A Science Experiment: Conductivity of Salt Solution with micro:bit

1.Introduction:

This project uses micro:bit and BOSON modules to build a salt solution conductivity experiment device. The metal probes of the soil humidity sensor are placed in pure water and salt solution respectively. The difference in voltage signals detected by the sensor reflects the difference in conductivity between the salt solution and pure water. This project also demonstrates two experimental methods: programmed and non-programmed. Please see below for details.

2.Materials：

Please see the end of the article for the hardware materials link.

3.Note：

It is recommended to use conical flasks of 50ml, as the mouth of this size flask can accommodate the probes of the soil humidity sensor.

4.Wiring:

Connect the hardware as shown in the diagram, install the batteries, and turn on the switches on the red power mainboard and the micro:bit expansion board. This diagram actually shows the final wiring of the two experimental methods. For convenience, we can prepare all experimental equipment here.

5.1Experiment (Non-Programming Version)：

（1）Set up the experimental device as shown in the figure, arrange two flasks, one filled with 50ml of pure water and the other with salt solution.

（2）Switch the module number: Press the button to switch to the module number i16 of the soil humidity sensor.

（3）Place the soil humidity sensor in pure water and observe the reading on the screen (about 200). This value represents the voltage signal. 200 means the voltage is very weak, almost nonexistent.

（4）Wipe the soil humidity sensor dry on a clean cotton pad, put it in salt solution, and observe the reading on the screen （about 600）. The voltage signal is much larger, indicating that salt solution is conductive. 

（5）Wipe the gold probe of the soil humidity sensor dry with a clean cotton pad.

Note: At this time, the highlighted part of the experimental device is in effect. We did not control through micro:bit programming, but only displayed the voltage signal value detected by the soil humidity sensor on the screen, thus completing the experiment.

In the non-programming version of the experiment, we distinguish the conductivity of salt solution and pure water by displaying values on the screen. You may feel that using values is not intuitive enough. In fact, we can also use programming to express the difference in conductivity between salt solution and pure water with patterns.

5.2Experiment (Programming Version)

（1）Connect the micro:bit to the computer

（2）Write a program with the following features:

          Mode A - Very weak conductivity, displaying a small heart; strong conductivity, displaying a large heart.

          Mode B - Displays real-time voltage values.

Please see the end of the article for the program files.

（3）Experimental steps:

          a. Press the button A on the micro:bit to enter pattern mode. Put the soil humidity sensor in pure water. The dot matrix screen displays     , indicating a weak voltage signal, which means that the conductivity of pure water is weak.

          b. Wipe the soil humidity sensor dries on a cotton pad, put it in salt solution, and the dot matrix screen displays     , indicating a strong voltage signal, which means that the conductivity of pure water is strong.

          c. Press the button B to enter data mode, and the dot matrix screen scrolls to display real-time data. (The reading at this time may be different from the screen, because when the micro:bit needs to display 3 digits at the same time, it will scroll to display, scrolling takes time, and the value update is slower, so it seems to be out of sync with the screen.）

 （4）Clean the metal probe of the sensor and place the sensor in the flask of pure water.

Note: In the programmed experiment, the detection value of the soil humidity sensor is input to the micro:bit through the branch module, and also input to the display screen module.

6.Outlook 

With micro:bit, we can also export experimental data for data analysis and explore scientific laws, such as the relationship between different salt solution concentrations and conductivity.

HARDWARE LIST

Sample Code