HELI TALK: DIY NIGHT BLADES

Apr 26, 2014 No Comments by

Make your own LED night blades

Making your own lighted blades really brings your night heli to life. You can add as many LEDs as you like and in whatever color pattern and spacing you choose.

In my March column, I discussed the basics of modifying a heli for night flying using LED strips. While there are some very nice commercial night blades on the market, this month I'd like to look at the mechanics for making our own. DIY night blades offer some very neat features. First of all, most of the night blades on the market are fairly “minimalist.” They provide enough lighting to look attractive and maintain orientation, but there's little question in my mind that more lights just look cooler. Second, you can tailor your lighting scheme to your own tastes. To build our night blades, we'll essentially be converting the blades themselves into circuit boards by applying traces of copper tape and then soldering LEDs and resistors to those traces. Finally, we'll add LiPo cells and be ready to go flying.

There are many variations for building night blades, and just about anyone who's built a couple of sets has their own tricks. Here we'll look at some options and outline the general procedure. It's beyond the scope of this article to cover every last detail, and I strongly urge you to spend some time watching some of the Youtube tutorials to be sure you understand the process. I'd be remiss if I didn't thank Donnie rogers for his help in refining my technique of selecting suitable components.

BLADE SELECTION

Here are the basic tools and materials required. A solder iron with a very fine tip is a necessity, but a soldering station like this one also allows you fine control over tip temperature. A digital multimeter will allow you to test the diodes, verify values of resistors, and make sure carbon blades aren't draining the LiPos. Magnifiers are a lifesaver when working with the tiny surface-mount components.

As noted, we make night blades by turning the blades themselves into circuit boards. One issue is that carbon-fiber blades are conductive, so you have to make sure the carbon is clear-coated anywhere it's exposed, such as at the leading and trailing edges. Otherwise, the conductive carbon will gradually drain your LiPo cells. Because of this issue, I like to use fiberglass blades because fiberglass is inherently non-conductive. It may lack the ideal stiffness for real smackdown 3D, but for my style of sport flying, it's good enough, and fiberglass blades are much less expensive as well.

TRACE LAYOUTS

There are a number of options for laying out the traces and LEDs. The most popular method is to run a positive trace along the top of the blade and negative along the bottom. Then traces are routed around the leading or trailing edge to bridge the gap and power the LEDs. My preference is to run parallel traces along the top of one blade and the bottom of the other, with the LED/resistor packages spanning the gap. This is visually attractive and avoids the exposed carbon at the leading edge while also keeping the blades balanced. Another option for dual-color blades is to run three traces: two positive, with a ground trace between. With this layout, a single resistor of the appropriate value can be soldered to each positive trace at the root of the blade. The only catch is that the resistor must have a high enough wattage rating to handle the total number of LEDs, but it saves you the trouble of soldering the tiny LEDs and resistors together.


This shows why a magnifier visor is a necessity; the surface mount devices (SMDs) are tiny. Fine tweezers are a big help in handling them. The LEDs must be installed with correct polarity, which makes a digital multimeter with diode testing capability very handy.

Resistor Values for LEDs

This simple spreadsheet calculates the correct resistor value for any given LED using the forward voltage and maximum current. The resistors don't have to be perfect, so I round up to the next readily available value — in this case, 33 ohms and 82 ohms.

Forward VoltageCurrent (mA)Resistor Value
LED.com Part No.Desc.ColorTypMaxTypMaxCalc ΩActual Ω
Use max battery volts and max current to calculate resistor to get brightest LED
Battery max volts: 4.2
EPT2012RWF-ALEDWhite3.34.020303033
EPT2012SECKLEDOrange2.12.520307082
EPT2012SURCLEDRed1.92.520307782
EPT2012PBC-ALEDBlue3.34.020303033

Here are the basic electronic components. The surface-mount resistors and LEDs come in tapes for feeding into commercial “pick and place” machines. The 3-pin sockets double as charging and arming ports, and the male 3-pin plugs are made from header pins.

After applying the copper traces to the blades, the next step is to mark the location of the LEDs. Be sure to mark them as accurately as possible so that both blades will have the same CG span-wise. To simplify the job, I install LEDs on the top of one blade and the bottom of the other. As long as they balance, this works nicely.

RESISTOR VALUES

Soldering the LEDs and resistors together is the most challenging step. What works best for me is to pre-tin all the locations and then solder the resistors on. The LEDs are then butted up against the resistors, soldered to the trace, and then soldered to the resistor. Rosin solder flux makes this job much easier.

Here are two layouts I've used with success. The 550 blade has two traces, but since each LED has its own resistor, you can use whatever colors you like. For the 450-size blade, I used my alternate three-trace layout, with the LEDs staggered to whichever trace gives the correct voltage. With the resistors located at the root of the blade, the LEDs are easier to install.

Since a charged lipo cell exceeds the voltage rating for the LEDs, resistors must be used to step the voltage down, so an important step is to determine the values for these resistors. if the resistance is too high, the LEDs will be dimmer than necessary and can fade out entirely as the lipo discharges. if the resistance is too low, the LEDs will have a shortened service life and may even fail outright.

Fortunately, there's a simple formula for determining resistor value for any given LED. all you need to know is the forward voltage and max current rating for each LED, and these are available on its data sheet. With these in hand, we use the formula:
(Vb − Vf) ÷ I = R

Vb is the battery's fully charged voltage (typically 4.2V), Vf is the LED's forward voltage, which is available on its data sheet, i is the max current in amps, and r is the target resistor value in ohms. So, for an LED with a forward voltage of 1.9 volts and a max forward current of 30ma, the calculation is:
(4.2V − 1.9V) ÷ .030A = 77Ω

You can round the resistor value up slightly to err on the safe side, and for this particular LED, i would use a readily available 82-ohm resistor. Run these calculations for all the LEDs you plan to use, so that you can order the correct resistors for each color (see table). As for the power rating of the resistors, you simply multiply the max current of the LEDs by the voltage of a fully charged cell. typically this means:
4.2V * .030A = 0.126w

So, if you're using one resistor per LED, 1/8-watt resistors are perfect. if you're planning to run a string of parallel LEDs with a single resistor, the power capacity should be multiplied by the number of LEDs. For instance, four LEDs in parallel would require a 1/2-watt resistor.

One final comment here: LEDs with higher forward voltage will dim and go out soonest (as voltage drops). Even if you opt for lower-voltage colors like red or orange, it's good to mix in at least one or two higher-voltage LEDs to act as a low battery warning.

The LiPo cell is attached to the trailing edge of the blade with two-sided tape. I recommend automotive-grade 3M VHB. The 200mAh cell shown here will power the six LEDs on each blade for well over an hour. The 3-pin socket allows you to charge the LiPo, but when the jumper plug bridges the two outer ground pins, the LEDs light up. Be sure to cover the LiPo with shrink wrap to help secure it; the tape alone isn't strong enough.

PUTTING IT ALL TOGETHER


With everything collected, we're ready to start. First, apply the copper traces to the blades. Lots of fliers use the copper tape sold for stained glass work or wiring dollhouses, which is cheap and widely available. I prefer copper trace repair tape. It costs a little more, but the adhesive is much stronger, and it's available in 1/8-inch widths, saving the tedious step of slitting it down.

Lay out the traces as accurately as possible. The more accurate the layout, the easier the blades will be to rebalance when they're complete. Once the traces are applied, carefully burnish them down to make sure they're fully bonded to the blades.

Before installing the LEDs, mark their spanwise locations accurately so that the two blades are as closely matched as possible. Then apply a tiny amount of rosin flux and a small dot of solder at each location.

Now you're ready to install the LEDs. If you're installing one resistor at the blade root, the LEDs can simply bridge the gap between the traces. If you're using a dedicated resistor for each LED, I recommend soldering on the resistors first and then butting the LEDs up against them.

With the LEDs mounted and tested, you're ready to install the LiPo cell at the root of the blade. I use 3M VHB automotive tape for this. You can then add the traces and charging/arming socket as shown in the photo. Each trace intersection must be soldered as shown. The 3-pin design shown allows you to charge the LiPo through the center and side sockets, and to turn the blade on by bridging the two ground sockets — simple, reliable, and lightweight. It's important to cover the LiPo with a sleeve of shrink wrap to secure it in place; the tape alone is not strong enough.

LAST STEPS

Once the blades are completed and tested, secure each LED with a drop of CA. Finally, balance the blades carefully before your first flight. If you ever need to replace the LiPo cells, you can simply slit the shrink wrap, unsolder the leads, and gently pry the cells loose. Clean up the tape residue, and you're ready to install new cells.

Properly maintained, DIY night blades can last a long time, and they look fantastic in the air. And when your buddies inevitably ask, “Where'd you get those?” you can smile and say, “I made 'em!”

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