# Coupled System of Dual-Axis Clinostat and Helmholtz Cage for Simulated Microgravity Experiments
## 2 Design and Construction
### Electrics and Sensors of the Clinostat  
Electric power is provided to the microcomputer and LEDs with cables that are run inside aluminum profiles and axles. Due to rotations of the construction elements, special slip rings with up to 12 connectors were used (KYM12B type made by Asiantool(r) (New Taipei, Taiwan)).  
In the cultivation area, diverse sensors are applied to monitor the experiment. Two temperature sensors (MCP9808 (Adafruit(r))) at opposite positions measure the temperature. Light-intensity sensors (APDS9960 (Adafruit(r))) are placed on basket guides. The soil moisture sensor is made of two aluminum rods mounted parallelly and distanced with plastic spacers. The three-axis LIS3 DH (Adafruit(r)) acceleration sensor is mounted in thecenter of the sphere to monitor the microgravity state. Additionally, two cameras are added to take pictures of cultivated plants.  
The LED illuminator, shown in Figure 5a, consists of 24 red (PK2N-3 LME-HSD, 650-670 nm, 3 W), 10 blue (PM2E-3 LBS-SD, 455-475 nm, 3 W), and 5 white diodes (PM2E-3 LVS-R7, 3 W), manufactured by ProLight Opto(r) (Taoyuan City, Taiwan). Each one has a maximum power of 3 W. The total mass of diodes with radiators equals 273 g. The number of diodes and colors was chosen in such a way to mimic the spectrum supporting  
Figure 4: The static version of clinostat offers the opportunity for natural Earth gravity experiments and comparative studies.  
Figure 3: A 3D view of the cultivation sphere and the construction details.  
the photosynthesis of plants [34]. Light intensity is controlled with a 12 V pulse width modulation (PWM) signal. Each diode has its radiator to remove heat. Diodes are mounted on the upper half of the sphere and point toward the basket. The position of diodes was examined by Monte Carlo simulations to find the uniform illumination area (Figure 5b).  
The main clinostat computer (Raspberry RPi4) controls the motors, light intensity of diodes, and water pump flow. The second computer (Raspberry Pi Zero W) was mounted on the sphere to collect signals from sensors and cameras. The general view of clinostat signal flow is given in Figure 6. Details about computer (cultivation, controller) signals are seen in Figure 7. Details about the relay block and the opto-isolation block are explained in Figures 8 and 9, respectively. The control of a pump by the RPi4 computer met some electric incompatibility. The current that can be taken from the RPi4 IO pins is too low for powering devices, so a solution with a BC547 transistor was applied to pass enough amount of energy. Similarly, the lighting control circuit uses the IRF540N MOSFET to control the LED's lighting system.  
Figure 5: The LEDs mounted in the upper semi-sphere (**a**) and the simulated light-intensity spatial distribution at the cultivation plane showing uniform illumination (**b**).  
Figure 6: Blocks diagram of the signal flow for the clinostat system. The server is an external desktop computer.  
Figure 8: Details of current supply of the pump where transistor base (Q1, BC547) is controlled by the pump IO signal, and where the electric relay K1 is applied.  
Figure 7: Electric diagram of connections and control signals (**a**) for the cultivation sphere computer, Raspberry Pi Zero W, and (**b**) for the main clinostat computer, Raspberry RPi4 (on the right).