Light dependant resistor

Light energy falling on a suitable semiconductor material also causes a change in resistance. The semiconductor material of a light-dependent resistor (LDR) is encapsulated as shown in Fig. 3.93 together with the circuit symbol. 

Transformer ignition circuit for bomb calorimeter. The switch is a momentary-contact push switch that is only on when it is held down. Release the switch if ignition does not occur after about 5 s (the pilot light will indicate when the fuse wire bums through). The choice of resistor R will depend on the type of calorimeter and the length of fuse wire to be used (a current of 3 to 4 A is desired). 

It is important to note that thermal noise, although dependent on the frequency bandwidth, is independent of frequency itself. I or this rea.son. it is sometimes termed whi c fwise by analogy to white light, which contains all visible frequencies. Also note that ihcrmal noise in resi.siivc circuit elemonis is independent of the physical size of the resistor. 

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