The document describes an electronic letter box circuit that uses a 555 timer IC to detect when a letter is placed inside. When the letter blocks the light falling on an LDR, the LDR resistance increases, lowering the voltage to pin 2 of the 555 IC below 1/3 of the supply voltage. This causes the 555 output at pin 3 to go high, lighting an LED to indicate a letter is present. The circuit uses an LDR, LED, 555 timer IC, and battery to provide a low-power way to detect when something is placed in the letter box blocking the light to the LDR.