This Arduino project constantly measures the temperature, records the median of recent measurements at a defined sampling period, takes user input from two buttons to read and store data as well as change settings, and outputs to a 16-by-2 liquid crystal display (LCD), the serial monitor, two LEDs, and a piezo buzzer.
The temperature is measured approximately once per second by a TMP36 Temperature Sensor. A formula is used to convert the analog input from the sensor to a temperature in degrees Fahrenheit. This formula relies on the Arduino power supply voltage. Because the voltage can vary, especially if a discharging battery powers the Arduino, it is measured every 30 seconds.
Due to variations in the Arduino power supply and the precision of the analog input measurements, the measurements include noise that can cause each measurement to vary up to several degrees from the previous one. To reduce the effect of the noise, each measurement is stored as a float value in an array of the 30 most recent measurements. After each measurement, the median of the array (excluding any unfilled values) is calculated. The median is then printed to the serial monitor and the LCD, and LED 1 glows for about 0.2 seconds to indicate that a measurement has been taken.
In addition to an array of the 30 most recent measurements, the program stores 900 values in a history array as bytes stored in the static random access memory (SRAM). The most recently measured median temperature is added to this array at a defined sampling period, and LED 2 glows for one second. The sampling period is by default set to 4 minutes, allowing approximately 60 hours of samples before the history is full. Once the history array fills, LED 2 glows continuously to alert the user.
The main screen of the LCD shows the time elapsed since the program started and the most recently calculated median temperature. A brief press of the left button enters the settings menu, and a brief press of the right button enters the data view menu.
Anywhere in the settings menu, pressing and holding the left button for approximately one second will exit the settings menu and return to the main screen. Brief presses of the left and right buttons move between the screens of the settings menu.
The first screen shows the settings menu title as well as the current free SRAM available in the Arduino for debugging purposes.
Pressing the left button shows the Backlight Toggle option. Pressing and holding both buttons for approximately one second either disables or enables the LCD backlight.
The next screen to the left is the Sound Toggle option. When sound is enabled, the piezo buzzer plays a 3-kHz beep whenever a button is pressed or the holding of a button or buttons is complete.
The final screen is the LCD Reset option, which re-initializes the LCD. This is necessary for the LCD to work properly in some cases after a connection to the LCD is broken and reconnected.
Anywhere in the data view menu, pressing and holding the right button for approximately one second will exit the data view menu and return to the main screen. Brief presses of the left and right buttons move between the screens of the menu.
The initial screen shows the menu title and the sampling period in seconds.
Moving to the right of the initial screen shows the values stored in the history array. Pressing and holding the left button for approximately one second will jump to the last recorded sample in the array.
To the left of the initial screen are read/write actions which can be activated by pressing and holding the left and right buttons.
Read to SRAM prints to the serial monitor on the computer, if connected, the sample number and value, separated by a comma, of each sample stored in the history array.
When the Arduino loses power or resets, all data in the SRAM is lost. Fortunately, the Arduino also has electrically erasable programmable read-only memory (EEPROM), in which data stored is not lost on resets. Because the EEPROM has a limited number of write cycles, it would not be practical to record every sample in the EEPROM. However, for cases where it is necessary to reset the Arduino after recording samples but before transferring the data to the serial monitor (e.g., when the Arduino is being powered by a battery instead of a computer during data collection), the program includes options to write to and read from the EEPROM. Write to EEPROM saves the current history array to the EEPROM. Read from EEPROM prints to the serial monitor on the computer, if connected, the sample number and value, separated by a comma, of each sample stored in the EEPROM.
The read/write actions can take up to several seconds depending on how many values are involved. In order to avoid delays to other program actions such as temperature measurements, the program implements pseudo-parallelism by occasionally pausing long read/write actions, executing a cycle of the main loop, and then resuming any in-progress read/write actions.