Rainbow LEDs in a Ring of Power

Introduction

While this is not an exact copy of Frodo’s ring of power, its visual effects are noteworthy.

Of course, some poetic license was taken in the design, especially in integrating the power supply. Alkaline LR41H or zinc-air 312 hearing aid batteries provide a reduction in size while producing enough current to power the integrated circuit and LEDs. Various ring sizes are covered by two flexible printed circuit board layouts.

This can be a challenging project. You will need to build a flexible printed circuit board with surface mount components. It’s not difficult once you develop the necessary skills. Or, maybe you have a friend with this skill-set that can help you. If you’ve never done this before, you will need to acquire some tools. I’ll walk you through the steps if you’re a beginner.

The first step is to determine which printed circuit board to order. Measure the diameter of the knuckle on the finger where you plan on wearing the ring. I found that the best ring fit was about 2mm larger than this measurement for the larger sizes. Do a quick print of that ring and check the fit (maybe you’ll need the next larger or smaller size).

Order the Printed Circuit Board

Unzip the LED_Ring_Files.zip.  The short board fits 14mm to 18mm rings, while the long board covers the 19mm to 25mm range.  Order the printed circuit boards (PCBs) first, since this could take 2-3 weeks.  Send the “LED_Ring_Short.brd” or the “LED_Ring_Long.brd” file out to be fabricated.  I use OSH Park as they will accept a BRD file to build a printed circuit board.  Drag the BRD file into the website link and follow the directions.  The first pages on the website will show the various layers of the boards for your verification.  You’ll also see a proposed price for a standard 2 layer PC board.  After clicking to the ORDER page you’ll see the price for the standard two layer board.  You want to order the “flex” boards so check that box.  You’ll get three copies of the short boards for about $16.10 or $18.10 for the long boards.  If the price is half that value you may have the wrong product selected. OSH Park builds three boards per order “one to mess up, one to build, and one to give to a friend”.

Order materials

Now do an internet search for “magnifying glass with light and stand” if you don’t already have something like this?  And, you’ll need a soldering iron, so search for “soldering iron with fine tip” if you don’t have an equivalent. 

Finally, search for LR41H or 312 batteries and get a card containing at least 3 (enough for one ring). The alkaline batteries are lower capacity but survive longer when unused. The zinc-air batteries have a higher milli-amp-hour rating but will dry out after a couple months once the seal tab is removed.  The trade-off is the total time you expect to power the ring before fresh batteries are needed.

Looking at the “Bill Of Materials” file you can decide upon the electronic parts for the PCB.  I like DigiKey as a source for components.  They are quick to fill your order so you’ll have a week or more before placing this order after ordering the PCBs. 

You can copy the manufacturer’s part number into an internet search and find an alternate supplier.   Or, copy the DigiKey part number from the bill of materials and paste it into the search box on the website.  In the component window fill in the quantity you want to purchase, and click “Add to Cart”.   The quantities for the top four listed in the BOM (i.e. the SPST switch, the SN74HCS04DR IC, the 1uF caps, and the 51k resistors) are enough for three rings. There is a price break at a quantity of 10 so you’ll have extras.  What color LEDs you would like in your ring?  Note: you can mix colors as shown in the video.   The rings use 12 LEDs and 12 series resistors, so, order the resistor values that match the LED color(s) you choose.  You could order a couple extra if you plan on losing any during assembly.  Note: these surface mount parts are too small to be handled with your fingers so order a set a tweezers if you don’t have a fine pointed pair.  Having some of the “Desolder Braid” i.e. “solder wick” on hand can be useful for cleaning up any excess solder that may short IC pins.  Finally, don’t forget to purchase some low temperature solder paste. Small connections on these flex printed circuit boards can lift when using higher temperature solder.

Print the Ring

While waiting for the components and printed circuit boards to be delivered you can print the ring and covers.  I slice with a 0.25mm Z step.  Print the ring with transparent or translucent plastic so the LEDs can shine through.  I printed the first two layers with filament that complemented the color of the LEDs then switched to transparent for the remainder.  That way the top and bottom of the ring had color but the color faded to clear toward the center section where the LED were located.

The battery compartment cover may be tight so give it a couple swipes on the edges with some sandpaper. The thumb nail notch allows it to be easily removed.

Assembling a PCB

Note on the board plot (LED_Ring_BRD.jpg) the RED traces are top metal while BLUE traces are bottom metal.  The schematic (LED_Ring_SCH.jpg) allows you to match the component names to their position on the PCB.

Tape the ends of the flex PCB down onto a block of wood with the top metal up to hold it steady under your lighted magnifying glass.  We’ll start with the LED current limiting resistors R4 to R15.  The schematic says the resistors are 1k ohms (1000 ohms) but depending on the color of the LEDs you chose they may be a different value. The resistors are packaged in a paper tape with a top plastic cover strip.  Hold the tape over a sheet of white paper and start peeling the plastic strip off using the tweezers.  Slowly peel the plastic strip back until you have 12 resistors on the white paper.

Find R4 on the PCB and deposit some of the solder paste on the two “gold colored” pads.  Note: use a light touch on the plunger of the solder paste dispenser tube, pull the plunger out slightly to stop the flow.  If you get too much, use a toothpick to transfer some to the next resistor’s pads.  Then touch the hot soldering iron to the paste and pad until the solder melts and wets the pad “i.e. it is tinned”.  If you may need more solder paste, pull the hot iron away and let the pad cool. Touching the hot iron to the solder dispenser can melt the paste in the nozzle and create a clog.  Once the solder wets the pad pull the soldering iron away.  Now with the tweezers pick up a resistor and hold it on the PCB aligned with the two copper (gold plated, but now tinned) contact pads.  Touch the hot soldering iron to the tinned pad and the end of the resistor so that solder flows over the junction and quickly pull the soldering iron away.  Touch the hot soldering iron to the other pad and resistor end.  Is the resistor flat against the PCB?  If not, lightly push down on the resistor and touch the solder joints again so the resistor is in place.  This is can be challenging if too much heat is applied for too long since the solder on the other end can melt and the resistor can come loose.  If this occurs, you can move the resistor around using the tweezers and the tip of the hot soldering iron.  Note: the metal traces on this flexible PCB material can come loose if too much heat is applied.  Repeat the process soldering R5 to R15.

Get one of the 1uF capacitors and solder it down at C4 using the same technique you employed with the resistors.

The LEDs only emit light when current flows in one direction.  You need to determine the diode orientation with respect to the terminals.  On the DigiKey website after you’ve entered the part number there is a link to the “Datasheet”.  Open the datasheet and toward the top (some have it at the bottom) you’ll find a diode symbol like on the schematic and a polarity indicator that shows the terminal orientation.  Note: this polarity indicator is on the bottom of the LED since the top is clear to let the light out.  On the PCB between the pads there is a diode symbol (triangle/”V”) that indicates the correct diode orientation on the board.  Note: if you are using the yellow LEDs, the triangle symbol on the back of the LED is reversed from the direction indicator shown on the red and orange LEDs. The blue and green LEDs just have a bar on the bottom while the white LEDs use the larger metal contact as the anode connection “i.e. the one next to the resistor”.  Solder the LEDs D1 to D12 down using the same soldering technique you used on the resistors and capacitors. 

The integrated circuit needs to be oriented correctly when soldered down.  The location of pin number 1 is shown on the flex PCB by the dot next to the “U1” part label.  Get one of the SN74HCS04DR IC’s out of the packaging and look carefully at the top (maybe under the magnifying glass).  There may be a small dot in one corner to indicate pin number 1.  Or, sometimes one side of the IC package is chamfered at an angle to indicate the row of pins that contains pin one.  On the PCB tin one of the corner pads by melting some solder paste on it.  Using the tweezers hold the IC down on the board with all the IC pins aligned with the pads on the PCB.  Touch the tinned pad with the hot soldering iron until it wets the IC pin and then pull the soldering iron away.  Is the IC flat to the board and still aligned with the pads?  If not, touch the tinned pad with the soldering iron and nudge the IC into alignment.  Now add a bit of paste and solder another IC pin down at one of the other corner pads. With two pins soldered down you can nudge the IC into alignment if needed by heating one pad at a time. Once the IC is in alignment add solder paste and solder the remaining pins to their pads.  If you get two much solder on a pad there’s a good chance of it bridging to the next pad and shorting the IC pins.  In this case hold the end of the “desolder braid” down on the offending lump of solder and heat it with the soldering iron.  The hot solder will wick up into the braid to hopefully remove the short.

Now turn the board over and solder the R1 – R3 and C1 – C3 components into place.

Finally, solder the switch in place on the front of the board.  The pins on the switch can line up with the pads on the board in two orientations.  It doesn’t matter which orientation you choose.

After completing the board’s assembly do a final check that the all components are soldered with a good joint and all the components are flat on the board.  A Q-tip and some IPA can be used to clean off any un-melted solder balls.  If you don’t plan on using the solder paste in the near future seal it up in its packaging and put it in the freezer.  That will slow deterioration until next time.

A quick board test can be performed by connecting 3 volts between the (+) and the (-) terminals to see if the LEDs flash.  Note: if you are using the green, cyan, and blue LEDs they may be faint with this voltage.

Assembling the Ring

Take three of the 312 batteries (if you’re using this type) and peel off the brown sealing tab.  Let them set for a minute for air to start energizing them.  I found that it took me longer than a minute before they were installed so you may not need to wait.

Starting with the switch, slide the flexible PC board into the switch notch in the ring while bending the board to fit the IC into its cavity.  It may help to also line up the LEDs around the rest of the ring at the same time.  The last part is the long gold colored battery contact pad.  As this slides into its slot you may find it’s too long for the smaller ring sizes.  Fold it back (gold-copper on the outside) so it’s just long enough to fit against the side of the battery holder cavity and slide the doubled section into the PCB slot.

Insert three batteries into the battery holder.  Note the (+) and (-) terminal directions (there are polarity symbols in the bottom of the battery holder that act as air vents).  Don’t [put the batteries in backwards since this LED circuit doesn’t have any protection against reverse polarity.  At this point try pressing the switch to test the operation and ensure that everything functions correctly.

Glue on the ring cover with a couple beads of super glue and press in place to cure.  Slide the battery holder cover on.  You may need to lightly sand the sides but you want a snug fit.

When the ring is on your finger press the switch button with your thumb to activate the “Power” function.

Enjoy your LED power ring…..