The LDR becomes less resistive when light is applied, and the brighter the light the lesser the resistance. A typical one can have 100 ohms in bright conditions and 1,000,000 ohms in dark. Thus by coupling one to a fixed resistor we can have a voltage divider that  can  vary a voltage according to the intensity of the light at the LDR. A supply has to be provided to this device to operate it.

Light Detectors.


As the final comment in this topic, we should look at the actual components that react to light and  thus transduce optical signals into electrical ones. These are termed detectors. In no particular order these are;


1. The Phototransistor



We met the phototransistor previously. The device is merely an NPN transistor which has no base. Instead light falls upon the junction and acts as the input to the gate. The amount of light falling to the junction controls the current at the collector, and by attaching a resistor to the collector we can control the voltage at the junction. In this way the phototransistor provides detection and amplification in converting the optical signal to electrical. When in an analogue device, the phototransistor is used as an amplifier, when in a digital one a switch.

2.The Photodiode


A photodiode can be used in a photovoltaic mode, in which case light striking it produces a photocurrent the flow of which outwards is restricted . thus a voltage builds up and a current begins to flow in the opposite direction producing a photovoltaic effect.  The photoconductive mode, in which the diode is reverse biased, gives a faster response time but produces more noise. Thus an ordinary photodiode has serious shortcomings in telecommunications and an improved version is the PIN diode, which has an intrinsic layer between the P and N junctions. This makes it highly sensitive to light and gives it a response time of less than 1 nanosecond.  Unlike transistors, the diode does not produce amplification, unless it is an APD or avalanche photo diode. In this high velocity electrons collide with others to release more electrons and produce a successive action. Diodes can be used with or without a  focusing lens , with they are more sensitive but have a narrower viewing angle, without sensitivity decreases greatly but the viewing angle increases considerably.

3.The Photoresistor or Light Dependent Resistor (LDR)

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