Like every year around the same time, I hurried to get my Christmas tree all set up and the first thing we do when the tree is 'standing' we like to hang the lights in the tree. Okay, better first test them before putting all 50 of them in the tree. Yep! Working beautifully. I started a carefully planned organization of the lights so they would be evenly divided over the branches. Now the second string of lights, tested, yep working. In the tree with them. Putting the plugs into the receptacle and... oh no-- one series of them are on in full glory, all the others are out. Annoyed I tried to 'fix them' by trying to push each bulb further into their sockets. Still no go.

It was a crime trying to pull all the bulbs out of their sockets to measure them for continuity. Funny enough, and against the law of nature, it was not even the last bulb in the string of 50 which was defective, but number 41.
I put a new bulb in it, and yes here we go, they all light up beautifully. Alright! Happy again I again hung them in the tree. Finally the big moment arrived, as soon as I plugged them in they would shine in all their glory. Right? Oh no! The second I plugged in my lights only the first series of bulbs lighted up, same as before. All my work for nothing. Sigh...

In the mean time it was already way past midnight and so I decided for my next attempt to wait till next morning. Irritated and very annoyed I went to bed. However, I was so irritated that I couldn't sleep immediately and so was thinking of a smart way to get to the defective bulb the easy way. All over sudden I got it; if the bulb was not lit, there was no current draw either and up to the defective bulb I would measure the 115V AC (phase). Now I knew the solution, I almost fell asleep satisfied right away.

The next day I had to get some groceries in I noticed new xmas lights for a small price. $5.95 for a string of 100 lights, and with a CSA and UL sticker. Wow, I thought for that kind of money I might as well forget the repair and buy a new set. So I did. Coming home I plugged the new lights into the receptacle and yes, all 100 were doing fine.

Happy again with the new lights I again hung them in to the Christmas tree, not suspecting that this could be another rotten day. After fiddling with the lights to get them all neatly organized in the tree the moment had arrived to plug them in and awe at the fascinating beauty of those little lights. Yes? NO! Not again. Isn't this to explode out of your skin! Angry I was looking for a solution, but there was none. I finally decided to put a circuit together on a piece of experimenters board from Radio Shack.

The heart of this little "CIRCUIT" is established by a hex inverter IC, the MC14069.
By positive feedback to the input, the first inverter acts as an analogue amplifier, which amplification can be adjusted a bit via the 50K trim potentiometer.

To get the correct polarity on the basis of the transistor a second and third stage inverter have been added the same way. The others I put to the positive input voltage of the 9-volt battery.
When you touch a voltage carrying wire, with the antenna connected to pin 1 of the MC14069, the led will light up. The antenna is just a sturdy small piece of wire.

Armed to the teeth with this little tester I re-investigated the cords. At the first try I of course picked the wrong wire; the neutral (0). The moment I tried it on the other wire (phase) the led came on right away. I followed the cord from bulb to bulb sliding the piece of antenna wire over the cord until I hit the broken xmas bulb and the led went out. Aha! Finally got the bloody little sucker! The broken bulb showed voltage on one site of the wire (led on) and none at the other end of the bulb (led off). This little tester can also be used for other AC applications, like checking for broken wires behind the wall and stuff.

If you have questions about this circuit, please direct them to Jan Hamer or visit his website in the Netherlands (if you can read Dutch).

Published & Translated from Dutch into English with permission of Jan Hamer, The Netherlands.