The main electronic component for the capacitive touch sensor circuit is the 555 timer IC. As shown in the circuit schematic diagram, the 555 timer is wired as a triggered monostable timer circuit. The monostable circuit is quite simple in construction, and it consists of three timing electronic components—the 555 timer IC, R3, and C1. The bypass capacitor (C2) helps minimize noise from the timer's internal switching circuits. The pull-down resistor (R2) is included to assure that 0 V DC is applied to the Arduino pin D2 when a trigger signal is not present. The transistor (Q1) and the pull-up resistor (R1) provide an inverting electronic switch, which is used to trigger the 555 timer monostable circuit. The 555 timer-triggered monostable circuit is shown in the following diagram:

To trigger the 555 timer monostable circuit, either zero or negative voltage is required at pin 2 of...