The 2N3904 or S39013 NPN transistor amplifies or increases the audio signal created by the Arduino. The transistor has an amplification value called “gain” used to determine the volume of an electrical signal. A typical gain value engineers use in designing simple amplifiers like this one is 100. The mini 8Ω speaker can be wired directly to pin D9 with a reasonable amount of volume, but the simple transistor amplifier increases the sound by a factor of 100, making the Theremin sound louder.
The block diagram in Figure 12-5 shows the building blocks and the electrical signal flow for the Theremin. A Fritzing software circuit schematic diagram of the Theremin is shown in Figure 12-6. As a reminder, circuit schematic diagrams use electrical symbols for electronic components and are abbreviated drawings of Fritzing diagrams.
Figure 12-5. The Theremin block diagram
Figure 12-6. The Theremin circuit schematic diagram
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