7. LED Printed Circuit Board

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LEDs (light emitting diodes) light up a series of Halloween pumpkins in a rainbow of colors.

One of the advantages of LEDs over candles is that the LEDs can be mounted anywhere in the pumpkin. For example, the LED lighting can be mounted underneath the top of the pumpkin, facing down. Try that with a candle!

Not only will overhead lighting provide a unique appearance for the pumpkin, but the individual LEDs will not be visible through the pumpkin’s mouth. Furthermore, it will be easier to access the LED light and battery by simply removing the pumpkin’s lid.

Although a solderless breadboard could be secured upside down in a pumpkin, the electronic components might fall out into the pumpkin goop. A better solution is to permanently assemble the LEDs onto a printed circuit board (PCB). Another advantage is that an LED PCB can be much more compact than a solderless breadboard.

Two-layer LED PCB laid out in PCB software.

The prototype PCB was designed in PCB layout software.

Each prototype circuit board is only 1.25 inches wide by 2.5 inches long by 0.5 inches thick (with mounted electronics). So, it should fit nicely even in a small pumpkin.

The printed circuit board prototype has a few optional extras that my circuit on the solderless breadboard does not have, such as a power switch and pushbutton.

There are extra holes on the board for test points and for minor rearrangements of the circuit. With a few additional wires, I can program the microcontroller to pulse or flash up to three columns of LEDs.

If the microcontroller portion is not needed, the PCB can be cut in half, making it only 1.25 inches square.

The LED PCB has room for 15 LEDs (five columns of three LEDs each). It isn’t necessary to populate each LED hole. Columns can be left blank and individual LEDs can be replaced by a bare wire.

Thick copper traces (which appear as large red areas on the image) are placed on the two layers underneath the LEDs to reflect light away from the board and to act as a large heatsink. An LED heatsink helps draw heat away from the LEDs, which may extend the life and brightness of the LEDs.

Circuit Board Prototypes

I ordered the prototype printed circuit boards from a PCB manufacturer on Thursday morning. Their service requires one day to manufacture and 2 business days of shipping. (The boards arrived one day early.)

Vacuum-sealed bubblewrap PCBs.

Vacuum-sealed bubblewrap PCBs.

The PCB manufacturer seems to have recently changed their bagging. The three PCBs arrived in a nice new vacuum-sealed bubblewrap.

Left: Cutting PCB panels on a table saw with diamond blade, using the fence as a straight-edge guide. Right: PCB dust particles contaminate the board.

Left: Cutting PCB panels on a table saw with diamond blade, using the fence as a straight-edge guide. Right: PCB dust particles contaminate the board.

I panelized the circuit -- which means there are multiple copies or multiple different circuits per board. I did this because the miniboard service is limited to exactly 2.5 inches tall by 3.8 inches wide. So, I was able to fit three copies of the Pumpkin LED circuit per board.

To separate the PCBs, I used a MicroMark Table Saw (#80463) with a diamond blade (#81524). The saw and blade are pretty good, but they are expensive.

Unfortunately, this method of cutting a PCB leaves a lot of dust. I first vacuum it off with a shop vac. Then, I polish it with a microfiber dust-free lint-free pure cotton cloth. (Nah, actually, I wipe it on my shirt.)

A cleaner, less-expensive choice for separating PCB panels is to use a shear. Micro-Mark has a Miniature Shear / Brake (#83213) that perhaps I'll get for Christmas.

The completed LED pumpkin candle light with Atmel ATTiny45 microcontroller is slightly larger than a 9V battery.

The completed LED pumpkin candle light with Atmel ATTiny45 microcontroller is slightly larger than a 9V battery.

The performance of the finished board is excellent. Due to voltage limits, only two blue LEDs can be installed per column, so a bare wire fills the spot of the missing third LED in four of the columns. The middle column contains three red LEDs (turned off in the photo) that the microcontroller can blink for a random spooky flash.

When a real pumpkin becomes available, I'll install it in the lid and take a picture. I think the board will be fine, but I may not have left enough room near the switch and battery connector for the screw heads.

Trick or Treat - Free LED Candle for Halloween

Through mid-October 2007, I held a drawing to give away three complete Pumpkin LED candles. (In the end, I actually decided to give away four, because I had an extra one.) Here are the winners and their choices:

Thank you to everyone who entered!

Conclusion

It is difficult to beat an ordinary candle on cost, disposability, and shelf-life. However, LED candles open up great decorative possibilities with their variety of colors.

This project should be fairly easy for most technical people to reproduce and experiment with. I encourage you to give it a try. It makes a great introduction to electronics.