In this project, I will show you a simple Electronic Letter Box, a circuit that can be used to indicate whenever you receive a mail (physical mail – like a letter). An LED is used as an indication in this Electronic Letter Box Project.

Usually, the LED stays ON.  But when a letter is dropped by someone into your letterbox, the LED stops glowing i.e. it is turned OFF. This indicates that there is a letter in your letterbox.

Electronic Letter Box Output GIF Image

Circuit Principle

The principle of the circuit is very simple. To identify the presence of a letter in the box, I have connected an LDR as well as the source of light. LDR, short for Light Dependent Resistor, is a special kind of resistor whose resistance varies based on the intensity of light falling on it.

Under low light or dark conditions, the resistance of the LDR is approximately 2 Mega Ohm, while at the time of bright light, its resistance will fall down to few tens of Ohms. In this circuit, the light source (an LED is used as light source) is adjusted with the LDR in such a way that the light from the LED will fall directly on the LDR and when any letter is placed inside the letterbox, it will obstruct the light from falling on the LDR. 

This change is detected with the help of a supporting circuitry consisting of 741 Op-Amp and CD4001 (Quad NOR Gate IC) and an LED is used to indicate the presence of a letter. 

Electronic Letter Box Circuit Diagram

The circuit diagram of the Electronic Letter Box Project is shown in the following image.

Electronic Letter Box Circuit Diagram 1

If the connections with respect to the Quad NOR Gate IC CD4001 is not clear, then the following image might be useful. 

Electronic Letter Box Circuit Diagram 2

Components Required

  • LM741 – 1
  • CD4001 – 1
  • Resistors
  • Capacitors
  • C1,C2(.1uF) – 2
  • LEDs – 2
  • LDR
  • 5V Power Supply
  • Mini Breadboard
  • Connecting Wires

Electronic Letter Box Image 2

Component Description

  • LDR – It is a device whose sensitivity is based on the strength of the light incline on it. With the increase in the strength of the light, the resistance of the LDR decreases and vice versa. When there is no light or in darkness, the resistance of LDR reveal in the range of mega ohms while in the presence of light it moves down to a few hundred.
  • LED – It consists of semiconductor which produces various colors of light as its output. LED produces narrow-spectrum light at the time it is electrically biased in the p-n junction at the forward bias state. When it is in the on mode, combines the electrons with holes and in the form of light energy releases.
  • LM741 – It is basically an all-purpose operational amplifier which is basically used to enhance the performance of the circuit over industry standards. They are straight forward plug-in substitute for the IC like 709C, LM210 also for MC1439 as well as 748 in many of the applications. There are many applications which make it almost foolproof. One is the overload guard on the input and output. Another one is the no latch-up at the time common mode range goes beyond and independent from oscillation.
  • Resistor – Resistor is attached in any of the circuits to restrict the flow of current. There are two varieties of resistors which are mentioned below.
    • Fixed Resistor – Its value of resistance is fixed.
    • Variable Resistor – Its value of resistance can vary. Suppose we have a resistor of 5K then its value varies from 0-5K ohm.

Electronic Letter Box Circuit Explanation

When anyone drops a letter in your letterbox, then this circuit will produce a visual alarm. Operational amplifier LM741, Quad NOR Gate IC CD4001 and as well as the LDR are the basic building block of the circuit. 

Operation of this circuit is very easy. You need to install the LDR and the LED at the different corners of the box so that the light from the LED will regularly fall on the LDR. As a result of it, the output of the OP-Amp LM741 will be HIGH.  

This high signal is given to Pin1 of CD4001, which is basically a NOR gate and it produces output as 1 when all the inputs are at a low state (which you can find in the truth table). So, the LED will keep on glowing point out that there is no letter in the drop box.

Electronic Letter Box Image 1

As soon as someone drops a letter in the box, light falling on the LDR is blocked and its resistance reaches high. Due to which the output of the Op-Amp becomes LOW. This LOW signal is given to Pin 1 of CD4001. Due to this, we will get Logic 0 in output Pin 3.

This, in turn, will get Logic 1 on the Pin 4. This is because the inputs to the second gate are given from Pin 3 and since both the inputs of the second gate receive logic 0, the output at pin 4 will be HIGH. 

Continuing the same way, finally, the output at Pin 11 will become HIGH and as a result, the LED stops glowing as an indication that there is a letter in the letterbox. 

The LED stays off until all the letters are taken out of the box and once all the letters are removed, the LED starts glowing once again. 


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