The circuit described here can be used as electronic letter box. When somebody drops letter in your letter box then LED connected to output stops glowing indicating you that you have letter. To detect the presence of a letter we have used a LDR and a source of light. It has resistance of about 1 mega ohm in total darkness and 2-5kohm in bright light. In this the source of light and LDR is so adjusted in letter box that light will fall directly fall on LDR so that when letter is drop inside the box it will block the beam of light and LDR will be under darkness.The circuit gives you a visual alarm when somebody drops a letter in your drop box. This electronic letter box circuit is based on operational amplifier LM741, CD4001 and LDR.An op-amp produces an output voltage that is hundreds of thousands times larger than the voltage difference between the input terminals.
Filed Under: Electronic Projects