Make this simple Electronic Mail Box and impress your friends. It gives beeps when a letter is placed in the letter box and intimates the arrival of Mail. The circuit is too simple and uses an LDR as dark sensor and White LED as illuminator.
The circuit’s operation is simple. IC CD4060 is a binary counter used for timing applications. Its outputs goes high and low based on the values of timing components C1 and R3. Its reset pin 12 is used to control the working of oscillator. If the reset pin is high, IC will be off and if the reset pin is grounded, IC oscillates.
LDR is the Light Dependent Resistor and its resistance varies depending on the intensity of light falling on it. In dark ,its resistance is as high as 1 Meg Ohm which reduces to few ohms in light. So when the light from the White LED falls on the LDR, it conducts to keep the reset pin of IC high and IC will be off. When the letter falls in between the LDR and White LED, the shadow of letter increases the resistance of LDR and the reset pin of IC goes to ground potential and it starts oscillations. LED blinks and the Buzzer beeps indicating the arrival of mail.
Fix White LED and LDR at the opposite sides of the letter box with exact alignment so as to fall light from White LED on the face of LDR. You can use a 100K preset in the place of R1 to set the sensitivity of LDR. Black tubing for White LED gives narrow beam of light and black tubing for LDR prevents day light falling on LDR.