Things used in this project
Hardware components:  
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Software apps and online services:  

Story
We have built a Scientific calculator having lots of functions like
 Basic algebraic operations (+, , *, %)
 Trigonometric functions (SINe, COSine, TANgent)
 Inverse trigonometric functions (arcSin, arcCos, arcTan)
 Logarithmic function
 Square root.
An arduino sketch is made to implement this. It calculates accurately upto six decimal places and display upto four decimal place.
Step 1: Components Required
The evive's colorful display makes it a bit ahead of other arduino calculators.
Components used :
 4X4 Keypad
 Pushbuttons of different colors (or different types to make it more user friendly)
 Breadboard
 Dupont (Jumper) Cables
 Breadboard Wires
 Resistors (for pulldown) (~1kOhm)
 Writeable tape
Step 2: Making the Simple Calculator
Firstly we will make a simple calculator that does simple operations like addition, subtraction, multiplication and division. We have used the A, B, C, D keys for the above operations respectively. For the 'equal to' operation we have used '#' key and for 'clear the screen' we have used '*' key in the keypad. In the attached image, connections for keypad are mentioned.
For the coding part we have taken help from the already build code at the arduino site
http://playground.arduino.cc/Main/KeypadCalculator...
Rest is just tweaking it to run by changing the pins etc. We have modified the code for 1.8" TFT (ST7735R).
But this was very simple!! Lets go ahead
Step 3: Reading Input Number with Decimal Point from Keypad
For both our input numbers the algorithm is basically the same.
Let the first number be a double named 'first'. We initialize it to be zero.
We make a Boolean 'isDecimal' and initialize it as false. This means that unless the decimal point is given as an input, the number is not a decimal.
Last, declare a float 'decimals' and set it as 10.0. We will use it for keeping a counter of our place after the decimal point.
Now if isDecimal is false, it means the number is not yet a decimal. Suppose you are storing your input number from the keypad as key.
You just need to update first=first*10+key.
But if isDecimal is true, the number is a decimal. You need to now update as
first=first+key/decimals, and decimals=decimals*10.
We keep repeating the above steps until the input for some operation is detected. Then we similarly detect the second number. Using the knowledge of the operation called, we operate the numbers and print the result when '=' is detected.
Remember to restate the values of decimals=10.0 and isDecimal=false after detection for a number is complete.
Step 4: Preparing the Breadboard Connections for the Other Functions
Now, we have to add more buttons for the scientific functionality. We have used some colorful buttons as shown in figure. Now assign them accordingly. Take care to assign similar buttons to similar functions.
Since evive uses Arduino Mega, we have used following pins:
 sine=Pin25
 cosine=Pin27
 tangent=Pin23
 log=Pin22
 arcsin=Pin24
 arccos=Pin26
 arctan=Pin14
 root=Pin15
Step 5: Printing the Calculation Log
We have printed the calculation log also. In the bottom of the screen you can print the calculation log by just remembering the first and the second numbers of the previous calculation.
Step 6: Building the Functions for other Operators
First try making the code for any one function (for example, sine function) using the inbuilt function 'sin(val)' of the arduino ide. Arduino ide also has preinstalled functions of cos, tan, inverse sin, inverse cos, inverse tan, log, square root. We have also added one more button for the decimal.
You can view the following image for reference for making different functions.
All the codes can be downloaded from GitHub. The code is well documented and self explanatory.
Step 7: Conclusion and Testing
The calculator works well!! We have also used evive's inbuilt buzzer to provide audio feedback in form of a beep while someone presses any button or switch. You can design the calculator which prints the results on the tft screen of the evive. Use different colors and sizes of text to decorate it. We can align the operants and the functions to make the appearance better.
Add more functions and dig harder to make it more complex :P
You can check more features and techSpecs of evive to make it more advanced calculator. Watch the video of evive here
Code
Github
Credits
evive team
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