Test Tools: A polarity tester

This is a simple but very handy little device to have around the home and workshop, which will help determine the polarity of two wires, or contact points of a DC circuit.

EDIT / UPDATE: If you wish to also use this circuit for voltages LOWER then 12 v or so, you may need to install a bypass switch over the resistor and to be safe, include a 1 k resistor in series with the switch.

Step 1 Gathering the parts

There aren't many parts to this. I didn't even use a pc board, although you certainly may. A small strip of one just a 4 -6 hole wide is sufficient.

R1 = 11 K 1/4 W resistor

D1 = 1 Bright Green LED
D2 = 1 Bright Red LED

D4, D5, D6, D7 = 4 1N4004 (or you can substitute 1N4005, 1N4007) Silicon Rectifier Diodes.

Misc: PC Board (optional), wire, case, Probes / Clips

Tools: soldering iron, needle nosed pliers, possibly hot glue and glue gun, etc.

Comments

[knexsuperbuilderfreak]

ah i rember whem i had a eletronic kit and i exploaded a led because i put it i wrong and this will prevent that great job

[Goodhart]

Thanks, if you will not be using it on anything high in voltage (above 12-24 vdc), you can lower the resistance value to around 760ohms - 1k so you will be able to see the LED

[thermoelectric]

Is there a need for the diodes? Since the LED's are diodes, I wouldn't think the extra ones are needed..

[Goodhart]

I am protecting the LED's, since I tested this with a fairly high voltage (in fact, it works best above 12 volts, but it can still be seen at 12. I even applied it directly to 110 vac without burning out the LED's

[thermoelectric]

Wow! I'm really suprised you could apply it directly to 110VAC without burning the LED's. Test it on 220VAC :-P

[Goodhart]

Because of the resistor size, the LED's do light pretty dimly at 12 v or less. The diodes also protect against reverse polarity hookup

[thermoelectric]

Cool

[drbill]

HAY ! Yours got more parts than mine. What's the deal ? I especially like the goop you used to cover it all. All in all I think yours is a better circuit. I like it ! Mine is only good for around 12v. or less. I like yours. I think I'm gonna use a rotary switch to change out the diodes though. Excellent work ! Thank You. DrBill KB1LZL 73's

[Goodhart]

I don't know what yours looks like so I don't know :-)

[Kiteman]

I have just acquired a bunch of LEDs that will light up red or green, depending on the direction of the current through them.

[Goodhart]

If they are the 3 legged type, this circuit can still be used, if only 2 legs, then a resistor would have to reside on one of the legs (or use two, with 1/2 value on each leg, but you probably already knew that).

[drbill]

The negitive leg.

[Kiteman]

I did, but I'm sure others didn't.

[navarrofeds]

thanks great job this is really useful

[Goodhart]

Thank you. One thing I did discover and neglected to mention is that the higher rated diodes I used, have a pretty high forward voltage drop, so if one does not plan on ever accidentally or on purpose using this on voltages higher then around 60 v or so, they could use lower rated (and lower voltage drop) diodes for D4-D7).

[navarrofeds]

thank you again I am going to try and gather some stuff this weekend to make one

[Gjdj3]

That's cool! I agree with Nacho, it may not look great but it sure looks useful!

[Goodhart]

It could easily be put in a small box, I just didn't have any at the moment. Especially now that I have installed a lower voltage switch for measuring things below 20+ V.

[Goodhart]

Thanks.

[Phil B]

In step 1 did your really mean to designate D1 and D2 as resistors? Did you mean LEDs or diodes? It is a neat circuit, and simple.

[Goodhart]

LED's, this happens when I rush to finish writing things up. My apologies, and it has been corrected: Thank you.

[NachoMahma]

. Great job.
. I don't think it will win any beauty contests, but it's still a great job. heehee
. Will it really work at 15VDC and 115VAC? That's quite a range.

[Goodhart]

Thanks, and yeah, it seems to do ok with both. It was so simple I just didn't bother with a box.....although, the continuity tester I am planing, one with sound, will be boxed