I almost never hand out business cards, but when I do, it would be nice to make an impact. Having seen several other cool PCB based business cards before, I wanted to set out and make one myself.


One problem with a lot of these pcb business cards is that while they did something cool, they still weren't something most people would really use repeatedly. I wanted mine to actually be useful to the person (including those without any technical experience), so they could keep it as a useful tool.

My idea came together as I had been reading up on efficient LED chips for grow light (ie for plants). I have also had a long time interest in practical solar power. And I also like RGB led lights for decoration. A flashlight is something people do use, one that doesn't require a battery change is nice, and one that can make a rainbow of different colors is pretty cool.

That became the concept: place three LEDs of colors red, green, and blue. Connect them to potentiometers to adjust their relative brightness and therefore final color. Power the system with a solar-charged battery.

There are two main design constraints: safety and cost. I don't want this starting on fire, and while it will be much more expensive than a paper business card, it would be nice to keep the cost somewhat reasonable.

This is the simplest circuit I have put on a PCB, but it still has a few quirks to keep in mind.

The solar panel is the most expensive part of the system. Mounting it was a major challenge. I tested a number of different solar panels, read more about that on my original blog post here, and came to the conclusion that the most expensive panel was both the fastest charging and most elegantly mounting of the lot (Any Solar SM141K10LV Monocrystalline Solar Cell 307 mW 6.91 V). However, almost all tested panels work to some extent. If it fits, has an open circuit voltage rating in the ~6-8V range, and produces at least 100 mW rated (the 26 mW tiny one I tried did charge, but not at a practical speed), you could probably use it.

Potentiometers are also surprisingly expensive in both price and board real estate. The ones I went with are being pushed close to their maximum ratings if LEDs are at maximum brightness, a point to note but not an issue so far in testing.

Perhaps the most fun part was designing a footprint that could handle a mix of LED chip sizes (mostly 3030s but also 2835) and then testing a bunch of different LED chips. I tried three different mixes of LEDs:

Note the color mixing is only visible on illuminated objects some distance from the board, if you stare at the front of the board your eye will clearly separate the three different LEDs of different colors as there is much more space between them than you would have on say, a color LED display.

I made more of the highest-efficiency board as I thought maximum brightness is the most practical application for most users, although it lost the fullest color mixing ability. With all three LEDs of high efficiency, even without an extra focusing lens, it produces a surprisingly bright light. It won't hold a candle to any commercial, high brightness LED torches, but it is quite usable for many tasks.

The most controversial part of this design by far is the lithium ion charging circuit. Having received reviews on reddit of this schematic and PCB, the two main feedback points were 1) it's not maximally efficient either in voltage conversion or energy harvesting 2) it might not be safe since it isn't a certified charging IC.

Those are both valid points. I already knew an energy harvester IC, as cool as they are, was off the table due to their high cost. This also isn't a design that needs to utilize every last milliwatt of available power as it is expecting only occasional use. I also used a non-switching regulator because I was looking for high efficiency at very low levels of light, but am not as concerned about maximal efficiency in full strong sun, when there is plenty of energy to be had from this panel.

Safety is another point. Lithium ion batteries can be dangerous when mismanaged, but they are not mystical devices with unknowable intentions. Here we stick to a trickle charge, keeping the current low by limiting the size of the attached solar panel. We also want the voltage to be steady and not too high, here around 4.2V. The resistors setting the linear regulator's voltage need to have a tight tolerance, and knowing the voltage drop of the forward diode at a tiny current is also important to understand.

As of me writing this post, I have three of these cards which have been sitting constantly in the sun of a windowsill for six months. No battery swelling has occurred and all have a measured battery voltage within 0.1V of 4.19V. Indeed I am quite proud to see one sitting at 4.198V (photo below), quite close to the lithium ion target of 4.2V. No excessive heating of the regulators has been observed either.

I can't and won't guarantee this is the safest possible design, but experience so far suggests it can be used safely. I still remind anyone I give a copy of this card that this is an un-enclosed electronic device and should be treated with some care.

Perhaps the greatest room for improvement here is some form of integrated undervoltage protection for the battery. Existing undervoltage protection consists of 1) the user noticing the LEDs are growing dim and ceasing use of the card and 2) the forward voltage dropout of the LEDS resulting in current consumption falling dramatically once the voltage falls below the forward voltage 3) this card likely only being used for short periods of lighting, and mostly sitting in the sun and charging.

My PCB manufacturer of choice assembled the passives for me, while I used a hot air gun and solder paste to mount the solar panel (heating from beneath), the LEDs, and the PTC fuse.

See my GitHub on this project with complete design files, and my other blog post for some more discussion.

Don't forget to change your name on the board if you make a copy, no running around pretending to be me!