Warning mailbox electronic project

A simple warning mailbox can be designed using electronic diagram below.
The circuit presented here show using a four 7-segment LED displays if mailbox is empty or full. In idle state, the transistor T2 conducts, the show is word Leer for assembly work, you must mount light barrier where mail enter in the box. LDR light beam is interrupted by the introduction of correspondence. In this case the transistor T3 pass briefly in conduction state and NAND gate bistable switch multivibrator N1/N2. As a result, the transistor T2 is blocked, while the T1 conducts. The word "POST" is displayed when there is something in the mailbox. This word remains on display until it is acted S1 button which returns circuit to its initial state.