LCD-LED Display

Electronic Letter Box Project Circuit


  • Introduction 
  • Circuit Principle
  • Electronic Letter Box Circuit Diagram
  • Components Required
  • Component Description
  • Electronic Letter Box Circuit Explanation


In this project, I will show you a simple Electronic Letter Box a circuit that can be used to indicate when ever 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 letter box, the LED stops glowing i.e. it is turned OFF. This indicates that there is a letter in your letter box.

Circuit Principle

The circuit’s premise is quite straightforward. I’ve linked an LDR and a light source to detect the presence of a letter in the box. Light Dependent Resistor (LDR) is a type of resistor whose resistance varies depending on the amount of light falling on it.

The LDR’s resistance is around 2 Mega Ohm in low light or dark settings, but it drops to a few tens of Ohms under high light. In this circuit, the light source (an LED) is set with the LDR in such a way that the light from the LED falls directly on the LDR, and any letter placed inside the letter box would obscure the light.

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.

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

Components Required

  • LM741 – 1
  • CD4001 – 1
  • Resistors
    • 10KΩ – 5
    • 1KΩ – 2
  • Capacitors
    • 0.1μF – 2
  • C1,C2(.1uF) – 2
  • LEDs – 2
  • LDR
  • 5V Power Supply
  • Mini Breadboard
  • Connecting Wires
Electronic Letter Box

Component Description

  • LDR – A device whose sensitivity is determined by the intensity of the light reflected on it. The LDR’s resistance diminishes as the strength of the light increases, and vice versa. When there is no light or darkness, the resistance of LDR reaches mega ohms, but when there is light, it drops to a few hundred.
  • LED — A light-emitting diode (LED) is a semiconductor that emits a variety of colours of light. When an LED is electrically biassed in the p-n junction at the forward bias state, it produces narrow-spectrum light. When it is turned on, it combines electrons with holes and releases energy in the form of light.
  • LM741 – This is an all-purpose operational amplifier that is utilised to improve the circuit’s performance over industry norms. In many applications, they are a simple plug-in replacement for ICs such as the 709C, LM210, as well as the MC1439 and 748. There are numerous uses, making it nearly flawless. The input and output overload guards are one example. Another is that there is no latch-up when the common mode range is exceeded and oscillation is not present.
  • Resistor – Resistor is attached in any of the circuit 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 resistor of 5K then its value vary from 0-5K ohm.

Electronic Letter Box Circuit Explanation

This circuit will emit a visual warning whenever someone drops a letter in your mail box. The circuit’s essential building blocks are the operational amplifier LM741, the Quad NOR Gate IC CD4001, and the LDR.

This circuit is relatively simple to operate. The LDR and the LED should be placed in opposite corners of the box such that the light from the LED falls on the LDR on a regular basis. The output of the OP-Amp LM741 will be HIGH as a result.

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 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.

Light falling on the LDR is stopped as soon as a letter is dropped in the box, and its resistance rises to a high level. As a result, the Op-output Amp’s becomes low. Pin 1 of CD4001 receives this LOW signal. We’ll get Logic 0 in output Pin 3 as a result of this.

As a result, Logic 1 will appear on Pin 4. This is because the second gate’s inputs come from Pin 3, and since both of the second gate’s inputs are logic 0, the output at pin 4 will be HIGH.

Continuing in this manner, the output at Pin 11 will eventually go HIGH, and the LED will stop glowing as a signal that there is a letter in the letter box.

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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