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17 rpm for the seconds hand means that the hour hand goes round in 3.5 minutes. Take a wheel with a diameter of a little more than 2 inches, mount it on the shaft and the robot will have a speed of about 2 inches per minute. I know, it's a snail's pace, but you can definitely see it move. And oscillator and motor together use less than 1 mA at 2 volts!
Moreover, the large reduction (the stepping motor itself runs at 500 rpm) makes for perseverance. The Photovore below is equipped with two clocks and has no trouble with a 20 degree incline - running continuously, under a heavy overcast, with the sun as its only source of power. The robot weighs 104 grams.


On a table next to a window the robot runs continuously during the daytime, even in the shade. But when it has to work itself from the middle of the room to the window, the solar panel will sometimes not even supply that one milli-amp. It makes sense to save up some energy, then run a short distance, repeating the process until enough light is found to run continuously. A large capacitor can be used for storage. The circuit shown waits until the solar panel raises the voltage over the capacitor (4700uF) above some 2.7 V before switching on the oscillator. If the voltage drops below about 2.2 V, the transistors turn off and the capacitor is recharged. The run/charge ratio depends on the amount of light. It is close to sunset when the run time drops to zero.

power supply diagram The circuit turns the robot on when the solar panel has charged the capacitor (4700uF) to about 2.7 volts, and off if the voltage drops below some 2.2 volts. The diodes and the first BC559 also limit the voltage supplied to the robot to about 4 V. The LED lits up when the limiter is active. If the voltage (in direct sunlight) still rises nothing will be damaged, but the clock stops. The robot doesn't feel hungry anymore...

Now let's look at the `brain' of the Photovore. The prototype seaches vigorously for better sunlight, while neatly sidestepping most obstacles. Apart from two clocks with wheels attached, it has two photodiodes (its eyes) and two feelers. The clocks turn clockwise. The robot can only switch them on and off. Yet it moves towards the light and it recoils from obstacles. How does it work? A series of illustrations provides the answer.

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