Light Dependent Resistors and Their Applications

Light Dependent Resistor (LDR)

• Definition and Symbol:

  • An LDR, or light dependent resistor, is a type of resistor that changes its resistance based on light intensity.
  • The symbol for an LDR is crucial to recognize for circuit diagrams.

• Concept of Resistance in LDRs:

  • In dark conditions:
  • Resistance is high.
  • In light conditions:
  • Resistance is low.

• Graphical Representation:

  • The relationship between light intensity and resistance can be represented graphically:
  • High resistance at low light intensity (dark).
  • Low resistance at high light intensity (light).

Use of LDRs in Circuits

• Practical Application Example:

  • Example: Mobile phone screen dimming.
  • When the phone is close to the ear, the LDR is in darkness, thus:
    • High resistance → Low potential difference across the lamp → Screen turns dim/off.
  • In light, when the LDR has low resistance:
    • Low resistance → High potential difference across the lamp → Screen lights up.

• Simplified Circuit Example:

  • Components:
  • LDR in series with a lamp (mobile phone backlight).
  • A cell providing a potential difference (e.g., 10 volts).
  • Behavior of the circuit based on light intensity:
  • Bright Light:
    • LDR has low resistance → Lamp lights up due to high potential difference.
  • Dark Conditions:
    • LDR has high resistance → Lamp dims or turns off due to low potential difference.

Other Applications of LDRs

• Garden Lamps:
  • LDRs are used in garden lamps to switch them on in darkness and turn them off in daylight, demonstrating their sensitivity to ambient light conditions.

Summary of Learning Objectives

• Recognize the symbol for a light dependent resistor (LDR).
• Describe how resistance of an LDR changes with light and dark conditions.
• Explain the practical use of LDRs in electronic circuits and everyday devices.