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Sunday, July 25, 2010




Physics
SPM
Transistor as Control Switch

Today i'm going to continue our discussion about light control switch by using transistor & LDR.Before we go further, firstly u must understand the characteristic of LDR?LDR is a type of resistor known as Light Dependent Resistor which is very sensitive towards light.Important point about LDR that u MUST remember:In a BRIGHT condition, its resistance DECREASE.In a DARK condition, its resistance INCREASE.Now lets have a look at the circuit below, describe what happen at NIGHT???
At night,

LDR's resistance increase much higher than the fixed resistor,
A higher voltage is supplied across the LDR,
This increases the flow of base current and switch ON the transistor,
The bulb lights up
So what if we want to switch ON the bulb in the morning & close it at night?Its simple, u just need to change the position between the LDR & the 10k fixed resistor.The same concept applies in Heat Control switch, except instead of using LDR we use THERMISTOR.Again, its important for u to memorise the chracteristic of a thermistor, i.e; at HIGH temperature - Resistance DECREASE, at LOW temperature - Resistance INCREASE

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