# Remote Low-Cost Differential Isolated Probe for Voltage Measurements
## 3 Probe Test and Validation
### Isolated Voltage Probe as a Remote Autonomous Instrument  
Once the operation of the probe has been verified at hardware level, as an instrumentation element, the next step is to equip the probe with the digitisation and RF data transmission process to acquire the information necessary to determine whether a signal coming from the electrical network complies with the EN 50160 standard, in order to assess the usefulness of the proposed probe as a remote instrument. Then, configured as an autonomous instrument, the probe can be battery-powered and managed by an Arduino Nano 33 IoT module. The SAM21D microcontroller in this module has a 12-bit 350 ksps ADC that can be multiplexed with up to 20 analogue input channels. The proposed probe only requires two ADC channels, one to digitize the voltage values monitored by the probe, plus an additional input to monitor the battery status. Although monitoring the battery status does not require a high sampling rate, for code simplicity, both input channels have been configured at the same sampling rate. The probe has been designed to adequately monitor 50 Hz voltage signals and verify compliance with EN 50160 up to the 25 th harmonic. Thus, the ADC sampling rate must be able to properly acquire  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{4}{c}{**Odd Harmonics**} & \multicolumn{2}{c}{**Even Harmonics**} \\ \hline \multicolumn{2}{c}{**Not Multiples of 3**} & \multicolumn{2}{c}{**Multiples of 3**} & \multicolumn{2}{c}{**Intek Voltage (U\({}_{\text{n}}\))**} & \multicolumn{2}{c}{**Relative Voltage (U\({}_{\text{n}}\))**} & \multicolumn{2}{c}{**\(n\)th Order**} & \multicolumn{2}{c}{**Relative Voltage (U\({}_{\text{n}}\))**} \\ \hline
5 & 6.0\% & 3 & 5.0\% & 2 & 2.0\% \\
7 & 5.0\% & 9 & 1.5\% & 4 & 1.0\% \\
11 & 3.5\% & 15 & 0.5\% & 6...24 & 0.5\% \\
13 & 3.0\% & 21 & 0.5\% & & \\
17 & 2.0\% & & & & \\
19 & 1.5\% & & & & \\
23 & 1.5\% & & & & \\signals with a frequency up to 1250 Hz. In order to comply with the Nyquist-Shannon sampling theorem, the sampling rate is set to 50 ksps, so that the system can acquire up to 40 samples per cycle. After voltage acquisition, data can be transmitted to a host for their storage, processing, and display. For this, the MQTT (Message Queuing Telemetry Transport) [27] protocol over the Wi-Fi network for wireless data transmission has been selected. MQTT is a light  
Figure 16: Arbitrary function generator test and harmonic analysis. (**a**) Main voltage signal and harmonics 3, 5 and 7. (**b**) FFT of the oscilloscope of the signal of (**a**). (**c**) Main voltage signal and harmonics 10 and 25. (**d**) FFT of the oscilloscope of the signal of (**c**). (**e**) Main voltage signal and harmonics 2, 4 and 22. (**f**) FFT of the oscilloscope of the signal of (**e**). messaging protocol used in networks with low resources, as those applied in IoT networks. This protocol is machine to machine (M2M), and the client connects to a host used as a server or broker by publishing or subscribing to a series of topics. In this work, the broker (Mosquito) is in charge of managing the protocol. The proposed remote probe acts as a publisher client, sending data to the broker under a specific topic (myPi) myVoltageProbe in the validation test explained in Section 3.2.1). A PC running Paho MQTT Python Client is the subscriber client, requesting data from the broker under the same topic, which allows it to collect the information published by the remote probe. Figure 17 shows the connectivity diagram of the different elements for the evidence presented in the following subsection. The flowchart in Figure 18 shows the operation of the code in both the microcontroller integrated in the remote probe and the script developed in Python for PC. As can be seen, first the peripherals necessary for the development of the project are configured, and then the microcontroller is configured as the publisher of the relevant topic. It then acquires the data and publishes them in its topic. Figure 17: MQTT connectivity block diagram.