Will shorting the tweezers destroy the microcontroller or is there inherent protection in the 328p?

I see one issue with the code. The tweezers stay in one state a lot longer than they do in the other. As a result if you have the tweezers connected one the LED will light brighter than the other way (you might not even see the led light at all unless you reverse the connection). The code sets the tweezers to one polarity at the top of the code, then switches it back at once. It then goes through a test of the timer, and returns to the main loop before getting back to the top of the loop() function. There could be a 2:1 or 3:1 time difference between on states. You might want to put a few nops' in the middle of the polarity switch to even things out.

Edit - The sense resistor was not necessary. It seems to work fine with the ADC pin connected directly to the test pin

I took on some of the comments and came up with a working breadboard prototype that uses an ATTINY85 and lights an LED on the probe tip that is in contact with the anode. I also timed the code so the LED under test lights evenly regardless which way round it is in the tweezers. I used PB2 and PB4 as the switching outputs to test the LED under evaluation, PB3 to sense the voltage on PB2, and PB0 and PB1 to drive the polarity indicating LED's. With a little logic and careful setting of the sense voltage break points, the code can tell which probe tip is connected to the anode and light the appropriate indicator. The code uses analog read to determine the system voltage each time it is booted up and calculates break points based on the current voltage. This is a simplistic percentage calculation based on some emperical testing of LED's I had laying around in an attempt to account for varying battery voltage. It would probably make more sense to calculate the ranges based on expected voltage drops across a range of LED's and the sense resistor for different voltages that might be seen, e.g. from a discharging battery. I plan to use a boost regulator and power it from a 1.5V cell so it shouldn't change once it is dialed in.

This is a very simple circuit needing only the uC and power to work as per the original post and a current limiting resitor (optional) and a couple of indicator LED's to add the polarity indicating function.

I have tried it on 3mm and 5mm TH LEDs in red, green, blue, white, yellow, IR, UV and smd green and blue and it works with them all. They are all run-of-the-mill LED's so it may not work with some others.

I have made test probe tweezers before by etching copper clad boards and screwing them to a plastic insulator. I plan to build another set with the circuit etched into the tweezer arms and use all smd components which I already have. The cost is a few bucks for the parts. I wish I had ordered some of the cool colored FR-4 copper clad last time I bought material. It would be perfect for this.

Thanks Pete for posting the project. Here is my code.

 //LED Tweezer on ATTINY 85 adapted and expanded from Sparkfun example by Pete Lewis
 //10/07/2012 Cary R Reeves

 //Pin Assignments
 int sense = 3;
 int tweezerA = 4;
 int tweezerB = 2;
 int outLED1 = 0;
 int outLED2 = 1;
 int range1Upper = 0;
 int range1Lower = 0;
 int range2Upper = 0;
 int range2Lower = 0;
 int VCC = 0;
 int voltageTest = 0;

 void setup()
   {
    pinMode(sense,INPUT);
    pinMode(tweezerA,OUTPUT); 
    pinMode(tweezerB,OUTPUT); 
    pinMode(outLED1,OUTPUT);  
    pinMode(outLED2,OUTPUT);
    digitalWrite(outLED1, LOW); 
    digitalWrite(outLED2, LOW); 

    // Read VCC and calculate break points
    digitalWrite(tweezerA, LOW);  
    digitalWrite(tweezerB, HIGH); 
    VCC = analogRead(sense);
    range1Lower = VCC*0.06;
    range1Upper = VCC*0.4; 
    range2Lower = VCC*0.55;
    range2Upper = VCC*0.95;
   }
  void loop() 
  {
    // Set tweezer voltage

    digitalWrite(tweezerA, HIGH);
    digitalWrite(tweezerB, LOW); 

    voltageTest = analogRead(sense);
    if(voltageTest > range1Lower && voltageTest < range1Upper) 
         {
         digitalWrite(outLED1,HIGH);
         }
    digitalWrite(outLED2,LOW);  

    // Reverse tweezer

    digitalWrite(tweezerA, LOW); 
    digitalWrite(tweezerB, HIGH);  

    voltageTest = analogRead(sense);
    if(voltageTest > range2Lower && voltageTest < range2Upper) 
         {
         digitalWrite(outLED2,HIGH);
         }
    digitalWrite(outLED1,LOW);

 }

Throwing a microcontroller dev board at a problem like this seems like overkill. I've got one of these LED Testers and it works great. And you don't need to worry about leaving the power on.

Interesting tutorial, but I thought if you sourced power to an LED the wrong way it would blow it out. Is it a "trick" of the 'fast switching' aspect?

Also, when learning how to do simple LED circuits I found you can do switching with a couple 2N3904 transistors, would that work? Save about $15 on the Arduino, but lose the cool alarm feature...

I may have to build one of these, just for the 'neat' factor of it! Thanks!

I commented on this in the news post that covered this tutorial ( http://www.sparkfun.com/news/926 ) and will just add a bit here:

It would be nice if there were actually a polarity indication so that not only does it not matter which way around you have the leads, it'll tell you whether the positive lead is at the anode, or at the cathode.

It addition, the polarity detection code can then stop the switching and hold that polarity, for testing blinking and RGB-program LEDs. They may also be fine with a high enough switching frequency, though.

That same code could be used to deal with bipolar LEDs (which would be forward based in either direction, simply emitting different colors) by changing to a lower switching frequency.