Xmas Ornament Upgraded With LEDs

A friend of mine owns an (almost antique) Christmas ornament. It is a cute little tree within a picture frame; everything is snowy white. At the end of each branch there is a tiny light bulb, also referred to as pea bulb. There are 40 bulbs in total, arranged in four groups. Each of the groups consists of ten light bulbs connected in series. The four groups are connected in parallel and, via a long cable, to a wall wart delivering a nominal output voltage of 24 V AC, as shown in the circuit diagram above (that looks more complicated than it really is :-)

Since the light bulbs are connected in series within the groups, if only one of the bulbs in a group burns out which, of course, happens from time to time, the complete group becomes dark. Finding the faulty one within the group of ten is rather difficult, and finding matching replacement bulbs - voltage, current and mechanical dimensions - is next to impossible. To make matters worse, in the present case not all of the light bulbs were still present since they tend to get lost over time. The photograph above, btw, shows the unit with only the last group functioning.

Since there was a lot of emotional value attached to this object, my friend was very reluctant to part with it.

I suggested to him that one might replace all of the bulbs by 3 mm LEDs. This was rather careless on my part because it was, if possible at all, connected with quite a bit of work for me.

1. Multimeter
2. Continuity tester – this is often contained in your multimeter
3. Small wire cutter
4. Soldering iron, solder
5. 4 permanent marker pens with different colors
6. Hot glue gun
7. Drill bit with the same diameter as the wires from the wall wart
8. Small power drill
9. Some bare wire (approx. 0.6 mm dia.)
10. 36 high-efficiency, warm-white LEDs, diffused lens (or clear lens + some Scotch-Brite)
11. 4 resistors 15 kOhm, 1/4 W
12. Some matrix board or strip board cutoffs; tools to work on the matrix board, such as jigsaw, rasp/file or sand paper
13. 1 DIL bridge rectifier (e.g. B40D)
14. 1 electrolytic capacitor 47 uF (or 50 uF), min. 50 V
15. Some heat shrink tube
16. Some white paint (e.g. Tipp-Ex) or white heat shrink tube
17. In addition, if you want to measure the forward voltage of one of your LEDs (they will be more or less the same within a batch of several LEDs), you need a fresh 9 V (6LR61) battery, a matching contact clip and a series resistor. The value of this resistor depends on the desired forward current and is determined below.

Nowadays, virtually everybody knows about the longevity and the high efficiency of LED lamps compared with conventional, incandescent light bulbs.

Since I remembered having a sufficient number of white 3 mm dia. LED leftovers in my junk box, I suggested to him to substitute the light bulbs by LEDs.

When starting such a task, there are several different facts to consider:

1. The voltage and current rating of the original bulbs was about 2.4 V at 40 mA each.
2. Light bulbs work OK no matter whether they are supplied with alternating current (AC) or direct current (DC). For LEDs, in contrast, a DC supply is mandatory. Producing DC from the wall wart's AC output is not exactly rocket science, we'll manage that.
3. White high-efficiency LEDs require much less current than light bulbs for the same brightness, but they need a DC voltage of around 2.7 to 3 V to illuminate – which is somewhat higher than the one of the AC light bulbs. We will see later that the existing wall wart could nevertheless be re-used for this project.
4. Due to the non-linear forward voltage/current characteristics of the LEDs, the forward current needs to be limited. This can simply be done by using a series resistor (always assuming that the supply voltage is more or less constant).
5. Since LEDs must never be connected in parallel without individual current limiting series resistors, each of the four groups needs its own resistor.
6. The LEDs not only need to be matched electrically to the available hardware, but they also have to fit mechanically.

Points 4 and 5 above dictate that a series resistor is installed per group. Without major surgery this can only be done by substituting one lamp per group by a resistor. The complete ornament will then be populated with only 9 LEDs and one resistor per group, so for four groups we need 36 LEDs plus 4 resistors, as shown in the diagram above

If you want to check the forward voltage of an LED at a particular current, you need one of your LEDs, a 9 V (6LR61) battery (or a power supply adjusted to 9 V DC), a series resistor, and a DC voltmeter connected in parallel to your LED. Refer to the second diagram above. Assuming that the LED's forward voltage will be around 3 V, the resistor should have a voltage drop (U) of about 6 V. For checking the LED at a current (I) of, say, 1 mA, the value of the series resistor (R) is then calculated using the following formula (Ohm's law, anyone?):

R = U / I = 6 V / 1 mA = 6 V / 0.001 A = 6000 Ohm = 6 kOhm

The next-higher value in the E12 series is 6.8 kOhm. This is OK, since the forward voltage of the LED will perhaps be a bit less than 3 V anyway. If you manage to find 6.2 kOhm resistors from the E24 series this is ok as well. And if you want to have a different current, you can now calculate the series resistor you need by yourself, it isn't witchcraft really.

However, when playing around with LEDs, there are two things you absolutely need to take care of:

1. Never operate an LED with a forward current larger than the maximum current indicated in its specs, and
2. Never use it in opposite polarity! Standard LEDs can only survive reverse voltages of around 5 V or even less!

The no-load output voltage of a mains transformer (or the wall wart, in our case) is always somewhat higher than the nominal voltage (which is called 'nominal' because it is output when the transformer feeds the nominal load, i.e., when it delivers its nominal current). In our case the no-load voltage was 28.1 V AC, in contrast to its nominal voltage of 24 V AC.

Although my LEDs were specified for a maximum forward current of 25 mA, I considered the brightness for this application sufficient with a forward current of as low as 1 mA (which is good for longevity as well as low power consumption). I tested one of my LEDs with 1 mA DC and measured a forward voltage of 2.7 V DC.

Multiplying this voltage by 9 (since there are 9 LEDs connected in series per group), I found the required minimum DC supply voltage of 24.3 V.

When rectifying (using a bridge rectifier) and smoothing (with an electrolytic capacitor), the AC voltage is converted to a DC voltage of (28.1 V AC x 1.41) – 1.4 V = 38.3 V DC. This is by far high enough for supplying 9 white LEDs in series, and thus it satisfies the conditions from the points 1-3 above.

You wish!

Right in the beginning there are two logistical challenges:

1. Finding which socket is part of which group, and
2. Finding the positive terminal of each socket.

This made me think quite a bit.

I started by removing all the light bulbs from their sockets and from their plastic fixtures; the fixtures will be reused, the light bulbs go to the junk box. I found that the plastic fixtures of the ornament can accept my LEDs nicely and without any mechanical problems (point 6 above: check!). But since LEDs are DC only, both the fixtures and the sockets at the end of each tree branch have to be color-coded.

In at least 99% of all cases, the positive (Anode) terminal of an LED is a bit longer than the other (negative/Cathode) terminal. In addition, the negative terminal within the LED case always ends in a kind of tiny bowl (which can, btw, be seen easily in LEDs with a clear case), and normally there is a flat section at the lower end of the transparent case, near the Cathode terminal. When inserting the LED into the plastic fixture, mark the position of the positive (i.e. the longer) terminal using a permanent marker, bend the wires up towards the 'live' end of the LED and cut them somewhat shorter. The 2nd picture above shows these steps; when looking sharply at the LED at the leftmost position, you also can see the tiny 'bowl' that is shown in the 3rd drawing.

Then you cut the two wires coming from the wall wart, a few centimeters from the wall wart, and define which one will be the future positive wire (e.g. mark it with a red dot, a knot, or whatever you like).

Connect one test lead of your continuity tester (or Ohmmeter) to the wire elected positive, and check to which one of the two terminals of the 40 sockets it is connected (i.e., shows continuity). Mark the first terminal you find connected with the input wire at the inside of the socket with a permanent marker (you want the outside to remain clean and white).

In order to find out which socket is next within the group of 10 sockets, you insert a bit of bare wire (about 0.6 mm dia.) into one of the empty plastic fixtures instead of a lamp (picture 4 above; you could also use a narrow strip of matrix board with some wire added). Plug it into the socket you located before so that it connects its two terminals. Then you check with your continuity tester again which one is the next socket/terminal showing continuity, and again mark it with your pen.

Now you have located the positive terminals of two sockets of one group already. Proceed in the same way by adding one more fixture with a bare wire, search for the next positive terminal, and so on. In the end you will have the positive terminals of ten sockets marked, and you have 9 fixtures with the bare wire installed. They will be used again for the three following groups.

Repeat this process with the three remaining groups (you can use the fixtures with the bare wire you used before, one after the other). Don't forget to use markers with a different color for each group so that you can distinguish the four groups from each other.

In order to convert the alternating current (AC) from the wall wart to direct current (DC), the wall wart needs the addition of a rectifier and a smoothing capacitor, as shown in the circuit diagram (1st picture) of step 2.

I recommend using a small piece of matrix board for good mechanical stability, like the one shown in the pictures above. I used a bridge rectifier in a dual in-line (DIL) package such as a B40D together with a 47 uF electrolytic capacitor having a minimum voltage rating of 50 V. To make it as small as possible you can feed the capacitor's terminals through the same donuts as the rectifier's positive (+) and negative (–) terminals before soldering. Electrolytic capacitors often have the positive terminal a bit longer, and normally the side of the negative terminal is marked by a wide strip and '–' signs. Connect the + terminals of rectifier and capacitor together, and the – terminals as well.

Make sure to provide mechanically sturdy connections for the input and output wires. To do so I drilled four holes with about the same diameter as the outer diameter of the wires, then threaded the wires through these holes before soldering – refer to the drawing above. You might even attach the wires to the matrix board by using some hot glue after soldering.

Remember: On the input side there is AC, no polarity needs to be observed. On the output side, you connect the wire you defined as positive in step 3 to the rectifier's/capacitor's positive terminal, the other one to the negative (or Ground) terminal.

I recommend insulating this assembly with some heat shrink tube. It is already present at the far right of the 4th picture above. Slide it in place and shrink it using a hot air gun. If you shouldn't have any fitting heat shrink tube, use at least some electrical tape to wrap - and insulate - it all.

Still not finished?

Right, you need one series resistor per group, as already mentioned above. So what value should they have? There already was an example for calculating a series resistor in the 'Solution' paragraph above – but let's repeat it here for clarity.

The difference between the required voltage (9 x 2.7 V = 24.3 V DC) and the available voltage (38.3 V DC) is 14 V. You need a resistor (R) having a voltage drop (U) of 14 V at a current (I) of 1 mA (= 0.001 A). According to Ohm's law (U = R x I), a resistor (R) can be calculated from the voltage (U) across it, aka the voltage drop, and the desired current (I) through it using the following simple formula:

R = U / I = 14 V / 1 mA = 14 V / 0.001 A = 14000 Ohm = 14 kOhm).

Select the next-higher value from the E12 series, which is 15 kOhm. You need four of them. A power rating of 1/4 W is sufficient. The tolerance is uncritical in this application, 5 or even 10% is fine. If required, the resistor's value can be adapted to different voltages and different forward currents by using the formula R = U / I given above.

And now there are two different cases. If you still have all the bulb fixtures available, you can simply insert each of the four resistors into a fixture, and plug each of the fixtures into one socket of each group. If, however, some of the bulb fixtures should be missing, you can cut narrow, short strips of double-sided, copper-clad PCB material. Solder the two resistor terminals to one side each so that the PCB strips will contact the two terminals within the sockets. Some experimenting might be necessary for reliable contacts. The polarity needs not be observed when plugging in the resistors, you can ignore the marks within the socket for this case.

BTW: As opposed to the circuit diagram with the 36 LEDs given in the 'Solution' section above, the position of the series resistor within the LED group may be freely selected, as long as there is one resistor per group. The current flowing through the group remains the same regardless of the sequence of the components (provided they still remain connected in series).

To disguise the resistors you can either paint them white (why not with some drops of a correction fluid, such as Tipp-Ex), or perhaps use short pieces of white heat shrink tube.

Replace the bare wires in the nine fixtures you used before for locating the sockets' positive terminals by LEDs. Populate all 36 sockets with LEDs, making sure to match all the marks within the sockets with the ones on the LED fixtures.

2nd BTW: The LEDs I had available had clear lenses and did, therefor, radiate mostly in forward (axial) direction – not ideal for the current application. I made them kind of diffuse by carefully sanding them with an abrasive pad (Scotch-Brite or similar) and succeeded in getting a more circular radiation pattern. This is of course not necessary if you use LEDs with a diffused lens from the beginning. Apart from that, I only had cold-white LEDs available; I'm quite sure the ornament could look still somewhat nicer with warm-white LEDs.

(Left: before, right: after the modification)

Thanks to the use of LEDs instead of light bulbs, the ornament now consumes considerably less power than before, and the LEDs will live happily ever after.

And, most important, both my friend and his wife are very enthusiastic about my modification!

... it will, of course, be somewhat different. If you want to implement a similar upgrade to your own Xmas ornament, you will have to make different modifications with respect to the number of LEDs, resistor values, capacitor value and voltage rating, etc.

Your wall wart might deliver DC from the beginning. Then no rectifier is required, and you can use it as it is, as long as the output voltage is a few volts (say, 6 to 9 V) higher than the sum of the forward voltages of your LEDs in series, in order to send a constant current through the series resistor.

The most difficult step however, regardless of the number of light bulbs, will always be finding out the number of groups, the number of bulbs within a group, and then locating the positive terminal of each socket.

If you follow the instructions given above, together with your own brains and your own creativity, I am confident that you will find a solution that fits your ornament – EXCEPT IN THE TWO CASES BELOW:

1. ORNAMENTS THAT ARE DIRECTLY CONNECTED TO THE MAINS VOLTAGE, i.e., without a wall wart, MUST NOT BE MODIFIED AT ALL UNLESS YOU EXACTLY KNOW WHAT YOU ARE DOING, because this can be dangerous at least, and, in the worst case, can kill you! These ornaments are beyond the scope of this instructable.
2. Ornaments (with or without a wall wart) having an additional electronics box that makes your light bulbs alternate or flash will be more difficult to be modified. These are beyond the scope of this instructable as well.