Christmas light fault finding apply mostly to 2.5V incandescent lights bulbs, with 50 bulbs in serious in one strand. Newer LED's don't blow up easily; they just fade away very gradually. But in a "few" more years, my lights can be collectables! Otherwise the nostalgic value is there - your childhood, your child were born, their childhood.
There are non-contact voltage detectors for fault finding. But their range should be an inch or more for detecting wires behind walls. Imagine icicles lights when the original neat formation is gone, and strands are hanged over each other to increase the brightness and light density. Even for one strand, the wires with and without voltage due to faulty bulbs are twisted together.
The faulty bulb finder is an old fashioned high voltage probe, a long single piece of wire, and a plug without polarity so you can swap hot and neutral.
The bending of the probe is incidental and have no relation with the project. A probe is just a bulb plus a high resistance. You touch the red end, using human as the electrical return path. It is safe because the current is very small.
DO not touch the red end because it can now be at 120 V. It is easy to attach a wire to the red end with the screw and spring inside. You should take off the metal clip too as it is dangerous. You should not touch any metal part except for the tip part of the probe - the opposite when using a normal probe. You can tape the end with electrical tapes but after Christmas you can just take the wire out and use the probe again, until next year.
There are probes that looks the same for low voltage as in hobby electronics. You will burn the bulb if 120 V is applied to it.
I actually keep the pair of wire which can be reused. But make sure that only one of the wire is used, and insulate any bare wire of the other unused wire.
A full chain of Christmas lights may have several strands of about 50 bulbs each. For a fully working light, if you take off any one bulb, a section of the light will be off. That's a strand.
Now assume there is only one faulty strand. Procedure:
- plug in the fault finder in any convenient voltage outlet. Don't touch any bare metal or wire (except for the tip of the probe).
- take out about the middle bulb in the strand.
- short the two contacts in the bulb housing with the tip of the probe (this is safe)
- if the strand light up, this bulb is the faulty one.
- Otherwise, probe the two contacts in the bulb housing in turn and note if the probe lights up in each of the two cases.
- Let the state of the probe light be ON (or brighter) and OFF (dimmer). If the state of the probe is OFF-OFF, you need to change the orientation of the plug, swapping the hot and neutral position. Then you should get one ON and one OFF.
- If you can't get an ON state, there are multiple faulty light bulbs. You should pick any one side as the faulty side.
- The faulty bulb is on the OFF side of the strand. The wiring into the housing should clearly indicate which is the OFF side. If not, you can pick one side at random and assume it is the OFF side. Then see later if your assumption is right.
- Put the bulb back.
- On the faulty (OFF) side, take off the bulb which is mid-way between the last test bulbs or the end of the strand.
- Repeat from 3 until 4 happens, or in case of multiple faults, you will approach the faulty bulb on both sides.
Happy New Year!