• The light dependent resistor is an electronic component
whose resistance decreases with increasing incident light
intensity. This allows them to be used in light sensing
• LDR is also called photo resistor or photo conductor.
• The light dependent resistor uses high resistance
• When light falls on such a semiconductor the bound
electrons get the light energy from the incident photos. Due
to this additional energy, these electrons become free and
jump into the conduction band.
• The electron-holes pairs are generated. Due to these charge
carriers, the conductivity of the device increases, decreasing
1. Used in many consumer items such as camera light meters, clock,
radios, security alarms and street lights.
2. Used for infrared astronomy and infrared spectroscopy.
3. Used in sensitive light operated relays.
4. Used in light interruption detectors, automatic light circuit,
logarithmic law photographic light meter
8. SENSITIVE LIGHT OPERATED RELAY
• An application of LDR.
• The variable resistor VR1 is adjusted in such a way that for the permissible light, the relay is inoperative.
When light level increases beyond certain level, the resistance of LDR decreases. This provides proper
biasing to the transistor Q and the relay is operated.
• By interchanging the positions of VR1 and LDR, a circuit can be obtained which activates the relay
when light level drops below a particular value. This is light interruption detector
• These circuits can be used to control on/off of parking or street lights depending on the intensity of