# How to find a faulty bulb in a Christmas lights string

I have a LED Christmas lights string, which consists of two circuits of LEDs connected serially. It is working directly on 110V AC. Most LED sockets have 2 wires connected to them, some have three. There is a 110V socket on the other end of the string, so these can be chained together.

One half of the string went dark, so I suppose one of the LEDs on that circuit is bad, or its connection is faulty.

LEDs are non-removable (molded plastic socket with lens), and I hope I can trace the string somehow and find where the fault is. Obviously cutting insulation in 50 places in order to test each LED separately is not an option...

If there any sane way to find the fault, either by buying some equipment or building DIY one, or do I need to just replace 100 LEDs string because one went bad?

## locked by Dave Tweed♦May 13 '16 at 21:23

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• how seasonally appropriate... :) – vicatcu Dec 16 '11 at 19:37
• Don't cut the string in 50 places! Binary chop your way through and solve the problem in O(log N) time en.wikipedia.org/wiki/Binary_search_algorithm First divide into {50} -> {25,25} -> {{12, 13},{12,13}}, -> etc. – Toby Jaffey Dec 16 '11 at 20:47
• Check out this video: youtube.com/… Uses a small circuit and a buzzer to detect where the ac noise stops. – captncraig Dec 16 '11 at 22:36
• Your answer is here: Coming this year (2012) vimeo.com/37397543 ledkeeper.com – user9049 Apr 3 '12 at 23:14

I just saw a great and simple project that does just this:

The project is by Alan Yates: http://www.vk2zay.net/

As I understand it, it uses a high impedance gate of a JFET to detect fluctuations in the E-field in the wires due to noise on the mains. The signal is amplified using a BJT to make sound on a piezoelectric speaker. If a light is burned out it the E-field will exist on the wire going into the light, but, not on its exit wire. Using this principal it is easy to locate the burned light. He applies this to incandescent light string, but, the same principal would apply to an LED string.

• Thanks for the screenshot and the explanation. Will try building something like this. In all fairness, though, @CMP posted this video link as a comment above earlier... – haimg Dec 19 '11 at 17:14
• @haimg: Real fairness would be to give Alan Yates all the credit; I certainly tried to do this. I didn't see CMP's comment, but, I did go the distance to make this understandable without leaving the stack-exchange, which was his option too. I personally think that deserves the win, which apparently you agree with. So thanks! I highly recommend that everyone check out Alan's site. This guy really knows his way around the analog and RF. – Dave.Mech.Eng Dec 19 '11 at 18:22
• wow, this is way simpler than what I suggested... brilliant! – vicatcu Dec 20 '11 at 16:19
• maybe see also sentex.ca/~mec1995/circ/xmasbulb.html – cwd Dec 8 '14 at 0:22

How about using two needles (or pins) to "short" one led at the time by pressing through the plastic insulation?. Just saw that this is directly connected to the mains, so better use a transformer, plastic covered needles and an insulation mat

The JFET sniffer is great, but if you happen to be at your local electronics store with a single 399 dollar bill and there are no FETs available, you could buy an oscilloscope to do the mains sniffing.

All you have to do is to plug in the xmas light chain in such a way that the live conductor is the one that gets interrupted by the bulb sockets. In this way, by simply touching the insulation of the wire entering and exiting each bulb socket with the tip of the probe, you can see the ghost of the mains' live. Until you reach the first faulty bulb, that is.

This is the 'background E-field' sensed as a voltage by the probe's tip, when the probe is nowhere near the powered xmas lights (5 - 10 inches away are enough to avoid detection).

And this is the 'field' sensed on the live wire, before any bulbs (and with the light chain dark because of one dead bulb).

The probe was stripped of the ground clip and the retractable tip; you only need to touch the insulation with the tip. The oscilloscope's scales were set to 100 mV/div vertical and 2 ms/div horizontal. (With another, much older, set of lights with very thin wires I had to use 500 mV/div to avoid clipping and see the full sine wave).

Now, when you reach the first dead bulb you will see the ghost of the mains on one end and almost nothing on the other:

(Sorry about the following pictures, I used my cellphone and I cut out the most important part, i.e. the bulb).

You can approach the dead bulb by binary search, if you wish to go full scientific. When you have replaced the first dead bulb found, repeat until you find all dead ones (they will be on the remaining portion of the string away from the plug).

Once the chain is repaired, well, it will light up. But if your eyes are glued to the scope screen and you don't have a solvent at hand, you could still tell because you will be able to see the ghost of the mains sine on both ends of all bulbs.

Now, try to picture yourself on a ladder, with the scope held by a strap around your neck, trying to reach the lights at the top of the tree. Isn't it the most wonderful time of the year?

I've contemplated this myself a number of times... but honestly I've never gone through with it because it's so cheap (albeit environmentally irresponsible) to just go out and buy a new strand.

At any rate, one way I could envision doing it, were I to design a DIY method, would be to transmit a very narrow pulse signal down the "neutral" input, and measure the time it takes to get a reflection of the pulse at the source.

I would generate the pulse with a general purpose I/O pin of a microcontroller which I would subsequently configure as a tri-stated input. I would "listen" for the pulse with an A/D input pin on the microcontroller. This could probably even be the same pin of the microcontroller. You might also want to put a current limiting resistor between the microcontroller pin and the strand of lights.

Knowing how long the pulse took to be reflected, it should be a relatively simple calculation to figure out how far down the strand the broken circuit is. I think it would actually just be (to a close approximation):

$$length = \frac{speed\;of\;light \times measured\;duration}{2}$$

Now, this will probably only work if half your lights are functioning and the other half aren't. If all your lights are out, I would expect you'd get two (possibly) overlapping reflections, which would make the measurement kind of ambiguous. Interpreting the measurement would also require some knowledge of the circuit topology of your particular strand as well I would imagine, but it would at least give you something to go on.