Simple Polarity Checker




About: I am an 18 year old hobbyist and tinkerer with a deep interest in DIY recycling, electronics and robotics.

Ever encountered difficulty in knowing if you've connected the wires right? Tired of pulling out your multimeter every time? Need something more compact and simpler to help you out? If you do, then this Instructable is for you!

Read on to learn how to make a very simple, handy and useful polarity checker.

NOTE: Do not be afraid! You won't need to order anything. Everything used here can be found in old electronics and this is so simple that even beginners can make this.

Step 1: Getting Started: Things You Need

In order to build a simple polarity checker, you need the following ingredients:

  • Two (2) LEDs (green and red or any other color)
  • Four (4) rectifier diodes (1N4007)/ any other will do fine
  • One (1) 1 kilo-ohm resistor
  • A small breadboard (optional)
  • Wires
  • A small project box/enclosure
  • Soldering iron + Solder wire
  • Heat shrink tubing or heat-shrinkless sleeve
  • Some screws
  • Some cardboard or an acrylic sheet

Step 2: Making the Circuit

The circuit for the polarity checker is very easy, just follow the diagram while making it.

In the diagram, R1 represents the 1K-ohm resistor while the D3, D4, D5 and D6 represent the diodes (1N4007). The D1 and D2 are the red and green LEDs respectively.

Also, to make the test leads, you may use the leads from an old multimeter or make them yourself using the caps from old pens or thumbtacks (the ones with the colored plastic heads). As soon as I make them myself, I'll put it up here; that's a promise!

Another thing! I did not use a breadboard while making this, because I could not find one which was small enough to fit into my project box, but you can use a breadboard if you want to! It would make the job easier and save you the time it took me to cut the wires, solder them and then put on the wire sleeve.

If you are using a project box similar to mine which does not have a cover, then use the cardboard or the acrylic sheet to make. Simply trace the outline of the project box onto the cardboard or acrylic sheet and then cut it out. Align it with the screw holes of the project box and use a pin (or directly the screws) to make holes in the cardboard. A soldering iron or drill may be used to make holes in the acrylic sheet.

After completing the circuit, enclose it in your project box and tighten the screws!

Step 3: Finishin' Up!

Once you have made the circuit, test it by putting the leads on the terminals of any battery or DC power source. If the positive test lead is kept on the positive terminal of the battery and the negative lead on the negative terminal, then the green LED will light up. The opposite will occur (i.e. the red LED will light up) if the test leads are reversed (i.e. the positive lead on the negative terminal and negative lead on positive terminal).

If the LEDs do not light up, then:

  • Check if you have installed them correctly.
  • Refer to the wiring diagram in the previous step.

(The negative terminal of an LED is usually on the side on which the LED is flat, as in the given picture.)

Once you solve the problems (if there are any!), the polarity checker is ready! Please do not be afraid to connect it to higher voltage and current sources. I have connected this polarity tester uptill 12 Volts, 2.5 Amps and 6 Volts, 6 Amps and it has worked fine. The diodes and the resistor give a lot of resistance and hence, prevent damage.

P.S. If you need any help regarding this, want to make some suggestions for improvement or have any questions then please do not hesitate to contact me.

Good luck and Happy Polarity Checking!



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15 Discussions


1 year ago

Great and simple useful tool! I reused old tester leads soldered on a very tiny perfboad. Also used a single 2 color LED to have a single point of checking.


3 years ago

This looks neat, nice work !


3 years ago

when probe is in positive, red led will light up. when probe is in negative green will light up an red will be off. for me this circuit is better because it will not confuse me, every time red led is on this means that lead is positive and vice versa. :) in ac both led will light.

1 reply
Salman NaveedSidhT

Reply 3 years ago

That's exactly what the polarity checker does! I'm happy that it helped and inspired you to build one!

Salman NaveedRafayJ

Reply 3 years ago

The feeling is mutual, my friend. By the way, we need more Pakistanis on Instructables!


AC has no polarity. AC has no rules. AC bad. AC confusing. *crawls back into pit of DC powered electronics*
...this is why I'll never be an electrician...


Reply 4 years ago

it will work.. haha..
blinking on off, on off

Salman Naveedlnss

Reply 4 years ago on Introduction

As AC current changes polarity constantly, the polarity checker will not work. But as I have mentioned, it can be used in DC current with voltages of 12V and beyond.

Just4Fun Media

4 years ago

Interesting build. This could prove quite useful with some electronics projects I am working on.

Have a great day! :-)

1 reply

Since it draws its power from the source under investigation, that makes this very handy as there is no battery to wear down. Very good circuit design.

1 reply