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Just a tether to a power supply. But look at the copper traces for the yet to be made leg-sensors, and the four unused 3 mm holes in the upper frame. The Spider has been designed to carry any combination of brain and on-board power supply that will fit on a 12 x 6 cm pcb.
Even in somewhat difficult terrain - like my desktop or the garden - the Spider doesn't need much brains to walk. But the tasks and sensors I have in mind for him seem to require some classic computing; therefore a simple microcontroller is most appropriate. Ken Huntington (see "A reader contributes") pointed me to the new Atmel AT90S series. The fist one I will use is the AT90S1200. It has 1K flash program space, 64 bytes of eeprom and 32 bytes in general purpose registers. You also get a timer/counter, an analog comparator, a watchdog, an on-board clock generator and 15 programmable i/o lines in this 20-pin baby, which costs just a few dollars. It is satisfied with less than 2 mA at 3 V when active and has a serial interface for programming while in-circuit. Programming hardware can be nothing more than a cable with the right connectors! A free assembler is available; Ken and I are working on simple programming software.
The only real constraint is weight. While the spider walks reasonably well at an all-up weight of 158 grams, performance increases as the weight goes down. On the photo above you see 92 grams. The modest power requirements of the motors allow continuous walking in full sunlight when the spider is connected to a 4 x 8 cm Panasonic BP-378234 solar panel (nominally 3.2 V at 40 mA). But since I want performance in normal Dutch weather, I intend to use two NiMH rechargeables, giving 2.4 V and 70 mAh each. There may be a solar cell to reacharge them. But only if total weight - including brain, sensors and so on - can be kept well below 140 grams. That means using a pertinax pcb, nylon M3's for mounting, and whatever else adds lightness.
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