Ultrasound (US) is widely used in areas such as medicine, engineering, physics and geology, among others. In our medical physics course, we teach US, although students do not have practical contact with simple equipment, which reveals the core physical principles, and instead have demonstrations in a large group with hospital equipment, where the basic concepts are not clearly visible. We identified low-cost apparatus for pairs of students to perform some simple experiments about the principles of US, using PCs, Arduino Microcontrollers (already available in our lab), the Arduino HC-SR04 US sensor, and cheap materials found in the laboratory or at home. Next we developed some simple experiments for a two hour laboratory session: (i) estimation of the velocity of sound in air; (ii) approximate profile of the US beam; (iii) description of acoustic properties of different materials; (iv) concept of B-mode ultrasound, the profile of an object obtained from multiple scanlines. The experiments have a good educational value, but might be improved by using a stepper motor to emulate a sector format transducer to create a B-mode like outline, and using multiple sensors to introduce students to principles of beamforming.