are negative temperature coefficient (NTC) components.
- This means that if the temperature increases, the resistance of the thermistor decreases (and vice versa)
Resistance-Temperature Graph for a NTC Thermistor
A graph of temperature against resistance for a thermistor shows that as temperature increases, resistance decreases
- Thermistors are temperature sensors and are used in circuits in ovens, fire alarms and digital thermometers
- As the thermistor gets hotter, its resistance decreases
- As the thermistor gets cooler, its resistance increases
Relationship Between Resistance and Temperature for a Thermistor
The resistance through a thermistor is dependent on the temperature of it
Light-Dependent Resistors
- A light-dependent resistor (LDR) is a non-ohmic conductor and sensory resistor
- Its resistance automatically changes depending on the light energy falling onto it (illumination)
- As the light intensity increases, the resistance of an LDR decreases
Resistance-Temperature Graph for a Light-Dependent Resistor
A graph of resistance against light intensity for an LDR shows that as light intensity increases, resistance decreases
- LDRs can be used as light sensors, so, they are useful in circuits which automatically switch on lights when it gets dark, for example, street lighting and garden lights
- In the dark, its resistance is very large (millions of ohms)
- In bright light, its resistance is small (tens of ohms)
Resistance of an LDR depends on the light intensity falling on it