I/O Led Monitor

A light-emitting diode (LED) is a semiconductor diode that emits light when an electric current flows through it. It is a very useful device used in electronics. LEDs can be used for lighting, indicators, and even data communications. They are also very power efficient and they are used instead of traditional electric lamps because they can be as bright as lamps while consuming less power than the lamps.

Overview

I/O LED monitor is a tool that uses eight light-emitting diodes. It can be used to monitor microcontroller output pins or input devices such as switches. It can be used to detect a non-responsive output or even abnormalities in I/O lines. It can also be used in learning because I am going to use this in my future tutorials about microcontrollers.

Hardware

The I/O Led Monitor comes in two configurations. One is common anode and the other is common cathode.

In the common anode configuration, all the anodes of the LEDs are connected together.

In common cathode configuration, all the cathodes of the LEDs are connected together.

For both I/O LED monitor configurations, each LED is connected to a current-limiting resistor. The value for each resistor is 470 ohms. This value is chosen to limit the current through each LED within safe limits while keeping the brightness of each LED visible to the naked eye. I also chose this value so that the I/O LED monitor can work pretty well for both 5V and 3.3V I/O lines.

There are two conditions that must be met so that an LED would emit light visible to the naked eye. First, the voltage between the cathode and anode of an LED must be enough to forward bias it or to cause electric current to flow through it. This voltage is called forward voltage. For red LEDs, the forward voltage is around 1.6V to 1.7V. Remember that the voltage at the anode must be positive with respect to the cathode. This means that the voltage at the anode must be in higher potential than that of the cathode. Second, the electric current through the LED must be high enough so that the emitted light will be bright enough. Most LEDs are at maximum brightness at 20mA of electric current. However, 4mA is already enough.

The current through the LED is determined using the formula:

I = (Vs – Vled) / R

where

I = current through the LED

Vs = supply voltage

Vled = forward voltage of LED

R = current-limiting resistance

For example, the current through the red LED when the supply voltage is 5V and the current-limiting resistor has a resistance of 470 ohms is 7.23mA.

Formula:

I = (Vs – Vled) / R

Given:

Vs = 5V

Vled = 1.6 (red LED)

R = 470 ohms

Solution:

I = (5 – 1.6) / 470

I = 7.23 mA

Build it…

If you are interested to build the I/O LED monitor, you may download the Eagle files here.