The 2N3904 NPN transistor provides a signal to the Arduino, allowing it to turn on the piezo buzzer. Placing a metal object on the test probes allows electrical current to flow through the transistor, turning it on like a pushbutton switch. The 1KΩ (kilo-ohm) resistor provides a control voltage (+5VDC) to the Arduino, allowing it to turn on the piezo buzzer. Placing plastic objects on the test probes will not activate the transistor, the Arduino, and the piezo buzzer.
The block diagram in Figure 11-4 shows the building blocks and the electrical signal flow for the Metal Checker. A circuit schematic diagram used by electrical engineers to quickly build cool electronic devices is shown in Figure 11-5. Circuit schematic diagrams use electrical symbols for electronic components and are abbreviated drawings of Fritzing diagrams.
Figure 11-4. The Metal Checker block diagram
Figure 11-5. The Metal Checker circuit schematic diagram
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