Simple tester checks Christmas-tree lights
Why is it that you always test 48 bulbs before you find the bad one in a 50-light string? This simple circuit allows you to divide and conquer, greatly reducing the time it takes to find the bad bulb.
William Dias, Brown&Sharpe, North Kingstown, RI -- EDN, December 23, 1999
|
Why is it that you always test 48 bulbs before you find the bad one in a 50-light string? The simple circuit in Figure 1 allows you to divide and conquer, greatly reducing the time it takes to find the bad bulb. The circuit uses a pair of NE2 neon bulbs with current-limiting resistors. You can use a pair of Radio Shack 272-1100 bulb-resistor sets. It's convenient to house the tester in a clear piece of plastic tubing, with the probe tip emerging from one end and a light-duty power cord emerging from the other end. You place the bulbs in the tube such that one is close to the probe tip and the other is near the power cord, so it's easy to remember which bulb lit last. The probe tip connects to common point between the neon bulbs. It consists of thin spring wire with all but the last ¼ in. insulated. You use the bare tip to make contact with the crimp connectors in the base of the bulbs.
Series-string Christmas-tree lights come in two types. The first type is the continuous-series string (Figure 2a). In this configuration, one wire from the plug goes from bulb to bulb until it reaches the last bulb. A return wire bypasses all the bulbs and returns to the plug. The second type is the alternating- series string (Figure 2b). In this connection, one wire from the plug goes to the first bulb, and the other wire from the plug goes to the second bulb. The connections then alternate through the string. To troubleshoot a defective continuous series string:
-
Plug in both the tester and the bulb set.
-
Insert the tip of the tester's probe into the wire hole in the base of the first bulb. One of the neon bulbs should light; remember which one.
-
Move halfway down the set and insert the probe again. If the same neon bulb lights, then the problem is in the second half of the set. If the other neon bulb lights, then the problem is in the first half of the set. Either way, you are testing 25 of the 50 bulbs without breaking into a sweat.
-
If the original neon bulb lights, move halfway down the remaining part of the set and try again. If the other neon bulb lights, you must move back halfway to the last bulb you tested and try again. This process should allow you to find a bad bulb in a set of 50 in only seven steps. You know you have the bad bulb when inserting the probe tip into one side of the bulb lights one neon bulb and placing the tip in the other side lights the other neon bulb.
To troubleshoot a defective set with many bad bulbs, use the same process as above. At some point, you will reach the dead spot between two or more bad bulbs. When you reach this point, neither neon lamp will light. Back up, just as if the other neon bulb had lit. You know you have a bad bulb if the probe lights when you plug it into one side and nothing lights when you plug it into the other side. Replace this bulb and start over.
To troubleshoot an alternating-series string, you must work in pairs. Test the first bulb, and one neon bulb lights. Test the second bulb, and the other neon bulb lights. Now move down the set an even number of lights and test the next pair of lights. When you pass the bad bulb, the same neon lamp lights for both series-string bulbs. (DI #2457).
-
The marketing manager for your string of lights is counting on you to be cheap and lazy. Cheap: you will buy the cheapest set of lights you can (often cheap goes with low quality). Lazy: you will not attempt to fix your problem, you will throw money at it by buy a new strand.
Matt Swain - 2012-3-1 13:40:24 PST -
The last string of lights I tried to debug had multiple bad or loose bulbs, not to mention intermittents.
It's time to give up.
Kenneth Lundgren - 2011-22-12 12:20:33 PST -
There is an easy way that Designers use but may be too much for engineers. I get out my voltage sniffer, that I normally use to see in a glance if there is power in a panel, and follow the string. When you get to a bad bulb, the sniffer no longer sees power. Replace the bulb. If it lights your done, if not, sniff on down the line.
Stanley Lichtenberg - 2011-19-12 14:17:24 PST -
@Kyle B. Why should I waste resources and purchase another string, adding an only slightly bad string to the waste stream? On top of that, consider that when I am involved with putting Christmas lights up anywhere at all is a time when I would really NOT want to visit any venue that is selling the lights.
Besides that, if engineers did think just like everybody else, we would be really poor as engineers. MY engineering success comes from NOT thinking just like the rest of the people.
William Ketel - 2011-19-12 13:44:07 PST -
Hi
There is an easier way to find the open bulb.
Get your oscilloscope warmed up and plug in the non functional string of lights. Ground the scope. Place the tip of the probe near a socket. then "walk" the probe along the string if you start with the "neutral" there will be only small "hum" on the string until you get to the open bulb, where the observed hum suddenly increases.
That is the open bulb. You can use a binary search pattern, but any way you do it is about as quick, and no mental gyrations are required.
Make sure you test away from the return lead since it has big hum as well. I do it by placing the probe next to the bulb socket. Side one, then side two.
Hans J Weedon
Hans J Weedon - 2011-19-12 12:25:38 PST
