Arduino simple calculator

0. Introduction.

• Courtesy of NES Academic Club, University of Science - Ho Chi Minh City, Vietnam.
• This is the "firmware" to turn Arduino into a basic calculator, with the abilities to evaluate basic expressions (sin, cos, tan, sqrt supported); evaluate approximately value of single variable integral (using Riemann middle sum); and solve simple equations (quadratic equation, two-variable linear system of equations).
• Project created from scratch and originally maintained by hungngocphat01, with the exception of Keypad.h (courtesy of Mark Stanley, Alexander Brevig) and LiquidCrystal_I2C.h (courtesy of Frank de Brabander).

Prototype of an Arduino running this "calculator firmware" at NES Academic Club (2020).

1. Wiring

Device	Device pin	Arduino pin
Keypad	1 - 8	5 - 12
I2C LCD	VCC	3.3V
	GND	GND
	SDA	A4
	SCL	A5
Shift button	1	GND
	2	A0

2. Source code structure

• Calculator.ino: literally main.cpp.
• ExprCalc.h: shunting-yard algorithm implementation.
• Stack.h: implementations of sstack (String stack) and fstack (float stack).
• Screen.h: prototype of LCD printing function.
• Input.h: functions to examine and process keystrokes.
• Modes.h: functions to handle supported modes of the calculator.
• tokenizer.py: creates an array of tokens, for debugging purpose only.

3. Program flow

• The void loop() function calls the void menuMode() function. This function acts as the main function and the program execution will always stay in this function. When the function is called, the user have to choose the operating mode of the calculator.
  • If the user chooses COMP mode, void compMode() will be called.
  • The same thing happens with EQN mode (void eqnMode()) and INTERGRAL mode (void intgrlMode()). More details about these mode are available in 5. Working modes.
  • If the user presses the MODE button in COMP, EQN or INTEGRAL mode, the function which handles the respective mode will return. The program then goes back to void menuMode(), waiting for another mode to be choosen.

4. Input

• 4x4 matrix keypad.

• Shift button (separate button).

• Layout:

  1	2	3	+
  4	5	6	-
  7	8	9	*
  .	0	=	/

• Layout when shift button is pressed:

  ^	sqrt	pi	sin
  (	)	e	cos
  LEFT	RIGHT	Ans	tan
  	DEL	=	MODE

5. Working modes

5.1. COMP mode

5.1.1. How does it work

• The calculator receives input from the 4x4 matrix keypad, parses the tokens immediately as soon as they are sent from the keypad, and categorizes them into numbers and operators.
• The user can press DEL (Shift + 0) key to pop the last operand/operator out of the expression, or = to start evaluating.
• Then, the original infix expression is converted into a postfix expression and is evaluated (using shunting-yard algorithm).
• Finally, the result is displayed on the LCD screen if there is no error.

5.1.2. Output

• Evaluation result if there is no error.
• Error message if there is least one error occured:
  • Math Error: divided by 0, or sqrt of negative number.
  • Syntax Error: for instance

    1 * + 5
    4.2 - ( * 7 )
    etc.
    

5.2. EQN mode

• Capable of solving quadratic equation (ax^2 + bx + c = 0, complex solutions are not supported) and 2-variable system of linear equations (ax + by = c).
• Works similarly to COMP Mode.
  • User is prompted to choose the equation type.
  • Coefficients are asked. User can also enter an equation to be calculated into a value which substitutes for a coefficient.
  • The calculator will solve the equation and the result will be displayed on the screen.

5.3. Integral mode

• Not enough memory to implement.

6. What's working

• Keypad, LCD screen.
• COMP mode.
• EQN mode.
• Mode changing.

7. What's not working/features being added in the future

• Screen scrolling.
• Integral mode.

8. Debugging

• Following information will be printed to serial monitor:
• Postfix expression.
• Evaluated result.
• New mode (when mode is changed).
• Coefficients and solutions (EQN mode).