Summary

A simple electronic circuit and a printable housing to make an electrical polarity tester

Use it to check your electronic circuits. It will tell you which wire is live, which is ground, or even if it's AC.

Designed to work on circuits from around 3-15V.

Instructions

OVERVIEW

This is a simple polarity tester circuit that I use with my electronics projects. It's good for a quick sanity check, to see whether two connections have a voltage across them. The circuit tells me which of the two has the highest voltage, or if the voltage is AC.

The LED will shine:

GREEN if the probe to your RIGHT is positive, or
RED if the probe to the LEFT is positive.
YELLOW if the circuit is AC.

You hold the device with the two "probes" pointing away from you, and touch it to your circuit. Of course you may need to use some croc/alligator clips.

CONSTRUCTION

Build the circuit by taking a bicolor LED and soldering a 1500R resistor to its cathode (that's the SHORTER of the two leads).

I do that by making a loop in the lead as close to the body of the LED as possible, then clipping off the remainder of the lead. Then slide the resistor into the loop to hold it in place while I solder.

The anode lead of the LED now needs to be bent out sideways, and wrapped around a bolts, which act as one probe for the tester. Do the same with the lead from the resistor to make the second probe (see image).

The nut and bolt came from Home Depot: "4-40 x 1/4 x 3/32" according to the packaging.

I use two nuts per bolt, one on each side of the lead.

HOUSING

Place the electronics inside the 3D printed housed. When mounting the electronics ensure you have RESISTOR TO THE RIGHT (see image).

Another nut on each bolt is used to hold the electronics in place in the housing.

This means you will need six nuts and two bolts.

As the picture shows, the probe with the resistor is on the right. IF YOU GET THIS WRONG, THE LED POLARITY INDICATION WILL BE SWITCHED.

The cap can now be put on the housing. The two halves should latch together, but if you like, you can add glue to hold the housing closed.

See my YouTube video for additional assembly instructions: http://youtu.be/xc9GlTj9xAQ

WARNING

TESTED WITH VOLTAGES FROM 3 VOLTS TO 15 VOLTS PLEASE WATCH YOUR MAX OPERATING VOLTAGE

Most of your home hobby electronics will be in the 3 to 9 volt range, so the circuit is designed to tolerate those voltages. The resistor in series with the LED is 1500R, which is about 10times larger than you would normally use for current limit on an LED. This means the circuit shouldn't explode in normal operating voltages, but the LED will be a little dimmer than you might normally expect.

Polarity Tester