Electronic eye has much use in this electronic age, also Known as magic eye. It can be used as an automatic guest indicator at the door, if fitted on the bottom of the door entrance. Once it is installed at the door there is no need to install a call bell. It can also be used at homes or in banks as a burglar alarm.
This electronic eye circuit uses NOT gate from CMOS I.C CD 4049. CD 4049 contains six independent NOT gate in one package; we have used one only. NOT gate output goes high(1) when the input pin 3 is at lower then 1/3rd level of the supply voltage, conversely the output goes low (0) when it is above 1/3rd level. 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. The output has only two states high and low and can not remain in any intermediate stage. It is powered by a 9V battery for portable use. The circuit is economical in power consumption. Pin 1 is connected to the positive supply and pin 8 is grounded.
To detect the present of an object we have used LDR and a source of light. LDR is a special type of resistance whose value depends on the brightness of the light that is falling on it. It has resistance of about 1 mega ohm when in total darkness, but a resistance of only about 5k ohms when brightness illuminated. It responds to a large part of light spectrum.
We have made a potential divider circuit with LDR and 220 KΩ resistance connected in series. 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. This divided voltage is given to input of NOT gate.
As soon as LDR gets dark the voltage of input not gate drops 1/3rd of the supply voltage and pin 2 gets high and LED or buzzer which is connected to the output gets activated.