post indicator

Has the postman been yet or isn"t there any mail today? This question is asked daily by millions of people. As a rule, the answer is to go to the letterbox to have a look. The farther you have to go to find out and the emptier the letterbox, the greater your disappointment. The post indicator shows on four seven-segment displays whether it is worth the walk or not. Initially, the flipflop formed by N1 and N2 is reset and transistor T2 conducts causing the word "NONE" to appear on the display. When the light beam to the LDR is interrupted (when a letter falls through the letterbox) T3 will conduct briefly and trigger the flipflop. As a result, T2 will turn off and T1 will turn on. The display will then show the word "POST". The circuit will remain in this state until the reset switch S1 is pressed whereupon it will revert to its initial state. For reliable operation it is advisable to mount the lamp and the LDR as close possible to the actual aperture of the letterbox. Further to r...