A small, two wheeled robot that will pull itself into the shadows. Originally based off of the behavior and appearance of a cockroach, the design was later made larger to accommodate cheaper equipment. I decided to pursue this kind of project because it represents a movement in robotics. Recently roboticists have been turning to animals for inspiration for robots, specifically, robotic locomotion.
The robot consists of:
The robot has a large, flat torso with a segmented tail that tapers off at the rear, making a conical shape. The two motors will only pull the toy forward at a constant speed. The tail acts as a caster to keep the toy upright. A small servo with a rudder-like attachment on the front of the robot will pivot and make contact with the ground closer to one side of the robot. This creates drag on that side causing the robot to bear in that direction. The photoresistors are positioned on either side of the torso to determine in which direction it should turn to follow the darkness. The toy will react to the varying light levels, moving towards the area with the least amount of light.
The rudder method of steering was chosen as a cheap solution to accommodate waning time and budget. The original idea was that the motors would simply vary their speed to make a turn, but the Arduino did not have enough current to accommodate an on-off motor solution. That is, the digital pins can provide a 3.3V source, but the current is less than the 3.3V source pin. Additionally, a dedicated power supply would have to be implemented before any additional components are added; the robot began to malfunction because the USB was not robust enough to power the Arduino and all the other components on the toy.
The toy only works part of the time and simply needs a more reliable source of power to operate fully.
The photoresistors have variable resistance, acting as a very large servo in the dark and resisting less when exposed to more light. The photoresistor is a semiconductor that sits atop a bit of cadmium sulfide. When light hits the cadmium sulfite its electrons collect enough energy to jump to the conductive band, lessening its resistance.
When powered with a small voltage (3-5V) and placed in series with a pulldown resistor, the Arduino can monitor the photoresistor by observing the voltage between it and a pulldown resistor. The resistor can be between 1k and 100k (ohms) depending on the desired accuracy and the light levels the device will be working with. Note that the photoresitor is not a very accurate device and must be calibrated when being used in conjunction (its readings being compared against) with other photoresistors. I achieved this simply with an offset, as seen in the code.
The chassis assembly (wood, glue, foam) cost less than $3.00 in materials. The breadboard with wiring cost less than $6.00. My final design used only two photoresistors, which cost only $1.00 each from a supplier. The one micro-servo I used cost $3.00 from a supplier. The Arduino cost about $30.00.
It cost me under $50.00 to make this toy on my own in only a few days. If I were to manufacture it, I would first refine the design and make it nicer looking. I would also implement a few more small servos to animate a couple of eyes that would follow light or have some other cosmetic function. This enhancement would cost me only a few more dollars to implement.
In a mass-producing scenario, in purchasing most of the materials wholesale, I expect you could cut the cost of many components in half, easily. The breadboard could be replaced with a cheaper circuit board or omitted all-together as there are not many wires to account for.
Assuming I still use the Arduino in each toy, and that they still cost $30.00 each (no wholesale), you could produce this product for the $30.00 plus $5.00 for the other components. This is a generous estimation.
Video demonstrating the rudder reacting to a change in light.
Robot's breadboard. The Arduino sits beneath it. The two photoresistors can be seen on the top and bottom of the board. The 3-pin attachment on the right connects to the servo.
The final prototype complete with foam chassis.
The inspiration for the robots chassis; Moldorm from The Legend of Zelda: A Link to the Past. Image licensed under Fair Use by United States Copyright Law.