PrayBot deployed

With a few new ideas for the last mechanical bits having arrived during my shower this morning, and a subsequent frenzy of Dremeling and hot-glueing, the PrayBot was in place and functional for the start of Summer!

It was my goal to get it going today, the Summer Solstice. And thanks to some child care time donated by the Life Companion for Father’s Day, it has come to pass. It’s always wonderful to finish a project, but to have just finished this in time to have it spinning at noon on the day of the year when it’s most useful is more rewarding than I had even imagined. I’m spinning right along with it!

See more pictures and video on my gallery site.


OK, I’ll back up a bit. MAKE magazine recently did a feature on robots, including an article on building simple BEAM-style bots based on the Solarengine. The Solarengine is basically a solution to the problem that most photovoltaic (PV) solar panels don’t produce enough current to start even a small electric motor from a full stop. So, in order to drive a motor from a PV panel, you have to store up the charge for a while, then release it in a burst. That’s what the Solarengine is, and there are various ways to build one.

I tried one common variant of the circuit (using an FLED - a flashing LED - to trigger the current dump) with parts I had at home, for the hell of it. Didn’t work; the transistor feedback loop never shut off. The more common variant, the one described in the magazine, uses a 1381 chip as a trigger, but I didn’t have one and didn’t feel like mail-ordering it special.

The original bit

So, I designed my own trigger using an 8-pin PIC microcontroller. (schematic - BoostC source) It works fine, may not be as efficient (though I obviously can’t compare since I don’t have the other circuits), was what I had on hand, and best of all, it can be tuned quite precisely to match a given motor and load.

But what should it drive? Making another robot that drives itself around at random seemed redundant. Instead, I decided to make a solar-powered prayer wheel.

Tibetan tradition holds that if you paint a prayer on a flag, or a windmill, and the wind moves it, it has karmic benefit similar to that of a human being saying the same prayer. Regardless of what that means to you, I think we can all agree that moving marquees are more eye-catching than static signs, and a piece of art that does something dynamic is more interesting than one that sits still.

And what better prayer for a solar-powered robot than “THANKS FOR THE SUN!”? And of course, noon on the Summer Solstice - the moment when the sun is highest in the sky - is the most logical time to utter that prayer.

Nuts and glues

I did have some problems getting it finished and running this morning. Several schemes for attaching the spinning piece to the motor had already failed, and I thought this one was going to work, but the cyanoacrylate glue I used to attach the drive shaft to the motor output just didn’t hold up in practice. And, though I took pains to avoid stress on the wires connecting to the very flimsy contacts on the solar panel, one came off.

But, I still got it up and limping by noon. Since the glue join broke when I revved it up in full sun, the motor just slips on the drive shaft. This is kind of neat, though, because it allows the motor to spend more time spinning freely, kind of like a friction clutch, and imparts an eccentric motion to the spinner. Also it makes a lot of noise, which is cool. I mean, somehow it’s very cool - at times, it was getting this low-frequency resonance that, with some imagination, sounded like Tibetan throat singing. A stretch. But still, I think it’s a feature and I’m not going to change it.

Clearly I need to do a bit more to make the thing more permanent. But having it spinning merrily along on the day was still quite rewarding.

Disclaimer for curmudgeons

In case you’re wondering - I had it running both in my office window at 12:00 local time, which of course is Daylight Saving Time at the moment, and again outside at 1:00, which is “true” noon from a pagan perspective. So I’m covered either way.

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