Tale of the Monkey Light

By Dan Goldwater
Founder of MonkeyLectric

Garage Prototype Falls Modification Challenges

I'd like to share some of my experiences trying to take something I think nearly all Jameco customers are doing – electronics – and make it into a consumer product that operates outdoors in harsh conditions.

It was over five years ago that I made the first prototype of what would become the Monkey Light bike light. The Monkey Light is a persistence of visionPOV light – it makes patterns in the air as it moves. There are a lot of simple POV light kits like the Adafruit MiniPOV and I think it's almost a coming of age ritual for electrical engineers to make one of these. Attaching it to a bike wheel is fun: it makes the bike more visible and it creates what we at MonkeyLectric think is a cool visual effect.

I had only meant to make these lights for my own bike, but when I rode around everyone kept stopping me on the street asking where they could buy one. Eventually I started wondering, could I actually make and sell them? One of the biggest problems I kept thinking about was durability. People leave their bikes outdoors a lot and they ride them in snow, in the desert, on and off curbs, and they often don't maintain them.

Monkey Light's Award-Winning Video

How would I ever make a circuit board that kept working through heat, cold, vibration, dirt and sand? I think this was an interesting question – whether pushing into new applications for drones, robots, lighting or any other outdoor project. How long could it operate without maintenance?

I started by looking at all the other types of circuits I could find that operated in these conditions – inside my car, my GPS, a few other devices. The two main strategies you might find are (a) a tough, sealed case that keeps the electronics inside it protected and clean or (b) encapsulation in a solid block of plastic or rubber for more delicate stuff. You tend to see tough, sealed cases more often unless you really dig into the dirtiest underbellies of machinery. That's where you see more encapsulations.

Needless to say, I decided to use encapsulation to protect the Monkey Light circuitry. Encapsulation is more durable than a plastic shell, but it's hard to get it to look quite as nice. It's a decision that has led to a great deal of interesting engineering and ultimately to development of our own production line here in Berkeley.

At first, I just started spreading adhesives on top of my circuits. If you're just protecting a couple of things this can work really well. You can use silicone caulk, hot melt glue or epoxy. But, these are messy and just don't look good enough for a consumer product. I also quickly learned just how many plastics and coatings don't last in sunlight! Epoxies yellow and degrade in just a few months outside. I've seen more than a few art projects made with cast epoxy that sadly yellowed after a few weeks outdoors. For many projects, paint can help protect from UV and can clean things up a lot, but paint doesn't work for lighting projects like the Monkey Light because there are LEDs on the inside and we're trying to let their glorious colors shine through. I found an old movie projector with a giant arc lamp in it and used it to do "accelerated sunlight testing" on a bunch of samples.

Ultimately, we developed a process to mold our Monkey Light circuit boards inside a clear polyurethane rubber. We pour the liquid polyurethane into a mold, drop in the circuit board and submerge it. The polyurethane cures in a few minutes and the method is really durable. It isn't hard to do at a basic level. Getting it to look good and not make a mess was a really challenging process; it took over a year of refinement. For example, we put lots of holes in the circuit board so that bubbles in the polyurethane could escape.

We also had a lot of fun with vibration testing. Vibration tests are important for any product if you want to reduce maintenance and failure. The ways something breaks or comes apart are often very surprising. Certain vibrations can unscrew mounting screws or chafe through straps. Our Monkey Light includes a plastic battery holder with lots of springs and contacts inside it. We wanted to make sure this would last for years on a bike.

Our simplest (and ultimately most versatile vibration testing tool) is just a common reciprocating saw or Sawzall. You can use it in a lot of ways for vibration testing. We made some modified saw blades that have bolts or hooks at the ends of them, and then we just attached parts of our products to the saw and oscillated them rapidly. An inexpensive RPM meter can read the actual oscillation rate. With the oscillation rate and the stroke length you can calculate the G-forces applied. A $150 saw can easily test up to around 80G and you can strap it down and run it overnight.

We learned so much about what needed re-design this way! We made really small changes to our wire routing, the shape of the battery spring contacts and the plastic of our battery holder. We got our product to last 20 times longer in the vibration test with these subtle changes. (Tip: The saw oscillates in one direction. If you test your item in several orientations, you'll often find that it breaks differently. With a larger project you can attach the saw to different spots with simple linkages to get different vibration amplitudes.)

About MonkeyLectric

We brought our first product, M210 Mini Monkey Light, to market largely through funding from Kickstarter and soon followed with the M232 Light. These products are available in hundreds of bike shops around the world. If you're ever in Berkeley, come by to say hi.

Dan Goldwater was a scientist at the MIT Media Lab when he built his first art bike. Dan co-founded the DIY community site Instructables.com and the technology development firm, Squid Labs. Dan's work has been featured in Nature and in the Smithsonian. Dan's helmet is inspired by the Australian Magpie, a bird that is known to viciously attack the heads of cyclists; the best known prevention is to cover one's helmet in cable ties.

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