The unloaded PCB appears green because thin sheets of green plastic have been applied to both sides (otherwise the PCB would appear pale yellow). Called solder masks, these sheets cover all exposed metal other than the component pads and holes so that errant solder can not inadvertently short (or electrically connect) the printed wires. All metal surfaces other than the exposed pads and holes (i.e., the wires) are underneath the solder mask. Not infrequently, blue or even red solder masks are used.
Several different resistors are used on Digilent boards. Some are used to limit LED current, and some are used on inputs (like the button and switch circuits) to both limit the currents flowing to the main chip, and to help protect against electrostatic discharge (or ESD – more on this topic later). The resistors on the Digilent boards, like most resistors used in digital systems, are physically small because they will not encounter large voltages or currents. For these smaller resistors, the resistor value in Ohms is printed in microscopic numbers on the resistor body, not visible to the naked eye.
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