### An Arduino Investigation of the RC Circuit

Calin Galeriu, Cheryl Letson, Geoffrey Esper
2015 The Physics Teacher
285 The battery is then removed from the circuit and the capacitor, now charged with an electric charge Q 0 = V 0 C, is allowed to discharge through the resistor with the help of a switch (that is closed at time t 2 ). During the discharging process, the voltage on the capacitor is given by the formula: (3) The formula for the electric current stays the same, but this time the current flows in the opposite direction. Even though Eqs. (1) and (3) indicate that a capacitor requires an infinite
more » ... unt of time to reach the saturation (or zero) voltage, from a practical point of view the capacitor is considered fully charged (or discharged) when the difference between the experimental and theoretical voltage values becomes smaller than the instrumental error of the voltmeter. 7 The charging or discharging of a capacitor has important practical applications, for example, it can be used to measure unknown values of resistance or capacitance. In our investigation of the RC circuit, we have measured the voltage by means of the Arduino Uno board, with the help of the analog pins. These pins, internally connected to analog-to-digital converters with a 10-bit resolution, effectively eliminate the need for a handheld voltmeter. The experimental data are subsequently analyzed using CurveExpert Basic, 8 and the formulas for voltage and current can easily be checked. The experimental data can also be graphed in real time using MegunoLink Lite, 9 which produces an attractive visual display without the need for an oscilloscope. A word of caution is also in order. It is very easy to inadvertently destroy an Arduino Uno board. Ten common but fatal mistakes are described by Rugged Circuits. 10 Unfortunately, the solution proposed by Rugged Circuits, the Ruggeduino board, is not good for our study of the RC circuit because of the small leakage currents through the additional Zener diodes. We do recommend the Ruggeduino board for applications that use only the digital pins. The RC circuit under investigation consists of a capacitor (measured value 2.24 µF), a resistor (measured value 9.73 MΩ), an SPDT switch, and an Arduino Uno board. The switch is used to connect the RC circuit branch to either the Arduino's 5-V source (for charging) or the common ground (for discharging), as shown in Fig. 1 . The time constant of this circuit τ = RC is expected to be 21.8 s. For the study reported here, we have used a metalized polymer film capacitor, with an excellent ability to hold an electric charge. Electrolytic capacitors do not perform as well. Our students from the electronics program at Bay Path Regional Vocational Technical High School have first as-