20 de enero de 2015

A late christmas tree with LEDs

For this new year season, I and  my daughter made a christmas tree lit with LEDs. It's about 5 cm (3 inches) high. We finished it late, aproximately at January 10.

To fit the entire circuit inside the tree, we put a watch battery with the battery, a latching switch, several small red LEDs (3mm) and a normal yellow LED (5mm). It doesn't blink, you have to turn it on and off by hand. For the tree structure, we used the so called cold porcelain (autotranlation) mixed with dyes. (I only drew four red LEDs in the circuits to keep them simple.)

Circuit diagram

The battery holder and the smallest switch we got were designed to use in a breadboard or printed circuit. They were much smaller than the other versions we found, but it was difficult to solder them.

We cut the LEDs terminals that are made of hard wire and replace them with insulated thin wires, so that we can be arrange them at the correct position. The original terminals have different lengths to distinguish the anode from the cathode. When you cut them this distinction is lost, so we resorted to the old trick to connect the LED at the two possible positions. (Actually the shape of the inner parts of the LED also serves as a guide, but I trust more what the battery says.)

Circuit without tree

To get an idea of the length of the wires, the positions, and how we were going to place the components in the tree, I drawled it on paper and settle the components (the artistic conception is mine).

Circuit over tree blueprint

After a few tries to arrange the components, we put the switch in the trunk of the tree and the battery in the bottom so it can be replaced. The other option was to make a pot to hide the battery and switch, but the pot was too big.

The battery can be replaced at the bottom

The switch is in the trunk

The problem is that I am a Chemical Technician (at the secondary school) (I dissolve circuits, I don't built them :)) and it was the first time my daughter tried to solder. Moreover the components were designed to use in a breadboard or printed circuit, not to be soldered. So we had to remake several times each solder joint until all of them were good. In general the easiest method was to make a small loop at the tip of wire, adjust it to the terminal with pliers and then solder. After all this steps, one of the LEDs died, so the tree got only five red LEDs and a yellow one.

We keep moving the circuit to verify that all the solder joints are good and strong. For example one of the solder joints of the switch was loose and then the tree turned off or twinkle (it twinkle!) spontaneously. Luckily, we realized this when only we had made the base and the trunk, so we have just to break the first version of the trunk, resolder and remade the trunk.

Circuit with base and trunk

Most of the circuit was isolated but close to the terminals of the LEDs they have some uninsolated parts. Luckily the cold porcelain is an insulator (at least at 5V), so we only have to be careful to put some cold porcelain (of the correct color) between the terminals of LED and around some other points that had no insulation.

For the tree structure we made a light green base that looks like grass and dark brown trunk. Then we put a very dark brown cone, that has almost the final form of the tree. (I wanted to make the tree wider like in the artistic conception, but my daughter insisted with a more elongated shape.) After that we put some green flat "leaves" (in the form of droplets) and press them to make them stick. We also put some colored beads. At each step we were turning the tree on and off to besure we have not broken any part of the circuit.

Finished tree (off)

Finished tree (on)

Using the GIMP magic we get

Fake photo of twinkling tree :)

Comments and future projects

The cold porcelain worked fine. We could handle it for long time adding a drops of water. In case of errors it can be removed, without leaving all the components completely stained. After a day it's firm but slightly flexible, with one or two more days it hardens completely.

Cold porcelain is also an insulation (at least at 5V), so it was useful to enclose the circuit without problems. There is a very small distance between the terminals of the LEDs and we have also the solder joints and the tips of the wires that are uninsulated, so with the movement any of them can be short-circuited. (With a 5V battery it's not a problems.)

One of the LEDs died before we put the tree. I don't know whether it was caused by moving the terminals too much for soldering or we heated the LED too much while soldering or ...

One of the LEDs has a bad solder, so you have to give it a technical tap from time to time to fix it.

The battery holder is brittle and the cold porcelain got inside by the borders. I don't know if it will be easy to replace the battery, I hope it lasts several years ...

It doesn't twinkle! Let's see if we make one tree with a 555 next year, but before that I should find out how to connect a 555 (I can dissolve a 555!). It will be complicated because there are more components to solder.

As an electronics expert should have noticed, we did not put any resistors (I take full blame). The idea was to minimize the amount of solder joint to make the project easier. Moreover, all the solder joints of the resistors would have no insulator, so we should be more careful to isolate them with tape or cold porcelain.

The biggest practical problem is due that there are two LEDs colors. The photon of yellow light have more energy than the photons of red light, so the yellow LED needs more voltage than red, approximately 2.0V vs. 1.7V (in practice everything is a bit more complicated). Then the red LEDs steal current to the yellow LED and then it shines only a bit. For a future project, my idea would be to make an intermediate circuit, combining the resistors of the red LEDs on a single resistor and using another resistor for yellow LED.

Actual circuit

Correct circuit

Proposed circuit. Does it work?

We should try this simplified version on a breadboard before putting a cold porcelain tree around it.