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Circuit diagram

An explanation of the abbreviations is here if your need it.

The Photovore's brain is built around two NAND-gates and six inverters, four elements each in two 74HC00 IC's. Two photodiodes (the `eyes') and two feelers provide input for a Schmitt-trigger, determining which of the two stepping motors (clocks m1 and m2) receives pulses from the oscillator. The feelers have priority over the eyes, and the feeler connected to zero has priority over the one connected to plus. So if both feelers touch obstacles, the robot will try to push itself out of trouble.
The frequency of the oscillator can be increased with the 5M variable resistor until the seconds hand makes about 17 revolutions per minute. Faster running is possible, but then the stepper will just as easily turn in the wrong direction, which would rob the Photovore of its sense of purpose.

circuit diagram

The two 47uF capacitors convert the square wave from the oscillator into pulses for the stepping motors in the clocks. Their value depends on the steppers, so 47uF is correct only for the type of clock mentioned in the parts list. Other stepper-driven clocks (not those equipped with a synchronous motor) can be used if you match the capacitor to the load. Ohmic resistance of the stepper coil is a useful guide: At 300 ohms, 47uF was perfect, while a lady's watch having a resistance of 2K6 needed 4.7uF (Yes, a Photovore using lady's watches is possible :)
The circuit around the three transistors 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. Use the 250K variable resistor to adjust the switch-off level. You're OK if the clocks run well until they stop.
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 clocks stop. The robot doesn't feel hungry anymore...

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