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 endeavor, I will demonstrate a straightforward electronic mailbox circuit designed to signal the arrival of physical mail, such as letters. In this Electronic Letter Box Project, an LED serves as the indicator.

Typically, the LED remains illuminated. However, when someone deposits a letter into your mailbox, the LED ceases to emit light, meaning it switches off. This serves as a notification that there is mail in your mailbox.

Circuit Principle

The circuit’s principle is quite straightforward. I’ve established a connection between a Light Dependent Resistor (LDR) and a light source to discern the presence of a letter inside the mailbox. An LDR is a type of resistor whose resistance changes in response to the intensity of incident light.

Under low light or dark conditions, the LDR exhibits a resistance of approximately 2 Mega Ohms, but it decreases to a few tens of Ohms in well-lit settings. In this circuit, the light source, represented by an LED, is strategically positioned in such a way that its light directly falls upon the LDR. Any letter inserted into the mailbox obstructs this light.

This alteration in light intensity is detected through a supporting circuitry comprising a 741 Op-Amp, CD4001 (Quad NOR Gate IC), and an LED, which is utilized 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 is designed to provide a visual alert each time someone deposits a letter into your mailbox. The fundamental components of this circuit include the LM741 operational amplifier, the CD4001 Quad NOR Gate IC, and the Light Dependent Resistor (LDR).

Operating this circuit is relatively straightforward. Position the LDR and the LED at opposite corners of the mailbox so that the LED’s light consistently illuminates the LDR. Consequently, the output of the LM741 operational amplifier remains in a HIGH state.

This elevated signal is then fed into Pin1 of the CD4001, which functions as a NOR gate. It generates an output of 1 when all inputs are set to a low state, as indicated in the truth table. Consequently, the LED continues to emit light, indicating the absence of a letter in the mailbox.

When a letter is dropped into the box, it interrupts the light falling on the LDR, causing its resistance to rapidly increase. Consequently, the output of the operational amplifier becomes low. This low signal is received by Pin 1 of the CD4001.

As a result, we obtain a Logic 0 at the output Pin 3. Furthermore, Logic 1 emerges at Pin 4 because the inputs to the second gate are sourced from Pin 3, and since both inputs to the second gate are logic 0, the output at Pin 4 remains in a HIGH state.

Following this sequence, the output at Pin 11 eventually transitions to a HIGH state, causing the LED to cease emitting light, serving as an indication that there is a letter inside the mailbox.

The LED remains inactive until all the letters are retrieved from the box. Once all the letters have been removed, the LED resumes emitting light.


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