The HC-SR04 ultrasonic distance module is an inexpensive device to detect distances with centimeter accuracy, up to a few meters.

The device emits ultrasonic pulses of sound and listens for an echo. The echo time depends on the distance of the object that reflected the sound.

Transistors are just about everywhere in electronics. They are used in digital switches, amplifiers, voltage regulators, and many other simple components that work together in complex digital circuits. Two types you're likely to use are the bipolar junction transistor (BJT) and the field effect transistor (FET).

Both types of transistor have three pins. The pins on the bipolar junction transistor are called the emitter, collector, and base. On the field effect transistor, the pins are called the source, drain, and gate.

Stepper motors can move back and forth and lock into fixed positions, or steps, around the axis.

Stepper motors are common in machines that require precise and stable movement, like 3D printers and CNC machines.

Usually you will connect a stepper motor to your microcontroller by means of a stepper motor driver board, which has all the circuitry for controlling the motor. Then for you to control the motor, you just need to provide power and a signal to trigger the motor's intended position.

A shift register is a simple memory device that allows data to be "shifted" bit-by-bit into or out of the register.

What are shift registers used for?

One place you might use a shift register is to control up to eight components, such as LEDs, using only three Arduino pins instead of a separate pin for each LED. Moreover, some shift registers can be chained together to expose a huge number of inputs or outputs while using only a few pins on the microcontroller. Shift registers are also used for controlling 7-segment LCD displays and various other components.

Servo motors are an important part of many robotics projects. A servo can move to arbitrary positions within some range of motion, usually 180 degrees. You might think of them like mechanical elbows.

Most Arduino boards can only supply enough power to move very tiny servos (like the 9g variety for steering small RC vehicles) – and no more than one unless you are careful to move them one at a time. This is helpful for prototyping and debugging isolated parts of a larger robot before moving them off to a different circuit with external power.

At first glance, some rotary encoders look a lot like the potentiometers used as knobs on radios and other control boards. They both can serve a similar purpose in providing rotational information to a circuit. However, these components are fundamentally very different: