So every once in a while I have a power supply of which I do not know what the polarity of the wires is. Testing with a multimeter is off course an option. But I chose to make a polarity tester for this purpose.
The general idea behind it is very simple. Take two leds and solder them opposite wise, meaning positive lead from first led to ground lead of the other one. Now add a current limiting resistor and that's it. Now make sure you solder a RED wire to the postive lead of the green led and a BLACK wire to the ground lead of the green led. Just look at the diamgram below.
I soldered it on a prototyping PCB and that looks like the next picture
Next I designed an enclosure in Tinkercad. You can find the Tinkercad design in this link which I provide so you can edit it for your own purposes. I will also set a link to the STL file at the bottom of this story so you can print it right away.
The next picture shows you my box being printed. Maybe not completely obvious from the pictures but the wholes for the leds are in the bottom of the box, which therefore automatically is the top......
Now the RED wire is the positive lead and the BLACK wire is the ground lead. So look what happens if I put the BLACK wire on the positive connection of your power supply and the RED wire to the negative connection:
Indeed the RED led lights up as the connections are wrong !!!
Now look what happens if the connections are right. So the RED lead is on the positive lead of your power source (a CR2023 battery here) and the BLACK lead on the ground connection of your powersource:
YEAH !!! Thats it. Red for positive and BLACK for ground. Childsplay. This will work up to a considerable voltage before you will burn out the leds. But hey leds are cheap so dont worry.
I stole this idea from an instructable which you can find here.
All that's left now is to provide you with the STL file.
Polarity check STL