LED as a Polarity indicator

Simple and Useful LED Circuits

Due to the advantages like low voltages, long life, cheap, reliable, fast on-off switching etc, the LED’s are used in many applications. The various applications of LED are,

  • All kinds of visual displays i.e. seven segment displays and alpha numeric displays. Such displays are commonly used in the watches and calculators.
  • In the optical devices such as optocouplers.
  • As on-off indicator in various types of electronic circuits.
  • Some LED’s radiate infrared light which is invisible. But such LED’s are useful in remote controls and applications like burglar alarm.

Other simple applications of LED are :

1) LED as a Polarity indicator :

LED as a Polarity indicator

Sometimes you need to know which wire/node is positive polarity and which wire/node is negative polarity.The LED as a polarity indicator circuit is show in above diagram. In the diagram as you can see two LED’s are placed in opposite directions so when one LED is ON the other LED will be OFF. I have used Red and Green LED’s.  When node A is positive green LED glows while when node B is positive, Red LED glows. This indicates polarity of the input d.c voltage .

2) LED as a Continuity tester:


The above shown circuit is LED as continuity tester. This circuit is used for checking continuity of cables, connectors and switches. When A and B terminals are shorted, the LED glows due to battery connected inside indicating the presence of continuity.

3) LED as a High a.c voltage indicator:

High a.c voltage indicator

To prevent the reverse biased destruction rectifier diode D is used. The presence of voltage is instead of  series resistor, a capacitor is used whose reactance Xc which is ( 1/2 πfC )Ω limits the current. Advantage of using capacitor is that it does not consume any power hence total power dissipation is less.

4) LED as a Blown fuse indicator:

Blown fuse indicator

When fuse is working, it provides short circuit path bypassing LED circuit. So LED is OFF. If fuse opens the line voltage drives the LED through capacitor and thus LED glows. Thus blown fuse condition is indicated.

Not only these there are many useful applications of diode.
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