Sir,I need the PPT of dark sensor using ic555

LDRs are made from semiconductor materials to enable them to have their light sensitive properties. There are many types but one material is popular and it is cadmium sulphide (CdS). These LDRs or PHOTO REISTORS works on the principle of “Photo Conductivity”. Now what this principle says is , whenever light falls on the surface of the LDR (in this case) the conductance of the element increases or in other words the resistance of the LDR falls when the light falls on the surface of the LDR. This property of the decrease in resistance for the LDR is achieved because it is a property of semiconductor material used on the surface.

Here in this dark detector circuit, LDR is configured with 555 ASTABLE in such a way that 555 ASTABLE generates square wave when the light intensity goes below a certain level.

Working

Before going to explanation, the circuit should be assumed ON and is not buzzing in the presence of light. This condition of non-buzzing in the presence of light can be achieved by adjusting the 1MΩ trim pot. Now in the circuit one can observe a voltage divider with 1M, 100K on one side and LDR on the other, the reset pin is connected in the middle. The trimmer pot is said to be adjusted because to create enough resistance on the top branch of voltage divider to drop almost all the potential (+5v) in the top branch itself. This leaves a virtual ground at the middle of divider (reset pin). Since the RESET pin of 555 is a LOW LEVEL triggered, the timer IC will be reset mode continuously and so there will be no square wave output as it should be. From this we can conclude that in the presence of light the 555 IC will be in complete reset and provides no output.

Now when the darkness falls on the LDR, the resistance of the LDR increases drastically as explained in introduction, this increase of resistance in the second branch (one with LDR) of voltage divider will be enough to change the ratio of voltage sharing between the two branches of voltage divider section. Once this happen, the potential at the junction of voltage divider circuit rises from 0V to 2V (approximately). And similarly the voltage at the RESET pin rises. This rise of voltage will be enough to lift the 555IC from reset mode. Once this reset mode is lifted, the timer generates square wave output. So it is concluded that once the darkness falls on the LDR the square wave output is generated by the timer.

The square wave generated by the timer is fed to the PNP transistor to drive the speaker. So the speaker outputs sound in response to the square wave.