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There are 14 LEDs on the miniature ring light. Each LED is partnered in series with a current-limiting resistor. So, there are 14 resistor-LED pairs in total. I have several pages devoted to explaining simple LED circuits if you want to learn more.
Surface-mount electronic components are used on the LED ring light to maximize the number of LEDs in such a small area. Resistors are installed on the back side of the ring light, so as to not take away precious space from the LEDs on the front. The greater the number the LEDs, the smoother the light distribution.
Many hobbyists panic at the thought of soldering surface-mount electronics. But, it isn’t that difficult. Besides the ground wire and power wire, the ring light only needs resistors soldered on one side and LEDs soldered on the other.
Tiny plated-through holes or vias connect component pads from one PCB side to the other.
Each resistor on the back side of the PCB connects to its partner LED on the front side with a very small hole called a through-hole, thru-hole, or more accurately a “via”. The walls of the hole are plated with copper during manufacturing to create an electrical connection from one end to the other.
I used the smallest possible via in the PCB layout software, which is a 0.031-inch pad with a 0.014-inch hole. The program indicates that the hole may be filled in. That’s fine because there won’t be any external wires attached there.
A circumnavigating trace on the back side of the PCB has +V and a circumnavigating trace on the front side has ground. Therefore, the circuit goes from +V on the back to a resistor through a via to the front to an LED to ground.
Fifty white LEDs were purchased on eBay for $20 (including shipping), which is 40¢ each. That’s a pretty good price for wide-angle (>100°) 2500 mcd (millicandela) surface-mount white LEDs. But, they don’t come with a datasheet.
Testing a white LED for polarity and voltage drop on an LED test circuit.
So, I used my LED Tester Tool to determine which end of the LED is GND. It’s the end with the notch on it. Also, the constant-current circuit allows measurement of the voltage drop at a precise current. In this case, it is 3 V at 15 mA.
If the ring light is connected to a 5 V power source, the current-limiting resistor needs to be:
5 V power source - 3 V voltage drop = 2 V voltage drop across the resistor
2 V ÷ 0.015 A = 133.33 ohms resistance
2 V × 0.015 A = 0.03 watts of heat that the resistor needs to dissipate
If the ring light is connected to a 4 V power source (freshly-charged lithium polymer battery), the current-limiting resistor needs to be:
4 V power source - 3 V voltage drop = 1 V voltage drop across the resistor
1 V ÷ 0.015 A = 66.66 ohms resistance
1 V × 0.015 A = 0.015 watts of heat that the resistor needs to dissipate
Based on these calculations, I purchased part #71-CRCW1206-133-E3 and #71-CRCW1206-68-E3 surface-mount resistors from Mouser Electronics for $3 per 100.
These are 1% accuracy, 1/4 watt, 1206 package size, Vishay/Dale thick film chip resistors with resistances of 133 ohms and 68 ohms respectively. Accuracy isn’t critical and 1/4 watt (0.25) far exceeds the required 0.03 watt maximum calculation.
The 1206 package was selected because it is a reasonable size (0.12 inch by 0.06 inch) to hand solder. Moderate-sized resistors are good for practicing surface-mount soldering because:
There are many different methods for manual surface-mount soldering. Some people use clips to hold the component in place. Other people pre-solder the pads, place the electronic part on the pad, and then melt them into place by reheating the pad. And, some other people apply cold solder paste to the pads, place all of the electronics onto the board, and place the entire board into a toaster oven.
Here’s the method I prefer...
A thin line of silicone adhesive applied with a toothpick to bare spots on the PCB.
Place a small blob of DAP 100% silicone rubber adhesive (SKU #7079800698) onto a scrap piece of paper. Dip the tip of a toothpick into the silicone and drag the toothpick tip in the gap between the pads of the components to be soldered. Work on one side of the board only.
As the blob of silicone cures, it becomes tacky and lumpy. Narrow and awkward gaps should be touched first with the toothpick while the adhesive is still thin, with larger components (such as tantalum capacitors -- but not in this project) getting the lumpier dabs. At some point the blob on the scrap paper becomes rubbery and unusable, so only dispense small portions at a time.
On most circuit boards, it is best to start by soldering the shortest height components in the center of the board on the first pass. After soldering the first set, then place adhesive for components that are taller or on the outer portion of the board. It will easier to reach all of the pads with the soldering iron using this approach. The LED ring light doesn’t have different height or center components -- so it’s not an issue here.
Silicone is selected as a PCB adhesive because it adheres to fiberglass, it isn’t electrically conductive when cured, and it operates at up to 400° Fahrenheit. But, there are two downsides to using silicone adhesive:
Placing surface-mount resistors on a printed circuit board using tweezers. Although it is hard to tell, I was able to get silicone wiped in place for all fourteen parts.
Place the resistors onto the PCB using a pair of fine-tip tweezers. (No, don’t use the slanted makeup tweezers from the bathroom with the gunk all over them.)
The silicone adhesive is sticky and will gently hold the component in place. The part can be nudged into alignment with the tips of the tweezers. But, don’t slide the part around too much or else silicone may get on the ends of the part and onto the metal pads.
After positioning, give the surface-mount component a final tap on the top to force it down onto the pads. Sometimes the silicone adhesive can be thick enough to float the part slightly above the pads. It isn’t a huge deal if this gap is bridged with solder (after all, most through hole components connect in this manner). However, if you are very accurate in your positioning and you push downward, the adhesive can hold the part against the pads so that you can actually have electrical connectivity even before applying the solder!
Left: Components in place waiting for the silicone to dry. Right: Components after soldering.
With all of the components in place, let the silicone dry for a while. Unless I’m in a rush, I usually set the board aside and go and have lunch or watch some Adult Swim on Cartoon Network. Let’s say 30 minutes curing time.
Test a component to see if still slides -- it shouldn’t. Although, note that even in the cured state the silicone remains slightly flexible. Think of window caulking.
This is your last chance to inspect the board before soldering. Use a magnifying glass. If a part came loose, is in the wrong position, or got coated in silicone, simply pull the part off of the board. The silicone can usually be rubbed off the pads and component. Use a pencil eraser if necessary.
With everything held in place, soldering is a breeze with a fine-tip soldering iron. The large surface-mount pads aren’t that much different than the soldering area of most through-hole components.
Finer pitch surface-mount components (not used in this project) may get solder stuck between pins -- which is called solder bridging. Apply no-clean liquid solder flux or solder flux paste and reheat the pins. Excessive solder can be pulled off with a solder wick or solder sucker, but small bridges will often self correct when reheated in the presence of solder flux.
After the resistors are soldered in place, inspect the board with a magnifying glass to ensure connectivity. Then, flip the board over to repeat the process for the LEDs.
The packaging for the LEDs I selected is PLCC-2 (Plastic Leaded Chip Carrier), which is 3.5 mm x 2.8 mm. That’s a decent size for hand-soldering.
Unlike resistors, white LEDs are polarized. In this case, the little notch needs to face GND.
Surface-mount LEDs hand soldered.
Two thoughts about the photograph above:
This brings up the other differences between surface-mount resistors and surface-mount LEDs. They’re more expensive to replace and the packaging can melt if you’re not careful. Worse still, unlike most components, exterior damage to LEDs can affect their operation since they emit light through their packages.
I wanted to show you my mistake so that you won’t feel bad if you also make some soldering errors. Don’t let that dissuade you. With practice, hand surface-mount soldering is fairly clean and easy.