Electrical polarity (positive and negative) is present in every electrical circuit. Electrons flow from the negative pole to the positive pole. In a direct current (DC) circuit, one pole is always negative, the other pole is always positive and the electrons flow in one direction only. In an alternating current (AC) circuit the two poles alternate between negative and positive and the direction of the electron flow reverses.

A light-emitting diode (LED) is a semiconductor device that produces light from electricity. LEDs last a long time and do not break easily (compared to incandescent lightbulbs). They can produce many different colors. They are efficient - most of the energy makes light, not heat.
An LED is a type of DIODE that makes one color of light when electricity is sent through it in the expected direction (electrically biased in the forward direction). This effect is a kind of electroluminescence.

A diode is an electronic component with two electrodes (connectors). It allows electricity to go through it only in one direction.

Today, the most common diodes are made from semiconductor materials such as silicon or sometimes germanium.

This polarity checker is super handy,and can indicate the polarities of a battery or it maybe LEDs etc.

The indication is done by blinking of led.

Step 1: Components Required

Few LEDs.

Few Resistances 330 ohms-1k ohm.

Battery we have used a 9V.

Female Connectors.

Breadboard or a PCB depends how you want to make it.

Couple of wires.

Step 2: Whats Happening

Connect the circuit as shown in circuit diagram..

Power Supply Polarity Checker

There are two LEDs both connected as the mirror of each other.Now as you might already know that LEDs are just simple diodes.Depending upon how you are connecting the LEDs one will become forward bias ie. it will allow current flow through it, and the other will become reverse bias ie. no current flow.The only purpose of connecting a resistor is to save the LED from fusing.The diode then forward biased will conduct current this is how the particular LED will glow.Indicating the polarity of the battery.

LED Polarity Checker

An extremely simple circuit,but still.

The positive terminal of the power source is connected to the resistor and so to the connecting port,the negative terminal to the other connecting port,thus completing the circuit.Now if someone inserts for example a LED,it wont glow until the diode inside it is forward biased,for this the positive leg of the LED has to be inserted in the port connected to the positive terminal of the battery.Now you can find the terminals even the legs are even.