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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.

Step 2: Basic Construction

One may, as I have done, solder all the parts together (use a heat sink, especially on the LEDs and the diodes) or you may choose to use a small pc board.

The Schematic shows the basic connection of the parts.

Step 3: Seal It, or Box It

I did not box this one, as I wasn't planning on using it with high voltages.

I merely sealed it in hot glue as pictured. It isn't very pretty, but it functions well.

Some tips as pictured, if you will be using insulated clips as I have, first thread the wire through the removed insulator as shown, and then solder / crimp it to the alligator clip.

If you plan on covering a large area with hot glue, it might be to your advantage to tape or cover in some ways, the device in case you need to use it outside in the heat, or inside repeatedly where it may get warmed by you hands or other source.

Step 4: Testing the Tester

First thing I did was hook it directly to a power supply to test the polarity of the tester.

The first picture shows that the Red wire attached to the positive portion of the PS plug, lights the green LED.

I then attached it to a known source directly to 115 v AC, and both LEDs lit, as would be expected.
Second Picture

Step 5: Final Step: Optional

As I did not make the original very good looking, I spruced it up just a little with some masking tape and a written in Title: Polrty. Tester

The last picture shows a 9 volt battery hooked up with the polarity reversed, demonstrating that this works also.

EDIT: I have installed a switch. Since the design made the 11 k resistor out of a 10 k plus a 1 k, I simply soldered the switch to bypass the 10 k resistor when closed. Opened, the current flows through all 11 k.

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
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
Is there a need for the diodes? Since the LED's are diodes, I wouldn't think the extra ones are needed..
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
Wow! I'm really suprised you could apply it directly to 110VAC without burning the LED's. Test it on 220VAC :-P
Because of the resistor size, the LED's do light pretty dimly at 12 v or less. The diodes also protect against <em>reverse polarity</em> hookup<br/>
Cool
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
I don't know what yours looks like so I don't know :-)
I have just acquired a bunch of LEDs that will light up red <em>or</em> green, depending on the direction of the current through them.<br/>
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).
The negitive leg.
I did, but I'm sure others didn't.
thanks great job this is really useful
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).
thank you again I am going to try and gather some stuff this weekend to make one
That's cool! I agree with Nacho, it may not look great but it sure looks useful!
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.
Thanks.
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.
LED's, this happens when I rush to finish writing things up. My apologies, and it has been corrected: Thank you.
. Great job.<br/>. I don't think it will win any beauty contests, but it's still a great job. heehee<br/>. Will it really work at 15VDC <strong>and</strong> 115VAC? That's quite a range.<br/>
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

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Bio: I am, most definitely older than 00010101 and to put it simply, still curious about nearly everything :-) I then tend to read and/or experiment ... More »
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