2.2.3. Electronics

Two operational amplifiers in instrumentation configuration are the base block of the acquisition system, as Figure 19 shows. We employ the KiCad software for the circuit design.

**Figure 19.** Two Op. Amp. instrumentation amplifier.

The voltage gain in the instrumentation amplifier is:

$$\mathbf{A}\_{\rm V} = \frac{\mathbf{v}\_{\rm o}}{\mathbf{v}\_{\rm i}} = \left[ 1 = \frac{R\_2}{R\_1} + \frac{2R\_2}{R\_1} \right] \tag{59}$$

By selecting R<sup>2</sup> = 100 kΩ, R<sup>1</sup> = 1 kΩ, and R<sup>G</sup> = 5 kΩ, the voltage gain is 141. With 34 mV as voltage input, we get:

$$\mathbf{v\_o = A\_v \cdot v\_i = 4.794\,\text{V}}\tag{60}$$

The final acquisition circuit has seven instrumentation amplifiers, with bias and gain trimmers for calibration. We design the printed board circuit as an ™Arduino Mega 2560 Shield, and assemble the components to the board by throw-hole soldering. We feed the circuits with a power system with two 18650 Li-Ion batteries in series, a backup pack, a Battery Management System (BMS); a 5V buck and a 12V boost converters. The Figure 20 shows the schematics. Finally, we add connectors for the MPU, OLED, and Bluetooth modules.

**Figure 20.** Power system with backup, BMS, boost, and buck converters.

#### 2.2.4. Electronics Casing

We export the KiCad printed circuit design to FreeCAD StepUp to design the case containing all the components, focusing on a compact configuration design. The two main electronic components are the Arduino Mega 2560 and the Orange Pi One single board computer. We place the components, such as the Dual Pole Dual Throw (DPDT) toggle switches, symmetrically on the box sides. Figure 21 shows the main sides and the final assembly of the electronics case.

**Figure 21.** Modular electronics casing.

Each box has attached components to optimize the space. We test every component, and then install the support structure.

### 2.2.5. Final Mechanical Assembly

The prototype consist of 45 3D printed parts, the union of main components is by an 8 mm steel threaded rod. The sub-assemblies uses M3 bolts and nuts. Figure 22 shows the assembly CAD.

**Figure 22.** Complete prototype.
