Saturday, August 15, 2015

Arduinos embedded within Homebrew Test Equipment ~ N6QW

More uses for the Arduino -- Test Equipment

There is always something on the bench! While I await some critical antenna parts still to come and a few parts I need for the two SSB transceivers, I thought I would turn my attention back to some investigations with the Arduino Nano coupled with TFT Color Displays.
I am amazed at how many sophisticated homebrew measuring devices  (many fitting in Altoids tins) are showing up in YouTube videos and on websites. Among these are Scalar Network Analyzers, Digital SWR Bridges, Audio Test Equipment and on and on.
In some of my own efforts I figured out how to add a moving bar S Meter to the bottom of a color display so that up until S9 the bar was one color and beyond S9 another color. Thinking that was pretty cool my attention turned to how would you add real time plots to a color display such as you might have in checking out a homebrew crystal filter (I know this is what a SNA does). But I wanted to see what makes things tick.
So my first thought was how to display a Sine Wave. We have two variables X and Y, where we can represent that y = sin(x). Fortunately the Arduino has built in math functions and so it is a simple matter to set a range of values for X and then have the Arduino calculate the Y values. Along the way we can add some axis' (ordinate and abscissa) and plot centerline.
As usual I thought all you needed to do was to set up the Arduino for the color display and write a couple of math equations and you are there. WRONG!!!!!! It took me nearly two days to figure out how to display a simple sine wave. You can see that below.
The problems I ran into was that I specified 320 points (width of the display)  for x starting at point x =14 so that it would fit within the axis. Well that netted multiple traces. By trial and error I found that if you only used  "x" at every 5 points that you got the plot below. Anything more or less gave a screwed up plot --Inexplicable is a really good word. Since the top of the display is considered y = 0 you must offset every value by + 120 (1/2 of 240 which is the number of vertical blocks) to place the plot in the middle of the screen. So then I thought this plot consists of rectangle data points, what does it take to "connect the dots" with lines. That never did work BUT I was able to create a plot similar to what you might see in a "two tone audio test" which is shown in the second photo. Below that is the sketch wherein I used Arduino 1.0.5 -- I do have 1.6.3 on a different machine.
So email me any thoughts or suggestions. If you know what you are doing, it is probably a pretty simple thing to homebrew your own test equipment --unfortunately I don't know what I am doing!!!!
Pete N6QW
This is the code I am using (it is OK to laugh).
/* From N6QW using the 240 X 320 TFT Color display
This is to test displating graphical information such as you would have with test instruments
#include <SPI.h>
#include <Adafruit_GFX.h>
#include <TFT_ILI9340.h>
#if defined(__SAM3X8E__)
#undef __FlashStringHelper::F(string_literal)
#define F(string_literal) string_literal
#define __CS 10
#define __DC 9
#define __RST 8
// Color definitions
#define BLACK   0x0000
#define BLUE    0x001F
#define RED     0xF800
#define GREEN   0x07E0
#define CYAN    0x07FF
#define MAGENTA 0xF81F
#define YELLOW  0xFFE0 
#define WHITE   0xFFFF
TFT_ILI9340 tft = TFT_ILI9340(__CS, __DC, __RST);
#include "Wire.h"
int val = 0;
int val1 = 0;
int x = 0;
int y = 0;
int old_val = 0;
int old_val1 = 0;
void setup() {
  #if defined(__MK20DX128__) || defined(__MK20DX256__)
        tft.fillScreen(0x07F0); // testing switching background colors
        tft.fillScreen(WHITE); // 0x07F0 = lt green, 0xF810 = rose, 0xF840 = rust, 0xF820 = orange
        tft.setRotation(1); // landsacape versus portrait
        tft.drawLine(14,0,14,230, BLACK); //X axiz
        tft.drawLine (14,230, 300, 230, BLACK); // Y axis
        tft.drawRect( 14, 118, 300, 2, GREEN); //centerline of plot & the 2 is the width of the rectangle --makes it look like a FAT line
void loop()
 for(int x = 5; x <320; x++){ //sets the range of values to plot along the X axis
 double sine = sin(x);
   x = x + 5 ;
   y =50*sin(x);
   old_val = x ; // for connecting lines to the dots at this point doesn't work
   old_val1 = y  ; //for connecting lines to the dots at this point doesn't work
   //tft.drawRect(x,y +120,  5, 5, MAGENTA); // the y+120 puts the y values in the center of the screen null this out for two tone test pattern
 tft.drawLine( x, y + 120,old_val , 120 -old_val1, BLACK); // two tone test null this out for sine wave