4) Circuit Devices

1. Light Dependent Resistor (LDR)

• Definition: A resistor dependent on light intensity.

• Behavior:

  • Bright light: Resistance decreases.

  • Darkness: Resistance increases.

• Applications: Used in automatic night lights, outdoor lighting, and burglar detectors.

2. Thermistor

• Definition: A resistor dependent on temperature.

• Behavior:

  • Hot conditions: Resistance decreases.

  • Cool conditions: Resistance increases.

• Applications: Useful in temperature sensors (e.g., car engine sensors) and thermostats.

3. Sensing Circuits Using LDRs and Thermistors

• Purpose: Control components like fans or bulbs based on light or temperature.

• Example Circuit (Fan Controlled by Thermistor):

  • Fixed Resistor and Fan: Connected in parallel, sharing the same potential difference (pd).

  • Power Supply: Pd is shared based on resistance:

    • As temperature rises, thermistor resistance decreases, taking a smaller share of the pd.

    • Fan pd increases, making the fan go faster.

• Bulb with LDR:

  • Connect a bulb across an LDR to control brightness with light levels.

  • In the dark (high resistance in LDR), pd across both components increases, making the bulb brighter.