"My next creation had to be battery powered, so I began to amass an impressive collection of DC motors, relays, and other diverse components while sorting out the design in my head. The end result was CRAWLER I, intended to be my junior-year science project, but the eagerly anticipated event was unfortunately canceled due to faculty indifference. I had also decided to build a tracked vehicle for improved maneuverability. A pair of 24-volt DC gearmotors from a aircraft surplus catalog were mounted on a 18- by 13-inch plywood base, driving left and right tracks fashioned from 1.5-inch rubber timing belts turned inside out. Control was again provided by electromechanical relays, but each motor had a centrifugal speed-limiting switch that could be adjusted to achieve precise straight-line travel. By adding an override circuit on the stationary side of the slip rings that fed the centrifugal governor, it was possible to momentarily boost the motor rpm to maximum. Skid steering was achieved by providing differential speed commands in this fashion or by stopping one motor altogether. The vehicle could also turn in place by reversing one track. The tough part in building an autonomous vehicle, of course, lies in how to control its motion, made even tougher in an era that predated microprocessors and low-cost sensors. I had in mind a platform that would drive around until it encountered an object, then alter course in an intelligent fashion. I also wanted it to automatically recharge the onboard lead-acid motorcycle batteries when they ran low. Like most engineers, I tackled the tougher issue first: automatic recharging. I settled on a beacon homing scheme with an ordinary 60-watt light bulb as the source. The scanning sensor was simply a cadmium-sulfide photoresistor mounted in the end of a 12-inch plastic tube, which was rotated in the horizontal plane of the beacon by a small DC gearmotor from a sign display. A special slip-ring sensor at the bottom of the rotating shaft was used to index the 0-degree position coinciding with the forward axis of travel. All the control logic was implemented with surplus mechanical relays. The photocell scanner rotated at a constant rate, with vehicle turning initiated by the shaft index sensor and halted by beacon detection (or vice versa). The first sweep was used to determine if the beacon was present, and if so, in which direction the vehicle needed to turn. A pair of triple-pole, double-throw relays was used to implement an electromechanical flip-flop to remember on which side the beacon had most recently been seen. CRAWLER I was therefore not only my first autonomous robot, but also the first to have an actual memory, albeit but a single bit! (Keep in mind this was a high-school science project back in the technology-starved mid-sixties.) For example, if the beacon lay off to the right side, the scanner would initiate a platform turn in that direction by stopping the right drive motor every time the collimating tube passed through the forward index (i.e., pointed straight ahead). As the sweep continued from left to right, the photocell would eventually detect the beacon, whereupon the control logic would restart the right drive motor. This process would repeat with every sweep. Each time the turning action would last for a shorter period than before, because the beacon would have moved closer to the 0-degree index as the robot turned toward it. When the beacon lay dead ahead, the sweep would start and then immediately stop the turn, resulting in straight-line travel." – H.R. Everett, ROBART I: In Retrospect.

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The Moth Cybernetic Model (1975) by M. Keul and H. Lohr, Elektor June 1975. "This is a design for a simple cybernetic model, based on an electric toy car, that will be attracted towards a light source like a moth, negotiating obstacles in its path. The car has two motors, one to propel it and one for the steering mechanism. The principle is quite simple. A light-sensitive element is mounted obliquely to the right at the front of the car. Normally, the steering motor keeps the car on a left turn, but this motor can be reversed by a relay, so that the car turns to the right. This happens each time the light-sensitive element receives light. If the car, travelling to the left, passes a light source, it swerves to the right in the direction of the light. However, it keeps turning to the right, so that after some time the element receives no more light. Then the car will automatically swerve to the left again until the element again receives light from the source, and so on. Thus the car zigzags towards the light source. It behaves more-or-less like a moth, hence the name of this apparatus." – Elektor june 75.

Beetle (1975) by Rossbach secondary school, Elektor, June 1975. "Beetles, tortoises and the like have often served as models for cybernetic machines which must also have a reasonable appearance. The beetle described in this article can 'see, hear and feel' and reacts to information in the form of sounds and movements. The animal has a memory and can get tired. The beetle is the first of a series of articles on cybernetic models. … In daylight (vertical light) the model moves slowly forward in a circle (counter-clockwise) with a diameter of approx. 80cm. If during its journey the beetle arrives in a place where there is less light (for instance in a dark corner or below a chair), it rests in the shade for approx. half a minute to a full minute and then resumes its circulatory journey. It is also possible that, instead of continuing the model decides to take a prolonged rest in the shade: it 'falls asleep'. This condition of prolonged rest can only be altered by external stimuli. The object casting the shadow is removed or the model is wakened by a loud sound (clapping). If the beetle 'sees' a horizontal light source on its journey, it makes a beeline for it. Any deviations from its course are automatically corrected. If the model hits an obstacle in its path, it gives a brief cry of fright and immediately shrinks back. This reverse movement is, however, preceded by a short turn, so that when the beetle resumes its forward movement after 3-4 seconds, it is positioned obliquely with respect to its previous direction of travel. Because the beetle can now no longer see the light source, it again starts a left turn, which results in a second collision with the obstacle, somewhat more to the left than the first time. Thus, after a few repetitions, extensive obstacles are also dodged, after which the beetle heads for its goal (phototropism)." – Beetle, Elektor June 75.

This song is entirely too short. 

Rayo - Mistake Made (Original Mix)

adj.Having a tendency to move in response to light. An animal or microbe that is positively phototropic will move toward light. One that is negatively phototropic will move away from light.