AmateurRadio.com

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; 
} 

66′ Dipole ready for the attic!

Over the 3 day weekend I finally squeezed in some time to get my 66 foot ladder fed dipole up in the attic.  One leg of the antenna is basically straight, but the other leg had to do a bit of zig-zagging through the trusses – it gets a bit crazy up there!

The ladder line drops down into my garage and then down to the basement where my shack is located.

Last night I built the BL2 1:1 or 4:1 balun from Elecraft.  The ladder line terminates at the balun and a 3′ piece of coax goes from the balun to my KX3.

Wrapped this all up last night about midnight – and then started doing some testing.  The internal tuner on the KX3 tunes ALL bands 40-10 meters almost down to 1:1 SWR.  Very nice.

I tuned around 40 meters and only heard a couple stations that time of night.  The noise level last night was between S4 and S5 – not sure if this is the normal noise level or not, but I suspect it is.

I did a bit of testing using the RBN – I called CQ on 40 meters for a few minutes and ended up with spots North, South and East of Kansas – good sign.  Then I switched antennas back to my 9:1 UNUN 30′ wire in the attic and did the same thing – so reports.  So good news is that my performance on 40 meters is GREATLY improved.  The reports on my 66′ Dipole were from 10-18 DB SNR on the RBN – all with 5 watts out of the KX3.

At 1:00 am local time I heard  K0GPA calling CQ – he was 559 here.  So I threw out my call and he came back to me with a 559 as well.  Turns out he was running a KX3 also – he was at 10 watts, and I was at 5 watts.  The QSB got him a bit, but I think he was using some type of loop – just missed what type.  It was a nice QSO with Bob – and it proved I was getting out!

So now I am looking forward to putting it through the paces a bit more and see just how much my reception will improve with more wire in the air!  I hope to at least have time to look at the waterfall tonight on 20 meters PSK31 to see what it looks like compared to my old antenna.

I will keep you posted!

My lunch time QRP session was the pits.  I didn’t work anyone and I kept getting jumpy SWRs.  The KX3 kept wanting to re-tune and I couldn’t get a decent SWR on 15 Meters, which had never been a problem until today.  Last night, I re-soldered the connections to my magmount as the shield connection snapped.  I brought the base in last night, cleaned everything up and re-soldered the wires to new connectors.  When I reassembled the pieces, I thought I had screwed everything down tight.  But when I took the Buddistick apart today at the end of my lunch time session, the threaded stud part of the magmount, that the radiator screws into, was turning pretty freely. Obviously, I didn’t tighten things as well as I had thought. I took care of THAT this evening with the help of two good Craftsmen Hex Grip wrenches.  Tomorrow should be better. Even if I don’t end up working anyone, the KX3 should be a happier camper than it was today.

Tonight however, was a different story. I worked W1AW/7 in WY and W1AW/0 in MO on 30 Meters, back to back. Then I hopped on over to 15 Meters just in time for the grayline to give me a hand.  Although I used QRO power (75 Watts), I was able to snare two good ones – E51KJW, South Cook Island and FW5JJ, Wallis and Futuna Islands.  I had to work FW5JJ twice, as the first time he had me in his log as “W2LO”. Big time dilemma. At that point, what do you do?  I knew he had me wrong, but I didn’t want to call again right away, so I went and worked E51KJW who got my call right the first time. After working Nakayama, I thought I’d give FW5JJ a second shot. I was worried that I would get the dreaded “WRKD B4”, but this time he heard me correctly and I clearly heard “W2LJ” this time. Two new DXCC entities in the log – Booyah!

Going to shut the rig down now and head upstairs to watch “The World Wars” series that is running on The History Channel.  I’ve been watching it since Monday night. So far it’s been a very good documentary assessing how World War II was really just a continuation of World War I. And although I always considered myself pretty familar with the histories of both wars, there have been some new tidbits of information that were new to me, which is always fun.

72 de Larry W2LJ
QRP – When you care to send the very least!

In a couple earlier posts, I wrote about making a homebrew Buddistick using Budd Drummand’s (W3FF) plans. His plans can be found on his page located here: https://sites.google.com/site/w3ffhomepage/.

I knew when I started building my station that I wanted a couple of portable antennas. I like making things, and decided to give his antennas a shot after deciding that I might be able to actually make an antenna that worked following Budd’s directions. I made the Buddistick first, then later made the Buddipole. Both are easy to make and work well. I use them both (not at the same time) for my main antenna right now, until I can get a full size dipole hung up, and coax run to the house etc. I am fairly new to the HF world and I don’t have my home station complete yet, so these antenna were a quick and easy way for me to get started on HF.

Pictured first are the components of the Buddipole. Starting at the top-left are the two 20 meter coils. To the right, the shorter coil set is for 15 and 17 meters. To their right is the “T” component for mounting the antenna on a painters pole. The black adapter changes the threads from PVC to the painters pole. In the middle are the two antenna whips, mounted in some CPVC with speaker wire coming out of the ends allowing for connections to be made to the whips. The bottom two pieces are CPVC arms with speaker wire running through them, and connectors.

The following picture shows the Buddipole set up with the painters pole extended. The length of the extended painters pole in this picture is about 10 ft.

The plans that Budd published has the coax line ending with the spade connectors. I bought a piece of 50ft coax with PL-259’s installed already on both ends and I did not want to cut one of them off, so I made the following adapter. It is simply a small project box from Radio Shack, with a SO-239 mounted in it and speaker wires connected to the SO-239 center and ground. The orange line is old fly fishing line for hanging the box from the antenna. I use a velcro strip for this.

Here is a table that I made showing how I setup the antenna and the resulting SWR measurements. I think the higher SWR on 20 and 6 meters is due to the proximity of the metal eve troughs that the antenna is near with extended. If I orient the antenna parallel to the gutter, the SWR goes up. My house sits approx SW-NE so pointing the red end of the antenna North puts it at about 45 degrees to the gutter and the SWR goes down. This is the most convenient place to set up the antenna within reach of the radio inside allowing for the 50ft run of coax. The whip column is how many additional sections of the whip I need to extend not counting the 1 section length it is with all the sections pushed in. I think if I got this away from the house a few more feet, the SWR would come down on 20 & 6m.

I have made SSB and PSK31 contacts on 20, 17, & 15 meters. I have not made contacts yet on 10 or 6. Someday I’ll do that.

Below are the graphs from my antenna analyzer showing the antenna setup for each band in the table above.

20 Meter

17 Meter

15 Meter

6 Meter

Overall I am happy with the antenna. I don’t expect it to work like a full size dipole, but for the ease of setup and take down, it’s working for me. I have talked to South America, Europe and all over the US with both  the home-brew Buddistick and Buddipoles.

My email address is on my QRZ.com profile. If anyone has questions, I’ll be glad to respond to any email.

Who’s up for some AmateurLogic.TV streaming Thursday evening. Tommy and George will be putting the Dayton Episode together. Begins around 6PM CDT, 2300 UTC.
The show won’t be released until next month, but you can get an advanced viewing as we put it together.
The stream is live now. Watch it here live:

Chat room is at www.amateurlogic.tv/chat

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());
}
}