Sensitiveness can be adjusted by this variable resistance. Variable resistance is so adjusted that it crosses potential of 1/3rd in brightness and fall below 1/3rd in darkness. This divided voltage is given to pin 2 of IC 555. We know that voltage is directly proportional to conductance so more voltage we will get from this divider when LDR is getting light and low voltage in darkness. We have made a potential divider circuit with LDR and 100K variable resistance connected in series. It responds to a large part of light spectrum. It has resistance of about 1 mega ohm when in total darkness, but a resistance of only about 5k ohms when brightness illuminated. LDR is a special type of resistance whose value depends on the brightness of the light which is falling on it.

To detect the present of an object we have used LDR and a source of light. Pin 4, 6 and 8 is connected to the positive supply and pin 1 is grounded. The circuit is economic in power consumption.

It is powered by a 6V battery for portable use. The output has only two states high and low and can not remain in any intermediate stage. So small change in the voltage of pin-2 is enough to change the level of output (pin-3) from 1 to 0 and 0 to 1. Conversely the output goes low (0) when it is above 1/3rd level. I.C 555 is connected as comparator with pin-6 connected with positive rail, the output goes high(1) when the trigger pin 2 is at lower then 1/3rd level of the supply voltage. This circuit uses a popular timer I.C 555.

Variable resistor in multisim 12