ELECTRONIC LETTER BOX
The ELECTRONIC LETTER BOX is a very useful gadget for homes and offices the circuit uses single Monolithic -555 in dual in line plastic package and a few external components.The main advantage of the letter box is we can find out the presence and absence of letter inside the box without opening the box.
LIGHT DEPENDED RESITOR
The L.D.R. is the most important part of this circuit.The Resistance of L.D.R.is miniumWhen the light falling on is maximum and its resistance is maximum when the lightfalling on is minimum.LD.R is connected to pin no2 of C-555.When the letter is insidethe letter box,the intencity of light falling on L.D.R.is minmium, so resistance is high , the voltage across the threshold pin no 6 goes above 2/3 VCC. The internal flip-flop resets and the output goes low and the red L.E.D is In the forwared biased condition emitting red light.In the absence of letter in the letter box, light falling on L.D.R. is maximum so resistance is low and the voltage across the threshold pin goes below 2/3 VCC. The internal flip-flop sets and its output goes high which forwared bias the green L.E.D. emitting green light.