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Son of Photovore

Son of Photovore

The little guy above is about 3cm high and 4.5cm wide. He weighs a mere 12 grams, yet he does everything the much larger Photovore is capable of, and does it faster, consuming much less energy. The minimum power needed is 80 micro-amps at 1.2 volt; about 1/10 of a milliwatt. That is enough to keep a wheel turning continuously, about as fast as the seconds hand of a clock. The power behind that motion is sufficient to keep the wheel turning, scraping the ground, even if the robot is blocked by an obstacle that escaped its feelers!

The secret: Son of Photovore has two lady's watches to drive his wheels. The stepping motors inside these watches are no longer controlled by the original electronics. The Son's `brain' provides them with a timing frequency about 55 times higher than they are used to, which is why the minute hands (coupled to the wheels) go round almost as fast as the seconds hand used to do.

Lady's watch and
   wheel The minute hands of the lady's watches (in red) are bent to drive the wheel shafts as shown. Small plastic beads are used as spacers, to keep the 0.5mm steel shafts in position; the frame is a piece of 0.5mm aluminum. The wheels (not visible) are made of foam and paper, with small bits of solder drilled into the foam to achieve balance.

The circuit diagram below shows how the watches are controlled. A schmitt-trigger built around two inverters takes input from the feelers and the BPW41 photo diodes. Thanks to the excellent properties of the BPW41 (don't substitute others unless you know what you are doing), the switch from one motor to the other occurs exactly when a line parallel to the wheel shafts points straight at the brightest light, almost regardless of the light intensity. The Son seeks the light and backs away from obstacles, just as his `father' does. A series of illustrations shows how it works.

Circuit diagram

I'm known as `Old Shaking Hands' around here, yet I found the task of modifying the lady's watches not too hard. Soldering SMD's (surface mounted devices) is also easier than you might think, providing you use /em thin /em solder (0.5 mm) and a good soldering iron with a sharp tip. If you want to have a go at building a `Son' of your own, here is the PCB layout I used:

PCB layout

When you're done, the PCB looks like this. The feelers are made of 0.3 mm steel wire, soldered to parts of the brushes of an old cassette recorder motor. Any tiny, solderable springs will do, of course.

The circuit board with the 3 electrolytic capacitors in the middle. The BPW41 `eyes' seem rather large now, compared to the surface mounted devices. circuit board

The other ingredients of this robot were foam (the ceiling tile variant), balsa wood, epoxy glue, 2-sided tape and a little patience. Not too much of the latter, though: total building time was about one day.