28 April 2015 User ReviewCheck your delivery options:
LDR (Light Dependent Resistor) is a resistor whose resistance varies inversely with the amount of light falling on it. It is also known as photo resistor, photocell, photo conductive cell etc. LDR are available in 5mm, 8mm, 12mm and 25mm dimensions.
LDR is made of high resistance semiconductor material. Semiconductor material used for the photo resistors is cadmium sulphide, CdS. When it’s dark, LDR has high resistance known as dark resistance. Usually dark resistance will be in the range of mega ohms. When light falls on LDR, resistance reduce to kilo ohms range.
Working principle: when light falls on LDR i.e. when photons fall on LDR, valence band electrons get sufficient energy to get excited to the conduction band. Incident photons must have energy greater than the forbidden energy gap of the semiconductor. When light having enough energy falls on the device, more electrons will get excited to conduction band. As the number of free electrons increases, more current flows through the circuit. Hence it is said that, as light falls on LDR, resistance decreases.
Here the LDR and R1 form a simple 'Potential Divider' circuit. When the light level decreases, the resistance of the LDR increases. When resistance increases w.r.t to the other resistor having fixed resistance, the voltage drop across LDR increases.