This week I studied the RC car robot and the parts that make it work. First, I built another RC robot so that pictures could be taken at each step, these pictures can be found in our repository or a quick and basic picture summary can be found here. This second robot only had one transistor and I did not hook the turning wheels to anything.
Because this robot didn’t do much, I decided to attach a photo-resistor to it. The photo-resistor was hooked up and programmed so that in bright light, the robot moved faster, and in low light, it moved slowly. As I experimented shining a flashlight on the robot, I noticed that once the light was too low, the robot couldn’t move while on carpet. The breadboard, Arduino, cardboard, and 9V battery simply made for too heavy a platform. The force of friction between the wheels of the robot and the surface it is on is given by F = Coefficient of friction between wheel and carpet(Mu) * mass(m) * acceleration due to gravity(g). The coefficient of friction between the wheel and a hard surface such as wood flooring is different than between wheel and carpet, so the robot could move more easily on wood. To move, the motor must generate a torque greater than this force of friction * the radius of the wheels. Thus, we cannot have these RC robots move very slowly without the risk of it not being able to move on a certain material. I think we’ll need slower moving, higher torque motors.
Next I decided to add push buttons to be able to turn the motor on and off rather than having to unplug the 9V from the Arduino. One button turns the motor on and the other turns it off. I’m not sure how to wire and program a single button to do this.
Finally I wanted to learn more about transistors. Bipolar transistors come in two types: PNP –Positive-Negative-Positive and NPN – Negative-Positive-Negative. The ones I have been using are all NPN. There are three pins on a transistor which match up to these negative and positive charges. The Base(Negative. Receiving from Arduino) and the Collector(Positive) and Emitter(Negative) pins. Transistors can act like amplifiers and switches for high power devices.
When using it like a switch, a small base current flows when the voltage between Base and Emitter is higher than 0.7 V (the voltage necessary for silicon devices) making Base more positive than Emitter. Once the voltage is high enough, a Collector current can flow, turning the device on. When using this for relays, solenoids, or motors, a diode is put in place to protect the transistor. These items are inductive loads and thus use magnetic fields, which after collapsing when voltage is switched OFF by the transistor, can cause kickback voltage. A diode only allows voltage to flow in one direction, so by placing it in the circuit as I have done, no voltage can travel back to the transistor. Transistors used as switches are reliable and inexpensive compared to using mechanical relays. Using a transistor like an amplifier and the Hfe value will tell you the DC current gain.
*Edit: I also downloaded Fritzing and drew a circuit diagram of my light sensitive, push button RC car robot.