LIGHT SENSOR

I have tried to use a comparator (LM358) to make a Light Sensor which gives output as high when a certain intensity level is crossed or vice versa but it has not given good output and I don't know the reason. So, I have to go for an alternative because the TASK is important at that time.So I have decided to go for a BJT (BC548) which is cheaper than the previous.This is the circuit which I have designed

Each and every component has its own physical significance. Let me explain briefly

The LED D1 always focus light on the LDR (light dependent resistor) whose resistance varies with light intensity. The potentiometer P1 is used as variable resistor in order to adjust the sensitivity of the LDR to the surroundings. When light falls on theLDR its resistance decreases and make the transistor(Q1) move into active region which may not sufficient to drive the load(L1) less than logic high voltage so I kept a transistor (Q2) for switching purpose operates in either cuttoff or saturation.

Simple Water Level Indicator

There is always a problem in our home whenever we switch on the motor and forget to switch it off, after the tank was filled with water and spilled out. So much of water and power is wasted. So, we have decided to solve the problem by making a simple circuit with transistors and a Buzzer.
              Let us find out the resistance of our tank water which about 53.7K which is very high.

             so we have used a transistor(bc547) for the purpose of amplification in CE mode with beta 110 ~ 220 which may not sufficient for the buzzer to drive directly. So we are using another BJT for switching. The configuration is in the form of a standard Darlington pair, which increases its current amplification capacity hugely.The current through buzzer is          β(β+1)I_b(Q1)

                       The purpose of Resistor and Capacitor is to insert some delay so that the transistor  is on for some time (R*C=2.2mS) and so the Piezo Buzzer starts ringing. The whole circuit is given 9V D.C power supply. I have used a 9V battery

UNDERSTANDING THE DATASHEETS OF BASIC GATES AND DECODER IC'S

Many of us use the logic gates for circuit designing. These gates can be made using transistors,diodes,resistors etc. It is not as  easy as it is said. But these gates are packed into ic's and are readily available in the market. We can use these ic's for our purpose. Mainly there are two types of ic's. One is TTL and other is CMOS. TTL stands for transistor-transistor logic whereas CMOS stands for complementary metal oxide semiconductors. TTL make use of the transistors and hence the logic levels are for low is "0V" and for high is "5V". But it is always not necessary that only 0V or 5V must be used. Logic low can be in the range of ( 0-0.8)V and logic high can be in the range of ( 2-5)V.
              CMOS make use of the MOSFET'S
remaining i ll post it soon.
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