So it's been a bit since I posted my last project (the motorcycle starter repair) and I have been quite busy since then. As such, I am still playing catch up on this website and, today, I will be writing about an older project that I completed back in April. This particular project goes by the name of Symet. I started working on Symet a few years ago when I first purchased Junkbots, Bugbots, and Bots on Wheels, by Hrynkiw and Tilden. I developed one version of Symet back in 2009 but it never quite worked right. Having been distracted by other things, I never sucessfully troubleshooted that version. Instead, I started a rebuild from scratch back in April and got a functioning robot in a couple weeks.
Essentially, Symet is a simple little circular robot that does little more than scoot around and bounce off of things. It uses its geometry, primarily, for guidance and navigation. It is powered by some small solar cells. The electricity produced by the cells is conditioned via an FLED solar engine that uses a flashing LED (FLED) to charge a bank of capacitors. The capacitors discharge on a timed interval allowing Symet to pop about a surface. The basic circuit schematic for the Symet is shown in Figure 1. It was provided courtesy of Solarbotics' Junkbots Apache server.
After getting everything hooked up (including my original motor) I was disappointed to see no activity in my unassembled Symet. I began taking current and potential (voltage) readings with my multimeter to see what was wrong. By the time all was said and done, I had to use math to solve my problem (as is usually the case in electronics).
Sizing the Motor
F = (A * s) / V
...where F symbolizes Farads (the unit of capacitance), A symbolizes Amperes (the unit of current), s stands in for seconds, and V symbolizes volts (the unit for potential). Originally I was using a motor that I pulled out of an old CD player. This motor was rated at a 3.5 volt operational load. This means that my capacitor bank was only capable of delivering 0.0066 uF * 3.5 V of current for 1 second. In other words, my capacitors could only deliver 0.0231 A, or 23.1 mA (milliAmps) to my motor over one second. Well my motor was rated to operate at 3.5 V and 200 mA or so. So, obviously, I was not delivering enough current to my motor.
initial assembly of the FLED engine.
time = F * V / A
time = 0.0066 F * 2.0 V / 0.012 A
time = 1.1 seconds
... 1.1 seconds in optimal conditions. Thus, all I had to do was swap the motor and the rest of the circuit should work. Sure enough, I plugged in the motor depicted in Figure 3, and my breadboarded Symet wiggled to life.
The first step is to assemble the two transistors, the resistor, and the FLED in a particular configuration. The resistor gets set between the collector of the 3904 transistor and the base of the 3906 transistor. The base of the 3904 transistor gets soldered directly to the collector of the 3906. The FLED cathode gets soldered to the emitter of the 3904 and the anode of the FLED gets soldered to the base of the 3906 (same node as one resistor end). The emitter of the 3906 will remain unanchored initially as it gets soldered to a conductive ring later. Similarly, the emitter of the 3904 will get soldered to the motor body later. Once these solder connections are made, you should end up with an FLED engine that looks similar to mine depicted in Figure 4.
from one of my 'helping hands' workstation tools.
solar array assembly. This simple geometry
ensures Symet will always have incident
light on its panels no matter its orientation.
Once I made this final connection, I held my breath while I shined a fluorescent lamp directly on Symet's solar panels. Sure enough, a few minutes later, the motor spun to life and Symet scooted across my workbench. Overjoyed at my final success, I glued the solar array in place (with an excessive amount of hot glue) and slapped a small wheel from inside a cassette deck to the motor axle. This gave Symet a better tilt and allowed it to hop a little more than a bare axle allowed. The final result is depicted in Figure 6 and in the video below.
Also, I learned how valuable it is to have a prototyping environment when starting a new project. Breadboarding the entire circuit and hot-swapping components allowed me to test parts and bits to ensure they were functional. It also allowed me to test my math before assembling anything. Diving straight into my original attempt without testing anything first was a doomed effort.
All in all, of course, this project was fun, and inspired me to do a lot more work. Since this effort, I've been slapping together an Arduino powered testbed robot which I should write up in the next couple of weeks. Until then, I hope you all had fun reading and learned something from my effort.
Best of Luck,
Brady C. Jackson