This Electronic Mail Box circuit generates music intimating the arrival of mail.
The circuit uses a high bright White LED and an LDR to detect the arrival of mail in the box. IC NE 555 is designed as a Schmitt trigger by shorting its trigger pin 2 and threshold pin 6. When the light from the LED illuminates the face of LDR, it conducts taking the pin 6 of IC low. Output pin 3 then remains low inhibiting the buzzer.
When a mail falls between the LED and LDR, light from LED will be blocked and the LDR becomes non conducting. This makes the threshold pin of IC high and its output goes high. The high output makes buzzer active and music will be heard. The music stops only when the letter is removed from the box. Keep LED and LDR at the opposite sides of the box.