Fun with LED. Simple Design 15


LED is a wonderful semiconductor without which a circuit is generally incomplete. LEDs are used mainly as indicators to show whether the output of a particular section of the circuit is working or not. Let us see some simple circuits using LEDs that you can try.

First see some basics of LED.

LED is a simple diode with light emitting property and hence the name Light Emitting Diode. Its encapsulation has two electrode posts with a chip. When the chip is energized, it emits Photons in the form of visible light. The colour of the light depends on the material in the chip. LED works in very low voltage as low as 1 volt. But the new generation White and Blue LEDs require minimum 3 volts to get sufficient brightness.

Internal of LED

The chip of the LED is the Gallium compound which emits the photons when the p-n junction is forward biased. Brightness is an important aspect of the LED. Human eye has maximum sensitivity to light near 550 nm region of yellow – green part of the visible spectrum. That is why a Green LED appears brighter than a Red LED even though both use same current.
LED has a semiconductor chip placed in its centre. The semiconductor consists of two regions namely a P region that has positive charge carriers and an N region with negative charge carriers. There are three layers in the chip. An active photon generating material is sandwiched between the P and N type materials so that photons will be generated when the electrons and holes combines. That is when a potential difference is applied between the P and N materials through the LED terminals, holes from the P layer and electrons from the N layer move towards the active material where they combine to produce the light though the phenomenon of Electroluminescence.
The colour of photon emission depends on the chemical doped in the active material. Chemicals like Gallium, Arsenic and Phosphor are used to give color to the light emission. Some of the chemicals used in LEDs are
1. Infrared LED- Aluminium gallium arsenide
2.Red LED – Aluminium gallium arsenide, Gallium arsenide phosphide, Gallium phosphide
3. Green LED – Aluminium gallium phosphide, Gallium nitride
4. Yellow LED – Aluminium gallium phosphide, Gallium arsenide phosphide, Gallium phosphide
5. Orange LED – Aluminium gallium indium phosphide
6. Blue LED – Indium gallium nitride, Silicon carbide, Sapphire, Zinc selenide
7. White LED – Gallium nitride based indium gallium nitride

LED Voltage Drop 

Wattage of an LED
LED is a current dependent device. Minimum 20 mA current is required to get sufficient brightness. If excess current is flowing through the LED, its semiconductor heats up and gradually deteriorate. This leads to poor performance and finally LED will be destroyed. Wattage of the LED is the forward voltage multiplied by the forward current. In high current LEDs, forward current can go up to 350 mA. In these devices the wattage depends on the forward voltage drop ranging from 1.8 volts to 4 volts. Therefore an average of 1 watt is found in high current LEDs.

LED Series Resistor


Light output
Light from the LED depends on the material used and the amount of current flowing through it. Luminous intensity of LED depends on the current flowing through it. Typical LED requires minimum 20 mA current to give sufficient brightness. Some semiconductor material is affected by temperature. High power Red and Orange LEDs suffer light loss at high temperature. The light output and viewing angle of LED depends on the semiconductor as well as the epoxy casing of LED. LED with high luminous intensity will have narrow viewing angle because light rays will concentrate into a beam. Diffused LED will scatter light so that intensity will be less.
LEDs have more life when compared to ordinary bulb. The life of LED depends on the physical parameters and operating current. When the light from the LED reduces to half, it indicates its degradation. High quality LEDs can operate more than 1 lakh hours but ordinary LEDs will not live more than 50,000 hours.

Simple circuits using LEDs.

1. White LED in 230V AC

This simple circuit can be directly connected to 230V AC to make a Night Lamp.

2. LED Continuity Tester


A single LED can function as a continuity tester to check the tracks of PCB, continuity of wires and more.

3. Fuse failure Indicator

This circuit can be hooked between the power supply and the load. When the fuse is intact, LED remains on since it has continuity for current path. When fuse blows, LED turns off.

4. Polarity Indicator

When the positive and negative polarity of the power supply and the load are correct, LED remains off. When the polarity of the load reverses, LED turns on.

5. Current flow Indicator

This is ideal for battery chargers. If current is flowing to the battery LED lights. It indicates whether the battery connector is properly connected or not with the charger.

6. Voltage Monitor

This circuit can be hooked between the Battery and the load to monitor the battery level. As long as the battery holds a voltage above the value of Zener, LED remains on. When the battery voltage drops below the Zener value , LED turns off. For example for a 6 volt battery, Zener value should be 4.7 V and R is 470 Ohms.

7. 12 V battery Monitor


This circuit can be used to monitor the voltage level in 12 V Lead Acid Battery. When the battery holds full voltage ( 13.8V) all the LEDs remain ON. Indications are like this
Battery Full voltage -13.8 V – All LEDs on
Battery Moderate voltage – 12.8 – Red and Yellow LEDs on
Battery Low voltage – 11.8V – Red only on.

8. Simple LED Flasher

A Simple Astable Multi vibrator using two NPN Transistors and two Capacitors and two resistors along with two LEDs. LEDs On/ Off alternately.

9. LED as a Zener


Usually we use Zener diode to get a fixed reference voltage. But an LED can also do the same job since each type of LED has a Forward voltage drop. The voltage from the Anode of the LED will be a fixed reference voltage. See table in the image to get the required reference voltage.

10. Regulated power supply using LED