Circuit devices

What is a LDR?

a resistor that is dependent on the intensity of light

How does an LDR work?

• In bright light the resistance falls

• In darkness the resistance is at its highest

What are LDR’s used for?

• Automatic night lights

• Outdoor lighting

• Burglar detection

What does the graph for LDR’s look like according to their resistance?

Inversely proportional curve

What is a thermistor?

a temperature dependent resistor

How does the resistance in a thermistor work?

• In hot conditions the resistance drops

• In cool conditions the resistance goes up

What are thermistors used for?

• temperature detectors

  • car engine temperature sensors

  • electronic thermostats

What does the graph for thermistors x resistance look like?

Inversely proportional curve

What is a sensing circuit?

a circuit that can be used to turn on or increase power to the components depending on the conditions they are in

What does a sensing circuit look like?

• thermistor

• fixed resistor

• component (fan)

How do thermistors and Lars work in a sensing circuit?

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

• The potential difference 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 higher the resistance, the more potential difference it takes

• As the room gets hotter, the resistance of the thermistor decreases and takes a smaller share of the potential difference from the power supply.

• So the potential difference across a fixed resistor and the fan rises, making it go faster

What circuits can LDRs and thermistors be used in?

Sensing circuits

How is the circuit above different to the sensing circuit in regard to LDRs or thermistors?

The bulb is connected in parallel to an LDR, the potential difference across both the LDR and the bulb will be high when its dark, and the LDR’s resistance is high.

The greyer the potential difference across a component, the more energy it gets. So a bulb connected across an LDR would get brighter as the room got darker.