2B-LETTER BOX INDICATOR
Sitting at a distance and without opening the letter box you can get the indication whether there is any letter in the box through this gadget. This project is quite helpful to aged as it saves their labour of going to the letter box or coming down the stairs when there is no letter in the box.
TECHNICAL DESCRIPTION AND WORKING
This project has been developed and tested O.K. in our TV Engineers Technical Lab. The components and PCB used in the projects published in our books are available for sale at resonable price.
The working of the project mainly depends on the LDR and timer IC 555. You can connect the indicator any where you like. The IC 555 is a most popular IC which can be employed in any analogue or digital circuit. Its INPUT/OUTPUT can be connected to any TTL or CMOS logic circuit. The LDR is connected in between pin no 4 of IC and supply. As soon as the light stops falling on the LDR , the information at pin no 4 of IC becomes low and also the information at pin no 3 becomes low as the IC resets. As a result the LED connected to the transistor BC158 glows. When there is no DAK or Letter in the letter box, the information at pin no 4 of the IC becomes high triggering the IC and the LED connected to the transistor BC 148 glows indicating ” no letter” in the box.
PRECAUTIONS WHILE ASSEMBLING THE PROJECT:-
*Keep LDR in such place so that the light falls directly on LDR.
*Use IC base for mounting of IC 555.
*Check +ve & -ve terminal of L.E.D. while mounting on PCB.
*While mounting transistor, you should connect collector , base & emitter properly.
*Please use resistance of the exact value as given in the circuit.
*First of all check all the components as per circuit.
LDR = 1
IC555 = 1
BC148 = 1
BC158 = 1
L.E.Ds GREEN = 2
L.E.Ds RED = 2
4.7K = 2
100K = 1