Circuit Devices

• LDR = Light Dependent Resistor
• An LDR is a resistor that is dependent on the intensity of light
• In bright light, the resistance falls
• In darkness, the resistance is highest
• They have lots of applications including automatic night lights, outdoor lighting and burglar detectors

  
• The resistance of a thermistor depends on temperatue
• A thermistor is a temperature dependent resistor
• In hot conditions, the resistance drops
• In cool conditions, the resistance goes up
• Thermistors can make useful temperature detectors, e.g. car engine temperature sensors and electronic thermostats

  
• You can use LDRs and thermistors in sensing circuits
• Sensing circuits can be used to turn on or increase the power to components depending on the conditions that they're in
• The circuit below is a sensing circuit used to control a fan in a room

  
• The fixed resistor and the fan will always have the same potential difference across them (because they're connected in parallel)

  
• The pd of the power supply is shared out between the thermistor and the loop made up of the fixed resistor and the fan according to their resistances - the bigger a component's resistance, the more of the pd it takes

  
• As the room gets hotter, the resistance of the thermistor decreases and it takes a smaller share of the pd from the power supply. So the pd across the fixed resistor and the fan rises, making the fan go faster
• You can also connect the component across the variable resistor instead

  
• If you connect a bulb in parallel to the LDR, the pd across both the LDR and the bulb will be high when it's dark and the LDR's resistance is high

  
• The greater the pd across a component, the more energy it gets

  
• A bulb connected across an LDR would get brighter as the room got darker