Considering that the suitable environments of the three fermentation systems are different, we try to design an automated fermentation device to assist our flora to stabilize in a suitable environment, and to detect the fermentation state and make autonomous adjustments, so that the system can be more stable.

The device we envisage is composed of three different tanks, which maintain three bacterial systems of cyanobacteria, nitrogen fixation, and Escherichia coli respectively. The colonies are immobilized, and the nutrient solution circulates among the three systems. The numerical control pump controls the exchange of substances between the flora and the inside and outside, and combines with the temperature control device to create a suitable fermentation condition.

Such a system needs a control center, for this, we have made a circuit board (PCB), which can detect and automatically control the temperature and flow, and can transmit the data to a personal computer.

Our circuit is extended and designed on the basis of Arduino, because it is an open source platform with good compatibility, which can be used more conveniently, and can also write corresponding programs according to needs. In order to better meet our needs, it is transformed as follows:

1. Use the analog-digital input port of the development board to expand the interface of several resistive sensors.
2. Use the digital output port of the development board to expand the management capability of large current, so as to control the thermal conductivity of the Peltier to flexibly regulate the temperature.
3. Use the digital output port (PWM wave) of the development board to achieve precise control of the pump to control the flow.

Circuit board we made

After attaching Arduino-micro: The board has four ports for connecting resistive sensors, two for adjusting the peristaltic pump, one for adjusting the Peltier, and micro-USB for transferring data to a computer

We hope to monitor the state of the fermentation broth in real time during the fermentation process, so sensor interfaces are reserved on the circuit board to receive data.

Through the voltage regulator chip, we get a standard 5V voltage, and then connect the sensor in series with the custom resistor, collect the voltage between the two, and calculate the resistance value of the sensor. We set the value of the fixed-value resistor to 10KΩ, which can fit most sensors. We can apply it to the monitoring of the physicochemical properties of fermentation broth such as temperature, ammonium concentration, pH, dissolved oxygen, etc., and obtain data to enable the system to make decisions.

As an example, here is how we implement temperature detection:

We plug the temperature monitoring semiconductor into the interface to form a parallel measurement circuit. We use NTC100K, which is a temperature sensor which is a heat sensitive semiconductor whose impedance decreases with increasing temperature and follow the rules:

(For NTC100K: `T_0 = 25^{\circ}c, R_0 = 10K \Omega`, `B = 3950K` )

Monitor the voltage of the port and calculate the resistance value of the NTC100K to calculate the current temperature.

NTC100K, a temperature detector

For biological safety reasons, we need to seal the bacterial liquid in a controlled system, so we choose the form of a peristaltic pump to control the in and out of gas and liquid. This structure is universal for different liquids and gases.

The liquids and gases remain inside the pipeline the entire time, and do not come into contact with other mechanical components.

In addition, by changing the speed and direction of the motor, we can flexibly control the smart direction and flow.

PWM speed regulation: In order to control the speed of the motor, the controller outputs a square wave to "simulate" different voltages, where the duty cycle determines its equivalent voltage.

The flow rate of the peristaltic pump under the condition of 12V full load is 50ml/min, so after adjusting the duty cycle, the delivery volume rate is:

`Q = \frac{1-\alpha}{6} \ ml\text{/}s`, where `\alpha` is duty cycle.

We choose semiconductor refrigeration sheet for cooling, because of its small size, no mechanical refrigeration, can be realized and heated by the same unit, and can be controlled by flow.

Consists of two parts: Heat transfer: `Q=PIt` , where `P` is the Peltier coefficient, `I` is the current, and `t` is the time. Joule heat: `Q=I^2Rt`

We built a sandwich heat exchanger with a fin in the middle, a liquid heat exchanger and an air fin on either side. By changing the magnitude and direction of the current, the temperature of the liquid can be controlled to increase or decrease.

We want the controller to be an open, interactive platform that can change the program as needed. It can be controlled automatically according to a program that has been written, or it can feed data back to a personal computer.

As shown in the figure is a real-time temperature monitoring feedback:

x-axis: time(S); y-axis: Temp(Celsius)