In this article you will learn:

• how to make sounds and play tunes;
• how a for...next loop operates;
• how to connect and use a digital temperature sensor.

Sounds in electronic projects are usually generated with an electromechanical or piezo buzzer. We’ll describe only the piezo type here, as they use much less power than the electromechanical types and are well suited for use in battery-powered circuits.

A typical piezo buzzer contains two main parts: a piezoelectric transducer and its driving circuit. The piezo transducer consists of a polarised ceramic material bonded to a metal disc.

A voltage applied to opposite sides of the disc will cause it to flex, so by applying a varying voltage, it can be made to vibrate and produce an audible sound. The built-in driving circuit in a buzzer generally applies a voltage that varies (or "oscillates") at a single frequency, resulting in a monotonous, irritating tone.

In conjunction with the sound command, PICAXE micros can generate a voltage at an output pin that oscillates at a programmable frequency. By connecting a piezoelectric transducer (without the inbuilt driver circuit) to that pin, we can therefore make our own buzzer for signalling purposes – and much more.

Making sounds

In use, three numbers must follow the sound command. The first is simply the output pin number that is to be driven and on the Schools Experimenter board, this will always be output 2. The second number is the desired pitch of the sound and can be between 0 and 127. As one might expect, a higher number generates a higher frequency signal. The third number is the length of the sound in approximately 10ms steps.

The simple program in Listing 1 shows how to use the sound command. In this example, three sequential tones of equal duration are produced. Listing 2 shows how the three lines can be combined into a single command to save PICAXE memory space.

Note that on the Schools Experimenter board, the piezo transducer and green LED share the same output (output 2), so the green LED will flicker as the sound is being generated.