The robot this code supports is an inexpensive three wheeled, 2 servo model that can be obtained on ebay or amazon. The part list includes:
https://www.amazon.com/gp/product/B00CAG6GX2/ref-oh_aui_detailpage_o03_s00?ie-UTF8&psc-1
You might consider replacing the Parallax part with a cheaper item. I had a couple of these laying around from before cheaper alternatives existed. The Parallax device requires 5V. If you do use another part, you might need to modify Robot::getDistanceInInches() and/or Robot::microsecondsToInches() to make it work properly.
These are awesome and cheap. Comes with two motors that operate with 3V - 6V. Assembly instructions were in Chinese, but there are youtube videos that help you figure out how to put the chassis together if you can't read Chinese.
Standard Arduino Uno, or a knockoff.
I mounted a SPDT switch on the chassis and wired it into the circuit to turn the robot on and off.
You'll need at least 9V to drive the motors and the Parallax ultrasonic. The three wheeled robot comes with a 4-AA holder which gives you 6V. The bot won't last 10 minutes with only 6V, the sensor starts giving false readings. Replace with a 6-AA holder which provides more than enough voltage (9 volts on fresh batteries) and works much better.
The control pin for the ultrasonic is wired to pin 5 of the arduino. For the L298N, I used the wiring as shown in the excellent instructions at https://tronixlabs.com.au/news/tutorial-l298n-dual-motor-controller-module-2a-and-arduino/
The rest of the wiring should be straightforward to anyone comfortable with using Arduino in breadboard designs.
The logic is simple. The Robot class is in C++ and provides the following functions:
long getDistanceInInches();
void reverse();
void forward();
void setSpeed(int wheel, int speed);
void stop(int wheel);
void turnLeft(long seconds);
void turnRight(long seconds);
The loop() function calls a function named wanderAndWait(). This function gets the distance sensed by the untrasonic. If less than 8 inches, it reverses the bot, then the bot is rotated in a random direction for a small random number of seconds. Then it then goes forward again. The result is the bot will navigate a room avoiding any objects that are able to be sensed ahead of the robot with the ultrsasonic.
Add some changes to the logic to try and detect that the robot is stuck in the forward state but is not moving (if the distance read over time is not infinity and is not changing, maybe the device is stuck on something and should back away as if it were sensing a close object).