AmateurRadio.com

Nice to see that SatNOGS won the hackaday prize this evening. A little sad that PortableSDR didn’t win as well. They both prove that Ham Radio is alive and kicking and has a very well rooted place in the 21st century……As if it was ever in doubt

Pinched from hackaday.com

I remember reading something about this on the Southgate ARC news a while ago. When I tried to find it I couldn’t. Thanks to Hackaday.io I found it again.

So what is it. The website has some big ideas on it but, to me it is a homebrew, simple Az El rotator using open source software and 3D printed parts. Something that, funding willing, I will be able to do over the winter. Info on availability seems a bit scarce but I’ve emailed regarding PCB’s.

Here’s a few links and a video

Hackaday.io Project page

Website

If you are like me, you appreciate electronic gadgets with dials and displays. So when I discovered this “USB detector”,  I thought to myself that I really always wanted to know the voltage as well as the current consumption of my USB devices. And since it is more or less impossible to connect a multimeter, this is exactly what I need.

The device fully satisfied my curiosity. Actually one surprising result was that the charger for my Samsung Galaxy Note 8 has a negative output resistance.

With a load it outputs 5.27 Volts as shown in the top image. Usually one expects the voltage to increase when the load is removed. But for this charger the voltage dropped instead to 5.13 Volts (second image). That should mean that there is the equivalent of a series resistance of (5.13-5.27)/0.98 which is about -0.14 Ohm.

I measured other chargers also, without finding a negative output resistance, so it seems as if it applies to this particular charger only. Out of curiosity, I also measured the current consumption of my Arduino Mega to 0.08 A without any shields connects.

The unit has two outputs which are different from each other. Output 1 is a fully functional USB port, while output 2 only connects DC power. What is that good for? Well, the epanoroma blog opened my eyes to the utility of this. If you charge your phone at some public place, then this feature isolates the data port of your phone. That may protect you from being hacked.

So there you see. The €5.13 were well spent and I even learnt something new by giving in to the temptation to click “Buy It Now” on Ebay.

But why does the Samsung charger have what amounts to a negative output resistance, is it by design or by accident?

Ever since the dawn of time coding has been a foreign language to me. I pick up bits and pieces but largely it gets forgotten or lost.

Tonights issue is about interrupts and debouncing buttons. What better thing to do whilst listening to the 6m white noise contest (also known as vhf from my qth).

The idea is simple. I made a shack clock from a gps and an arduino. I made it tell the time, tell me the position I’m in (no I don’t mean like that) an calculate the locator square. Now then all I want to do is link these together with a simple button press. Press the button and it changes from one function to the next.

Holy arduino, this isn’t straightforward at all. Buttons need debouncing and interrupts don’t like this or that. I feel a long development time in my future…still there’s not much on 6m and 2 contacts in the first hour isn’t going to win me any awards.

161.3 cm 27.0°  347.7 m/s

Ultrasonic distance sensors can find the range out to 2-4 meters and are popular in e.g. robotics. Here I look at how the accuracy can be improved by compensating for the variation of speed of sound with temperature. It actually varies quite a lot in air and around 0 C it is:

      c = 331.3 + 0.606 * T

where c is in m/s and T is in C. The formula is good to up to at least +/-30 C. There is also a dependence of humidity, but as it is so small it is neglected here.

The equation can be analyzed for sensitivity (a little bit of differentation, you know). The result is that a two-way range measurement creates an error of 1.8 mm/C/m. That means that with a 4 degree C error, the deviation will be 14.4 mm at a range of 2 meters. Not a lot, but more than the wavelength which is 9 mm at 40 kHz. Considering how easy it is to compensate for, then why not give it a try?

I have tested this with the inexpensive HC-SR04, but the compensation is really independent of the type of sensor. First let me analyze a typical example code for this sensor:

• For range in cm the code divides elapsed time in microseconds by 29. That corresponds to c=10,000/29=344.8 m/s. According to the equation above, this is the speed for a temperature of 22.3 C.
• For range in inches it divides elapsed time by 74. That corresponds to c=1,000,000/74 =13513.5 inches/sec =1126 feet/sec or 343.2 m/sec. This boils down to an assumed temperature of 19.6 C. That’s 2.7 degrees lower than the calculation for cm and means that the measurement in inches is lower than that in cm by 2.7*1.8 = 4.9 mm per meter of range. 

Temperature is 27.0°, but the program doesn’t use that value and assumes instead its standard value of speed of sound of 344.8 m/s and finds 160.0 cm.

I integrated an HC-SR04 program from Tautvidas Sipavičius with a code from the Arduino playground that uses the DS1820 1-wire temperature sensor (the last program on that page). In the process I had to convert the range estimation from integer to floating point arithmetic. This may give a speed penalty, but at least it runs fine on my Arduino Mega 2560.

The first image in this post shows that at a temperature of 27 C, the speed of sound is 347.7 m/s and the distance from my desk to the ceiling is found to be 161.3 cm. In the picture to the right, I have disabled the temperature compensation so the default velocity of 344.8 m/s is used instead and the estimated distance falls to 160.0 cm.

In the bottom picture, I have detached the 1-wire bus to the temperature sensor, so the program believes it is 0 C, and finds that the range drops to 154.5 cm.

154.5 cm  0.0°  331.3 m/s

Now, the HC-SR04 isn’t the most advanced of sensors. Other sensors may have a more accurate detection circuit that works more reliably and with better repeatability at longer ranges. I should also say that since I admitted a digit behind the decimal point in my code that wasn’t there in the original, my measurements wandered a bit from ping to ping also. In reality, I don’t think the HC-SR04 has much more accuracy than 1 cm. But it may have some potential for improvement as e.g shown here: “Making a better HC-SR04 Echo Locator“.

Combining a better detector with compensation for speed of sound variations with temperature should be what it takes to get the ultimate range sensor.

The Arduino sketch measures the range every 0.1 second and pauses for a second every 10 seconds to measure the temperature. The code is here (formatted with Hilite Me):

/*
 Ultrasound distance measurement with compensation for temperature
 Ultrasound sensor : HC-SR04
 Temperature sensor: DS1820
 LCD:                2 x 16 lines
 Created by merging code from
     http://www.tautvidas.com/blog/2012/08/distance-sensing-with-ultrasonic-sensor-and-arduino/
 and http://playground.arduino.cc/Learning/OneWire (Last program on this page)
 
 Sverre Holm, 30 May 2014
 la3za (a) nrrl.no
*/

#include <Wire.h>
#include <OneWire.h>
#include <LiquidCrystal.h>
LiquidCrystal lcd(8, 9, 4, 5, 6, 7); 
#define LCD_WIDTH 16
#define LCD_HEIGHT 2

#define FIXEDSPEED 0 // turn off temp compensation if == 1

/* Ultrasound sensor */
int pingPin = 12; 
int inPin = 13; 

/* DS18S20 Temperature chip i/o */

OneWire ds(24); // (4.7K to Vcc is required)

#define MAX_DS1820_SENSORS 1
byte addr[MAX_DS1820_SENSORS][8];

int RepeatTemp = 100; // Temp measurement is done every 100*0.1 sec = 10 sec



void setup() { 
  
  lcd.begin(LCD_WIDTH, LCD_HEIGHT); 
  lcd.print("init ..."); 
  
  //delay(1000);

   if (!ds.search(addr[0])) 
   {
     lcd.setCursor(0,0);
     lcd.print("No more addr");
     ds.reset_search();
     delay(250);
     return;
   }
   if ( !ds.search(addr[1])) 
   {
     lcd.setCursor(0,0);
     lcd.print("No more addr");
     ds.reset_search();
     delay(250);
     return;
   }
   
} 

int HighByte, LowByte, TReading, SignBit, Tc_100, Whole, Fract;
char buf[20];

int cntr = RepeatTemp;

void loop() {
  
  // *** Part 1: Measure temperature ***
  //
  byte i, sensor;
  byte present = 0;
  byte data[12];
  
  if (cntr == RepeatTemp)
  {
    for ( sensor=0; sensor<MAX_DS1820_SENSORS; sensor++ )
     {
       if ( OneWire::crc8( addr[sensor], 7) != addr[sensor][7]) 
       {
         lcd.setCursor(0,0);
         lcd.print("CRC not valid");
         return;
       }
  
       if ( addr[sensor][0] != 0x10) 
       {
         lcd.setCursor(0,0);
         lcd.print("Not DS18S20 dev ");
         return;
       }
  
       ds.reset();
       ds.select(addr[sensor]);
       ds.write(0x44,1);         // start conversion, with parasite power on at the end
  
       delay(1000);     // maybe 750ms is enough, maybe not
       // we might do a ds.depower() here, but the reset will take care of it.
  
       present = ds.reset();
       ds.select(addr[sensor]);    
       ds.write(0xBE);         // Read Scratchpad
  
       for ( i = 0; i < 9; i++) 
       {           // we need 9 bytes
         data[i] = ds.read();
       }
  
       LowByte = data[0];
       HighByte = data[1];
       TReading = (HighByte << 8) + LowByte;
       SignBit = TReading & 0x8000;  // test most sig bit -- only for C, not F
       if (SignBit) // negative
       {
         TReading = (TReading ^ 0xffff) + 1; // 2's comp
       }
       Tc_100 = (TReading*100/2);      
  
       Whole = Tc_100 / 100;  // separate off the whole and fractional portions
       Fract = Tc_100 % 100;
       
         if (MAX_DS1820_SENSORS == 1)
         {
            sprintf(buf, "%c%d.%d\337 ",SignBit ? '-' : ' ', Whole, Fract < 10 ? 0 : Fract);
         }
        else
         { 
            sprintf(buf, "%d:%c%d.%d\337C     ",sensor,SignBit ? '-' : '+', Whole, Fract < 10 ? 0 : Fract);
         }   
   
       lcd.setCursor(0,1); //sensor%LCD_HEIGHT);
       lcd.print(buf);     
     }
     cntr = 0;
  }
  //
  // *** Part 2: Measure distance ***
  //
 // establish variables for duration of the ping, 
 // and the distance result in centimeters:
 long duration;
 float cm;
 float c; // speed of sound
 // The sensor is triggered by a HIGH pulse of 10 or more microseconds.
 // Give a short LOW pulse beforehand to ensure a clean HIGH pulse:
 pinMode(pingPin, OUTPUT); 
 digitalWrite(pingPin, LOW); 
 delayMicroseconds(2); 
 digitalWrite(pingPin, HIGH); 
 delayMicroseconds(10); 
 digitalWrite(pingPin, LOW); 
 // Read the signal from the sensor: a HIGH pulse whose
 // duration is the time (in microseconds) from the sending
 // of the ping to the reception of its echo off of an object.
 pinMode(inPin, INPUT); 
 duration = pulseIn(inPin, HIGH); 
 //
 // estimate speed of sound from temperature:
 //
 if (FIXEDSPEED == 1)
 {
   c = 10000.0/29; // Original value for c in code
 }
 else
 {
  c = 331.3 + 0.606*Tc_100/100;
 }
  
 cm = microsecondsToCentimeters(duration, c); 
 
 lcd.setCursor(0, 0);
 for (i = 0; i < LCD_WIDTH; i = i + 1) {
    lcd.print(" ");
  }
 lcd.setCursor(1, 0); 
 lcd.print(cm,1); lcd.print(" cm");  
 
 lcd.setCursor(8, 1);
 lcd.print(c,1); lcd.print("m/s");
 delay(100); // measure range every 0.1 seconds
 cntr++;
 
 } 

float microsecondsToCentimeters(long microseconds, float c) { 
  // The speed of sound is 340 m/s or 29 microseconds per centimeter.
  // -- actually 29 microsec/cm = 10000/29 = 344.8 m/s, ie 22.3 deg C
  // The ping travels out and back, so to find the distance of the
  // object we take half of the distance travelled.
  return microseconds * c / 20000; 
} 

I’ve always enjoyed playing about with time. Accurate time is not really a fascination but I do like a clock to tell the time. The MSF 60khz time signal is one source and I have played about with that system with an Arduino and it works well, when there is a good signal for a whole minute. GPS time has always been a bit of a thing for me because you can set it to UTC and it’ll always show UTC and frankly there are a lot more libraries available to play with. GPS Tiny & GPS Tiny+ are two of those and this evening I ‘forked’ a sketch to use a cheapo off the shelf gps module to tell the time and date on a 16×2 LCD. Nothing spectacular but hey if I can do it then so can anyone. Here’s a short sweet video of it in action (near the window!)

sketch goes a little like this

#include <TinyGPS++.h>
#include <SoftwareSerial.h>
#include <LiquidCrystal.h>
/*
This sample sketch demonstrates the normal use of a TinyGPS++ (TinyGPSPlus) object.
It requires the use of SoftwareSerial, and assumes that you have a
4800-baud serial GPS device hooked up on pins 8(rx) and 9(tx).
*/
static const int RXPin = 8, TXPin = 9;
static const uint32_t GPSBaud = 9600;
// The TinyGPS++ object
TinyGPSPlus gps;
// The serial connection to the GPS device
SoftwareSerial ss(RXPin, TXPin);
// initialize the library with the numbers of the interface pins
LiquidCrystal lcd(12, 11, 5, 4, 3, 2);
void setup()
{
ss.begin(GPSBaud);
lcd.begin(16,2);
lcd.setCursor(1,0);
lcd.print("Tiny GPS+ Time");
delay(3000);
lcd.setCursor(1,2);
lcd.print("by Alex, g7kse");
delay(5000);
lcd.clear();
}
void loop()
{
// This sketch displays information every time a new sentence is correctly encoded.
while (ss.available() > 0)
if (gps.encode(ss.read()))
displayInfo();
if (millis() > 5000 && gps.charsProcessed() < 10)
{
lcd.print("No GPS detected");
for (int positionCounter = 0; positionCounter < 20; positionCounter++) {
while(true);
}
}
}
void displayInfo()
{
lcd.setCursor(4,0);
{
if (gps.time.hour() < 10) lcd.print(F("0"));
lcd.print(gps.time.hour());
lcd.print(F(":"));
if (gps.time.minute() < 10) lcd.print(F("0"));
lcd.print(gps.time.minute());
lcd.print(F(":"));
if (gps.time.second() < 10) lcd.print(F("0"));
lcd.print(gps.time.second());
}
lcd.setCursor(3,2);
{
if (gps.date.day() < 10) lcd.print(F("0"));
lcd.print(gps.date.day());
lcd.print(F("/"));
if (gps.date.month() < 10) lcd.print(F("0"));
lcd.print(gps.date.month());
lcd.print(F("/"));
lcd.print(gps.date.year());
}
}