# Wireless Strain Gauge for Monitoring Bituminous Pavements
## 2 Electronic Design of the Sensor  
Figures 1 and 2 show the synoptic and the gauge wiring diagram. For an application in bituminous materials, we choose a large gauge (type C2A-06-20 CLW-120 from Micro-Measurements (Wendell, NC, USA) [31]). The strain acts on the gauges placed in the test body. The strain gauge conditioning circuit shapes the signal for analog-to-digital conversion. A 10-bit analog-to-digital converter (ADC) is embedded in the microcontroller unit (MCU). It can convert two channels with a 1 kHz frequency sampling. The MCU also contains a thermally compensated voltage reference in order to calibrate the ADC. The measured data are translated into UART (an asynchronous serial communication protocol) and sent to a data grabber via a Bluetooth transceiver. In our case, data grabbers can be popular devices, like a laptop or smartphone, which is very convenient for in situ testing. Figure 1: Synoptic. The circuit is equipped with a supercapacitor, and supercapacitors have several advantages over traditional batteries. At \(-40\)\({}^{\circ}\)C, capacitance decreases around 10% for supercapacitors without any increase in leakage current. For lithium-ion batteries, it is about 20% at \(-20\)\({}^{\circ}\)C. They are also able to withstand a greater amount of charge and discharge cycles than batteries, which can lead to a higher reliability for the device. Thus, power management circuit and supercapacitors provide energy for a duration of 5 min of measurement. The wireless energy receiver uses the charge of the supercapacitors when the measurement circuit is turned off. This also uses the same technology as a phone wireless charger. Supercapacitors are charged at constant current (approximately 500 mA) to avoid an inrush current and improve service life. It is considered that the discharge is also performed at a constant current (approximately 120 mA). Supercapacitors are oversized by a factor of 1.5 to consider aging, self-discharge, and losses. To operate correctly, the circuit must be powered by a voltage between 3.3 V and 5 V. Supercapacitors are also able to handle higher current loads than batteries. This is important given that for a short measurement period, the sizing characteristic for batteries is the current capability rather than the energy capacity. This is conducted to the oversized energy storage. With supercapacitors, the volume required for energy storage is reduced if charging is possible between two measurements. The power management circuit has the following three functions: wake up the sensor, charge the supercapacitors, and supply the Bluetooth transceiver. To wake up the sensor, we use a reed switch, which is activated wirelessly with a magnet placed on the pavement when walking up the sensor is needed. The main advantage of this technique is zero energy consumption when the device is off. The charge is stopped using overvoltage detection. The MCU is used to drive the measurement channels and convert analog-to-digital and UART translation. The data logger (smartphone or laptop) receives raw data. The objective of such a setup is to reduce the MCU's tasks to improve the sampling frequency (event-based programming with hardware interrupts). At the end of measurement, the circuit is turned off by a software instruction that interacts with the power management circuit to avoid leakage or quiescent current when the device is off. The choice of an asymmetrical Wheatstone bridge (see Figure 2) with four unequal impedances is made to save energy. The Rtemp and the gauge form a divider bridge that delivers a third of the supply voltage (here VCC = 2.5 V). A third of the voltage is chosen to ensure correct biasing even for the negative part of the expected signal. Rtop, Rbot, and the adjustment potentiometer make it possible to adjust the voltage difference in the two dividing bridges so that the output of a differential amplifier corresponds to one-third of the full-scale voltage (FSV) of the ADC. The zero adjustment potentiometer is electronically tuned by the microcontroller at each start-up.