Electronic Mail Box Circuit

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.



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  • nadiahabdllahgmail-com

    We tried this however it work the other way around. Buzzer active when there is light and stop when it is blocked.

    • T.K.Hareendran

      Yes,you are right. It’s clear from the schematic that the buzzer works when light falls on the LDR. A sobering mistake (both in circuit & description) by the author D. MohanKumar. Try to swap the buzzer connection, ie. connect it between pin 3 of the IC and +9V rail. Also replace R2 with a 100K pot to adjust the detection sensitivity,if needed!

  • asdfgh

    what is D-mark100k??

  • Rohit Chandrawanshi

    Mail box alarm circlet

    9 volt btry
    100QC bajar
    IC 555
    3 LED
    2 trandistar

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