print – Experimental Engineering

Posted on November 2, 2017November 2, 2017 by The Engineer — 9 Comments

Arduino Milliohm Meter Build

During the rebuild of the wheelchair motors for the support trolley, I found myself needing an accurate milliohm meter to test the armature windings with. Commercial instruments like these are expensive, but some Google searching found a milliohm meter project based around the Arduino from Circuit Cellar.

Here’s the original author’s circuit diagram, paralleling nearly all of the Arduino’s digital output pins together to source/sink the test current, an ADS1115 ADC to take more accurate readings, with the results displayed on a jellybean 128×64 OLED module. The most expensive part here is the 10Ω 0.1% 15ppm reference resistor, R9.
I decided to make some small adjustments to the power supply section of the project, to include a rechargeable lithium cell rather than a 9v PP3 battery. This required some small changes to the Arduino sketch, a DC-DC boost converter to supply 5v from the 3.7v of a lithium cell, a charger module for said cell, and with the battery voltage being within the input range of the analogue inputs, the voltage divider on A3 was removed. A new display icon was also added in to indicate when the battery is being charged, this uses another digital input pin for input voltage sensing.
I also made some basic changes to the way an unreadable resistance is displayed, showing “OL” instead of “—–“, and the meter sends the reading out over the I²C bus, for future expansion purposes. The address the data is directed to is set to 0x50.

I’ve not etched a PCB for this as I couldn’t be bothered with the messy etchant, so I built this on a matrix board instead.

Since I made some changes to both the software and the hardware components, I decided to prototype the changes on breadboard. The lithium cell is at the top of the image. with the charger module & DC-DC converter. The Arduino Nano is on the right, the ADC & reference resistor on the left, and the display at the bottom.
The Raspberry Pi & ESP8266 module are being used in this case to discharge the battery quicker to make sure the battery level calibration was correct, and to make sure the DC-DC converter would continue to function throughout the battery voltage range.

Here’s the final board with the passive components installed, along with the DC-DC converter. I used a Texas Instruments PTN04050 boost module for power as I had one spare.

The bottom of the board has most of the wire jumpers for the I²C bus, and power sensing.

Here’s both modules installed on the board. I used an Arduino Nano instead of the Arduino Pro Mini that the original used as these were the parts I had in stock. Routing the analogue pins is also easier on the Mini, as they’re brought out to pins in the DIP footprint, instead of requiring wire links to odd spots on the module. To secure the PCB into the case without having to drill any holes, I tapped the corner holes of the matrix board M2.5 & threaded cap head screws in. These are then spot glued to the bottom of the case to secure the finished board.

The lithium charger module is attached to the side of the enclosure, the third white wire is for input sensing – when the USB cable is plugged in a charge icon is shown on the OLED display.

The inputs on the side of the enclosure. I’ve used the same 6-pin round connector for the probes, power is applied to the Arduino when the probes are plugged in.

Everything installed in the enclosure – it’s a pretty tight fit especially with the lithium cell in place.

The top cover has the Measure button, and the OLED display panel, the latter secured to the case with M2.5 cap head screws.

Finally, the measurement loom, with Kelvin clips. These were an eBay buy, keeping things cheap. These clips seem to be fairly well built, even if the hinges are plastic. I doubt they’re actually gold-plated, more likely to be brass. I haven’t noticed any error introduced by these cheap clips so far.

The modified sketch is below:

// ---------------------------------------------------------------------------------------------
//  Simple, accurate milliohmeter
//
//  (c) Mark Driedger 2015
//
//  - Determines resistance using 4 wire measurement of voltage across a series connected
//  reference resistor (Rr, 10 ohm, 0.1%) and test resistor (Rx)
//  - range of accurate measurement is roughly 50 mohm to 10Kohm
//  - Uses Arduino digital I/O ports to deliver the test current, alternating polarity to cancel 
//  offset errors (synchronous detector)
//  - 4 I/O pins are used for each leg of the test current to increase test current
//  - Averages 2 cycles and 100 samples/cycle 
//  - Uses a 16 bit ADC ADS1115 with 16x PGA to improve accuracy
//
//  Version History
//    May 24/15    v1.0-v4.0
//      - initial development versions
//    May 27/15    v5.0
//      - changed display to I2C
//      - backed out low power module since it seemed to cause serial port upload problems
// ---------------------------------------------------------------------------------------------

#include <Wire.h>
#include <SPI.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>
//#include <LowPower.h>

#if (SSD1306_LCDHEIGHT != 64)
#error("Height incorrect, please fix Adafruit_SSD1306.h!");
#endif

// ---------------------------------------------------------------------------------------------
//  I/O port usage
// ---------------------------------------------------------------------------------------------
//    serial port (debug and s/w download)    0, 1
//    I²C interface to ADC & display          A4, A5
//    positive drive                          2, 3, 4, 5
//    push to test input                      8
//    unused                                  9, 10, 11, A0, A1, A2, A6, A7
//    negative drive                          6, 7, 8, 9
//    battery voltage monitor                 A3
//    debug output                            13

#define  P_PushToTest  10       // push button (measure), active low
#define  P_Debug       13
#define  CHG           12

//  ADS1115 mux and gain settings
#define  ADS1115_CH01  0x00    // p = AIN0, n = AIN1
#define  ADS1115_CH03  0x01    // ... etc
#define  ADS1115_CH13  0x02
#define  ADS1115_CH23  0x03
#define  ADS1115_CH0G  0x04    // p = AIN0, n = GND
#define  ADS1115_CH1G  0x05    // ... etc
#define  ADS1115_CH2G  0x06
#define  ADS1115_CH3G  0x07

#define  ADS1115_6p144  0x00   // +/- 6.144 V full scale
#define  ADS1115_4p096  0x01   // +/- 4.096 V full scale
#define  ADS1115_2p048  0x02   // +/- 2.048 V full scale
#define  ADS1115_1p024  0x03   // +/- 1.024 V full scale
#define  ADS1115_0p512  0x04   // +/- 0.512 V full scale
#define  ADS1115_0p256  0x05   // +/- 0.256 V full scale
#define  ADS1115_0p256B 0x06   // same as ADS1115_0p256
#define  ADS1115_0p256C 0x07   // same as ADS1115_0p256

Adafruit_SSD1306   display(0);               // using I2C interface, no reset pin
static int         debug_mode = 0;           // true in debug mode

float ADS1115read(byte channel, byte gain)
//--------------------------------------------------------------------------------------
//  reads a single sample from the ADS1115 ADC at a given mux (channel) and gain setting
//  - channel is 3 bit channel number/mux setting (one of ADS1115_CHxx)
//  - gain is 3 bit PGA gain setting (one of ADS1115_xpxxx)
//  - returns voltage in volts
//  - uses single shot mode, polling for conversion complete, default I2C address
//  - conversion takes approximatly 9.25 msec
//--------------------------------------------------------------------------------------
  {  
  const int    address = 0x48;      // ADS1115 I2C address, A0=0, A1=0 
  byte         hiByte, loByte;
  int          r;
  float        x;

  channel &= 0x07;                  // constrain to 3 bits
  gain    &= 0x07;
 
  hiByte = B10000001 | (channel<<4) | (gain<<1);    // conversion start command
  loByte = B10000011;
  
  Wire.beginTransmission(address);  // send conversion start command
  Wire.write(0x01);                 // address the config register
  Wire.write(hiByte);               // ...and send config register value
  Wire.write(loByte);           
  Wire.endTransmission();

   do                               // loop until conversion complete
    {
    Wire.requestFrom(address, 2);   // config register is still addressed
    while(Wire.available())
      {
      hiByte = Wire.read();         // ... and read config register
      loByte = Wire.read();
      }
    }
  while ((hiByte & 0x80)==0);       // upper bit (OS) is conversion complete

  Wire.beginTransmission(address); 
  Wire.write(0x00);                 // address the conversion register
  Wire.endTransmission();

  Wire.requestFrom(address, 2);     // ... and get 2 byte result
  while(Wire.available())
    {
    hiByte = Wire.read();
    loByte = Wire.read();
    }

  r = loByte | hiByte<<8;           // convert to 16 bit int
  switch(gain)                      // ... and now convert to volts
    {
      case ADS1115_6p144:  x = r * 6.144 / 32768.0; break;
      case ADS1115_4p096:  x = r * 4.096 / 32768.0; break;
      case ADS1115_2p048:  x = r * 2.048 / 32768.0; break;
      case ADS1115_1p024:  x = r * 1.024 / 32768.0; break;
      case ADS1115_0p512:  x = r * 0.512 / 32768.0; break;
      case ADS1115_0p256:  
      case ADS1115_0p256B:  
      case ADS1115_0p256C: x = r * 0.256 / 32768.0; break;
    }
  return x;
  }

// ---------------------------------------------------------------------------------------------
//  Drive functions
//   - ports 4-7 and A0-A3 are used to differentially drive resistor under test
//   - the ports are resistively summed to increase current capability
//   - DriveOff() disables the drive, setting the bits to input
//   - DriveOn()  enables the drive,  setting the bits to output
//   - DriveP()   enables drive with positive current flow (from ports 4-7 to ports A0-A3)
//   - DriveN()   enables drive with negative current flow
// ---------------------------------------------------------------------------------------------
void DriveP()
  {
    DriveOff();
    digitalWrite( 2, HIGH);
    digitalWrite( 3, HIGH);    
    digitalWrite( 4, HIGH);    
    digitalWrite( 5, HIGH);
    digitalWrite( 6, LOW);
    digitalWrite( 7, LOW);
    digitalWrite( 8, LOW);
    digitalWrite( 9, LOW);  
    DriveOn();
  }

void DriveN()
  {
    DriveOff();
    digitalWrite( 2, LOW);
    digitalWrite( 3, LOW);    
    digitalWrite( 4, LOW);    
    digitalWrite( 5, LOW);
    digitalWrite( 6, HIGH);
    digitalWrite( 7, HIGH);
    digitalWrite( 8, HIGH);
    digitalWrite( 9, HIGH);   
    DriveOn();
  }

void DriveOn()
  {
    pinMode( 2, OUTPUT);      // enable source/sink in pairs
    pinMode( 6, OUTPUT);
    pinMode( 3, OUTPUT);
    pinMode( 7, OUTPUT);
    pinMode( 4, OUTPUT);
    pinMode( 8, OUTPUT);
    pinMode( 5, OUTPUT);
    pinMode( 9, OUTPUT);
    delayMicroseconds(5000);  // 5ms delay
  }
    
void DriveOff()
  {
    pinMode( 2, INPUT);       // disable source/sink in pairs
    pinMode( 6, INPUT);
    pinMode( 3, INPUT);
    pinMode( 7, INPUT);
    pinMode( 4, INPUT);
    pinMode( 8, INPUT);
    pinMode( 5, INPUT);
    pinMode( 9, INPUT);
  }

int CalcPGA(float x)  
// ---------------------------------------------------------------------------------------------
//   Calculate optimum PGA setting based on a sample voltage, x, read at lowest PGA gain
//     - returns the highest PGA gain that allows x to be read with 10% headroom
// ---------------------------------------------------------------------------------------------
  {
    x = abs(x);
    if (x>3.680) return ADS1115_6p144;
    if (x>1.840) return ADS1115_4p096;
    if (x>0.920) return ADS1115_2p048;
    if (x>0.460) return ADS1115_1p024;
    if (x>0.230) return ADS1115_0p512;
    else         return ADS1115_0p256;
  }

void BatteryIcon(float charge)
// ---------------------------------------------------------------------------------------------
//   Draw a battery charge icon into the display buffer without refreshing the display
//     - charge ranges from 0.0 (empty) to 1.0 (full)
// ---------------------------------------------------------------------------------------------
  {
    static const unsigned char PROGMEM chg[] =     // Battery Charge Icon
    { 0x1c, 0x18, 0x38, 0x3c, 0x18, 0x10, 0x20, 0x00 };
    
    int w = constrain(charge, 0.0, 1.0)*16;  // 0 to 16 pixels wide depending on charge
    display.drawRect(100, 0, 16, 7, WHITE);  // outline
    display.drawRect(116, 2,  3, 3, WHITE);  // nib
    display.fillRect(100, 0,  w, 7, WHITE);  // charge indication

    //battery charging indication
    pinMode(CHG, INPUT);
    if (digitalRead(CHG) == HIGH)
      display.drawBitmap(91, 0, chg, 8, 8, WHITE);
  }

void f2str(float x, int N, char *c)
// ---------------------------------------------------------------------------------------------
//    Converts a floating point number x to a string c with N digits of precision
//     - *c must be a string array of length at least N+3 (N + '-', '.', '\0')
//     - x must be have than N leading digits (before decimal) or "#\0" is returned
// ---------------------------------------------------------------------------------------------
  {
  int     j, k, r;
  float   y;

  if (x<0.0)                    // handle negative numbers
    {
      *c++ = '-';
      x = -x;
    }
  for (j=0; x>=1.0; j++)        // j digits before decimal point
    x /= 10.0;                  // .. and scale x to be < 1.0

  if (j>N)                      // return error string if too many digits
    {
      *c++ = '#';
      *c++ = '\0';
      return;
    }

  y = pow(10, (float) N);       // round to N digits
  x = round(x * y) / y;
  if (x>1.0)                    // if 1st digit rounded up ...
    {
      x /= 10.0;                // then normalize back down 1 digit
      j++;
    }

  for (k=0; k<N; k++)
    {
      r = (int) (x*10.0);        // leading digit as int
      x = x*10-r;                // remove leading digit and shift 1 digit
      
      *c++ = r + '0';            // add leading digit to string
      if (k==j-1 && k!=N-1)      // add decimal point after j digits
        *c++ = '.';              // ... unless there are N digits before decimal
    }
  *c++ = '\0';
  }

void DisplayResistance(float x)
// ---------------------------------------------------------------------------------------------
//    Adds the resistance value, x, to the display buffer without refreshing the display
//      - converts to kohm, milliohm or microohm if necessary
// --------------------------------------------------------------------------------------------- 
  {
    static const unsigned char PROGMEM omega_bmp[] =     // omega (ohm) symbol
    { B00000011, B11000000,
      B00001100, B00110000,
      B00110000, B00001100,
      B01000000, B00000010,
      B01000000, B00000010,
      B10000000, B00000001,
      B10000000, B00000001,
      B10000000, B00000001,
      B10000000, B00000001,
      B10000000, B00000001,
      B01000000, B00000010,
      B01000000, B00000010,
      B01000000, B00000010,
      B00100000, B00000100,
      B00010000, B00001000,
      B11111000, B00011111 };

    char  s[8];
    char  prefix;
    
    if (x>=1000.0)          // display in killo ohms
      {
        x /= 1000.0;
        prefix = 'k';
      }
    else if (x<0.001)       // display in micro ohms
      {
        x *= 1000000.0;
        prefix = 0xe5;    // mu
      }
    else if (x<1.0)         // display in milli ohms
      {
        x *= 1000.0;
        prefix = 'm';
      }
    else
      prefix = ' ';         // display in ohms
  
    f2str(x, 5, s);
       
    // display computed resistance
    display.setTextSize(2);
    display.setTextColor(WHITE);
    display.setCursor(0,20);
    display.print(s);

    // display prefix
    display.setCursor(85,20);
    display.print(prefix);
    
    // display omega (ohms) symbol
    display.drawBitmap(103, 18, omega_bmp, 16, 16, WHITE);
  }

void DisplayDebug(int a, int b, float x, float y, float Vbat)
// ---------------------------------------------------------------------------------------------
//    Adds debug info to the display buffer without showing the updated display
//      - Adds 2 ints (a, b) and a float(Vbat) to the top line and 2 floats (x, y) 
//      to the bottom line+, all in small (size 1) text
// ---------------------------------------------------------------------------------------------
  {
    // display x, y in lower left, small font
    display.setTextSize(1);
    display.setCursor(0,45);
    display.print(x,3);
    display.print("  ");
    display.print(y,3);

    // display a, b in upper left, small font
    display.setTextSize(1);
    display.setCursor(0,0);
    display.print(a);
    display.print("  ");
    display.print(b);

    // display Vbat in upper middle, small font
    display.setTextSize(1);
    display.setCursor(60,0);
    display.print(Vbat,1);
  }

void DisplayStr(char *s)
// ---------------------------------------------------------------------------------------------
//    Adds a string, s, to the display buffer without refreshing the display @ (0,20)
// --------------------------------------------------------------------------------------------- 
  {
    display.setTextSize(2);              
    display.setTextColor(WHITE);
    display.setCursor(8,20);
    display.print(s);
  }

#ifdef TESTMODE
void loop()
  {
    while (digitalRead(P_PushToTest))
      ;
    DriveP();
    display.clearDisplay();
    DisplayStr("Drive: +");
    display.display();
    delay(250);

    while (digitalRead(P_PushToTest))
      ; 
    DriveN();
    display.clearDisplay();
    DisplayStr("Drive: -");
    display.display();
    delay(250);

    while (digitalRead(P_PushToTest))
      ; 
    DriveOff();
    display.clearDisplay();
    DisplayStr("Drive: Off");
    display.display();
    delay(250);
  }
#endif
  
void setup() 
// ---------------------------------------------------------------------------------------------
//    - initializae display and I/O ports
// --------------------------------------------------------------------------------------------- 
  {
    DriveOff();                                    // disable current drive
    Wire.begin();                                  // join I2C bus
    display.begin(SSD1306_SWITCHCAPVCC, 0x3c, 0);  // initialize display @ address 0x3c, no reset
    pinMode(P_PushToTest, INPUT_PULLUP);           // measure push button switch, active low
    debug_mode = !digitalRead(P_PushToTest);       // if pushed during power on, then debug mode
    pinMode(P_Debug, OUTPUT);                      // debug port
  }
  
void loop() 
// ---------------------------------------------------------------------------------------------
//    main measurement loop
// --------------------------------------------------------------------------------------------- 
  {
    const float      Rr = 10.0;             // reference resistor value, ohms
    const float      Rcal = 1.002419;       // calibration factor
    const int        N = 2;                 // number of cycles to average
    const int        M = 50;                // samples per half cycle
    static long      Toff;
    double           Rx;                    // calculated resistor under test, ohms
    byte             PGAr, PGAx;            // PGA gains (r = reference, x = test resistors)
    float            Vr, Vx, Wx, Wr;        // voltages in V
    float            Rn;                    // calculated resistor under test, ohms, single sample
    double           Avgr, Avgx;            // average ADC readings in mV
    int              j, k, n;
    float            Vbat;                  // battery voltage in V (from 2:1 divider)
    char             serialbuff[10];        // Buffer for sending the reading over I²C

    display.clearDisplay();
    DisplayStr("measuring"); 
    display.display();

    // determine PGA gains      
    DriveP(); 
    Wr =  ADS1115read(ADS1115_CH01, ADS1115_6p144);
    Wx =  ADS1115read(ADS1115_CH23, ADS1115_6p144);    
    DriveN();
    Vr = -ADS1115read(ADS1115_CH01, ADS1115_6p144);
    Vx = -ADS1115read(ADS1115_CH23, ADS1115_6p144);

    //  measure battery voltage ... while drive is on so there is a load
    Vbat = analogRead(A3)*5.0/1024.0;    // 2:1 divider (5V FS) on 4.2v lithium battery

    DriveOff();

    PGAr = CalcPGA(max(Vr, Wr));           // determine optimum PGA gains
    PGAx = CalcPGA(max(Vx, Wx));

    // measure resistance using synchronous detection
    Avgr = Avgx = 0.0;                     // clear averages
    Rx = 0.0;
    n = 0;
    for (j=0; j<N; j++)                    // for each cycle
      {
        DriveP();                          // turn on drive, positive
        for (k=0; k<M; k++)
          {
            digitalWrite(P_Debug, 1);
            Vx = ADS1115read(ADS1115_CH23, PGAx);
            digitalWrite(P_Debug, 0);
            Vr = ADS1115read(ADS1115_CH01, PGAr);
            Avgx += Vx;
            Avgr += Vr;
            Rn = Vx/Vr;
            if (Rn>0.0 && Rn<10000.0)
              {
              Rx += Rn;
              n++;
              }
          }

        DriveN();                          // turn on drive, negative
        for (k=0; k<M; k++)
          {
            digitalWrite(P_Debug, 1);
            Vx = ADS1115read(ADS1115_CH23, PGAx);
            digitalWrite(P_Debug, 0);
            Vr = ADS1115read(ADS1115_CH01, PGAr);
            Avgx -= Vx;
            Avgr -= Vr;
            Rn = Vx/Vr;
            if (Rn>0.0 && Rn<10000.0)
              {
              Rx += Rn;
              n++;
              }
          }
      }
    
    DriveOff();
    Rx   *= Rr * Rcal / n;                 // apply calibration factor and compute average
    Avgr *= 1000.0 / (2.0*N*M);            // average in mV
    Avgx *= 1000.0 / (2.0*N*M);   

    // display the results ... battery icon, Rx measurement, debug info if requested
    display.clearDisplay();                // ... and display result
    BatteryIcon((Vbat-3.0)/(4.2-3.0));     // 7.5V = 0%, 9V = 100%
    //display.drawLine(0, 8, 127, 8, WHITE); //Draw separator line under icons
    if (n==0){                              // no measurement taken ...
      display.setTextSize(2);
      display.setCursor(51,20);
      display.print(F("OL"));
    }
      //DisplayStr("-----");
    else
      DisplayResistance(Rx);
    //Send Reading via I²C
      Wire.beginTransmission(0x50);
      Wire.write(dtostrf(Rx, 5, 5, serialbuff));
      Wire.endTransmission();
    if (debug_mode) 
      DisplayDebug(PGAr, PGAx, Avgr, Avgx, Vbat);
    display.display();                     // show the display
    
    // and then wait for next measurement request
    Toff = millis()+60000L;
    while(digitalRead(P_PushToTest))       // loop until measure button pressed
      {
        // Enter power down state for 120ms with ADC and BOD module disabled
        //LowPower.powerDown(SLEEP_120MS, ADC_OFF, BOD_OFF);  
        if (millis()>Toff)                 // after 7 seconds ...
          {
            display.clearDisplay();        // clear display
            display.display(); 
          }
      }
  }

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// ---------------------------------------------------------------------------------------------

// Simple, accurate milliohmeter

// (c) Mark Driedger 2015

// - Determines resistance using 4 wire measurement of voltage across a series connected

// reference resistor (Rr, 10 ohm, 0.1%) and test resistor (Rx)

// - range of accurate measurement is roughly 50 mohm to 10Kohm

// - Uses Arduino digital I/O ports to deliver the test current, alternating polarity to cancel

// offset errors (synchronous detector)

// - 4 I/O pins are used for each leg of the test current to increase test current

// - Averages 2 cycles and 100 samples/cycle

// - Uses a 16 bit ADC ADS1115 with 16x PGA to improve accuracy

// Version History

// May 24/15 v1.0-v4.0

// - initial development versions

// May 27/15 v5.0

// - changed display to I2C

// - backed out low power module since it seemed to cause serial port upload problems

// ---------------------------------------------------------------------------------------------

#include <Wire.h>

#include <SPI.h>

#include <Adafruit_GFX.h>

#include <Adafruit_SSD1306.h>

//#include <LowPower.h>

#if (SSD1306_LCDHEIGHT != 64)

#error("Height incorrect, please fix Adafruit_SSD1306.h!");

#endif

// ---------------------------------------------------------------------------------------------

// I/O port usage

// ---------------------------------------------------------------------------------------------

// serial port (debug and s/w download) 0, 1

// I²C interface to ADC & display A4, A5

// positive drive 2, 3, 4, 5

// push to test input 8

// unused 9, 10, 11, A0, A1, A2, A6, A7

// negative drive 6, 7, 8, 9

// battery voltage monitor A3

// debug output 13

#define P_PushToTest 10 // push button (measure), active low

#define P_Debug 13

#define CHG 12

// ADS1115 mux and gain settings

#define ADS1115_CH01 0x00 // p = AIN0, n = AIN1

#define ADS1115_CH03 0x01 // ... etc

#define ADS1115_CH13 0x02

#define ADS1115_CH23 0x03

#define ADS1115_CH0G 0x04 // p = AIN0, n = GND

#define ADS1115_CH1G 0x05 // ... etc

#define ADS1115_CH2G 0x06

#define ADS1115_CH3G 0x07

#define ADS1115_6p144 0x00 // +/- 6.144 V full scale

#define ADS1115_4p096 0x01 // +/- 4.096 V full scale

#define ADS1115_2p048 0x02 // +/- 2.048 V full scale

#define ADS1115_1p024 0x03 // +/- 1.024 V full scale

#define ADS1115_0p512 0x04 // +/- 0.512 V full scale

#define ADS1115_0p256 0x05 // +/- 0.256 V full scale

#define ADS1115_0p256B 0x06 // same as ADS1115_0p256

#define ADS1115_0p256C 0x07 // same as ADS1115_0p256

Adafruit_SSD1306 display(0); // using I2C interface, no reset pin

static int debug_mode = 0; // true in debug mode

float ADS1115read(byte channel, byte gain)

//--------------------------------------------------------------------------------------

// reads a single sample from the ADS1115 ADC at a given mux (channel) and gain setting

// - channel is 3 bit channel number/mux setting (one of ADS1115_CHxx)

// - gain is 3 bit PGA gain setting (one of ADS1115_xpxxx)

// - returns voltage in volts

// - uses single shot mode, polling for conversion complete, default I2C address

// - conversion takes approximatly 9.25 msec

//--------------------------------------------------------------------------------------

{

const int address = 0x48; // ADS1115 I2C address, A0=0, A1=0

byte hiByte, loByte;

int r;

float x;

channel &= 0x07; // constrain to 3 bits

gain &= 0x07;

hiByte = B10000001 | (channel<<4) | (gain<<1); // conversion start command

loByte = B10000011;

Wire.beginTransmission(address); // send conversion start command

Wire.write(0x01); // address the config register

Wire.write(hiByte); // ...and send config register value

Wire.write(loByte);

Wire.endTransmission();

do // loop until conversion complete

{

Wire.requestFrom(address, 2); // config register is still addressed

while(Wire.available())

{

hiByte = Wire.read(); // ... and read config register

loByte = Wire.read();

}

while ((hiByte & 0x80)==0); // upper bit (OS) is conversion complete

Wire.beginTransmission(address);

Wire.write(0x00); // address the conversion register

Wire.endTransmission();

Wire.requestFrom(address, 2); // ... and get 2 byte result

while(Wire.available())

{

hiByte = Wire.read();

loByte = Wire.read();

}

r = loByte | hiByte<<8; // convert to 16 bit int

switch(gain) // ... and now convert to volts

{

case ADS1115_6p144: x = r * 6.144 / 32768.0; break;

case ADS1115_4p096: x = r * 4.096 / 32768.0; break;

case ADS1115_2p048: x = r * 2.048 / 32768.0; break;

case ADS1115_1p024: x = r * 1.024 / 32768.0; break;

case ADS1115_0p512: x = r * 0.512 / 32768.0; break;

case ADS1115_0p256:

case ADS1115_0p256B:

case ADS1115_0p256C: x = r * 0.256 / 32768.0; break;

}

return x;

}

// ---------------------------------------------------------------------------------------------

// Drive functions

// - ports 4-7 and A0-A3 are used to differentially drive resistor under test

// - the ports are resistively summed to increase current capability

// - DriveOff() disables the drive, setting the bits to input

// - DriveOn() enables the drive, setting the bits to output

// - DriveP() enables drive with positive current flow (from ports 4-7 to ports A0-A3)

// - DriveN() enables drive with negative current flow

// ---------------------------------------------------------------------------------------------

void DriveP()

{

DriveOff();

digitalWrite( 2, HIGH);

digitalWrite( 3, HIGH);

digitalWrite( 4, HIGH);

digitalWrite( 5, HIGH);

digitalWrite( 6, LOW);

digitalWrite( 7, LOW);

digitalWrite( 8, LOW);

digitalWrite( 9, LOW);

DriveOn();

}

void DriveN()

{

DriveOff();

digitalWrite( 2, LOW);

digitalWrite( 3, LOW);

digitalWrite( 4, LOW);

digitalWrite( 5, LOW);

digitalWrite( 6, HIGH);

digitalWrite( 7, HIGH);

digitalWrite( 8, HIGH);

digitalWrite( 9, HIGH);

DriveOn();

}

void DriveOn()

{

pinMode( 2, OUTPUT); // enable source/sink in pairs

pinMode( 6, OUTPUT);

pinMode( 3, OUTPUT);

pinMode( 7, OUTPUT);

pinMode( 4, OUTPUT);

pinMode( 8, OUTPUT);

pinMode( 5, OUTPUT);

pinMode( 9, OUTPUT);

delayMicroseconds(5000); // 5ms delay

}

void DriveOff()

{

pinMode( 2, INPUT); // disable source/sink in pairs

pinMode( 6, INPUT);

pinMode( 3, INPUT);

pinMode( 7, INPUT);

pinMode( 4, INPUT);

pinMode( 8, INPUT);

pinMode( 5, INPUT);

pinMode( 9, INPUT);

}

int CalcPGA(float x)

// ---------------------------------------------------------------------------------------------

// Calculate optimum PGA setting based on a sample voltage, x, read at lowest PGA gain

// - returns the highest PGA gain that allows x to be read with 10% headroom

// ---------------------------------------------------------------------------------------------

{

x = abs(x);

if (x>3.680) return ADS1115_6p144;

if (x>1.840) return ADS1115_4p096;

if (x>0.920) return ADS1115_2p048;

if (x>0.460) return ADS1115_1p024;

if (x>0.230) return ADS1115_0p512;

else return ADS1115_0p256;

}

void BatteryIcon(float charge)

// ---------------------------------------------------------------------------------------------

// Draw a battery charge icon into the display buffer without refreshing the display

// - charge ranges from 0.0 (empty) to 1.0 (full)

// ---------------------------------------------------------------------------------------------

{

static const unsigned char PROGMEM chg[] = // Battery Charge Icon

{ 0x1c, 0x18, 0x38, 0x3c, 0x18, 0x10, 0x20, 0x00 };

int w = constrain(charge, 0.0, 1.0)*16; // 0 to 16 pixels wide depending on charge

display.drawRect(100, 0, 16, 7, WHITE); // outline

display.drawRect(116, 2, 3, 3, WHITE); // nib

display.fillRect(100, 0, w, 7, WHITE); // charge indication

//battery charging indication

pinMode(CHG, INPUT);

if (digitalRead(CHG) == HIGH)

display.drawBitmap(91, 0, chg, 8, 8, WHITE);

}

void f2str(float x, int N, char *c)

// ---------------------------------------------------------------------------------------------

// Converts a floating point number x to a string c with N digits of precision

// - *c must be a string array of length at least N+3 (N + '-', '.', '\0')

// - x must be have than N leading digits (before decimal) or "#\0" is returned

// ---------------------------------------------------------------------------------------------

{

int j, k, r;

float y;

if (x<0.0) // handle negative numbers

{

*c++ = '-';

x = -x;

}

for (j=0; x>=1.0; j++) // j digits before decimal point

x /= 10.0; // .. and scale x to be < 1.0

if (j>N) // return error string if too many digits

{

*c++ = '#';

*c++ = '\0';

return;

}

y = pow(10, (float) N); // round to N digits

x = round(x * y) / y;

if (x>1.0) // if 1st digit rounded up ...

{

x /= 10.0; // then normalize back down 1 digit

j++;

}

for (k=0; k<N; k++)

{

r = (int) (x*10.0); // leading digit as int

x = x*10-r; // remove leading digit and shift 1 digit

*c++ = r + '0'; // add leading digit to string

if (k==j-1 && k!=N-1) // add decimal point after j digits

*c++ = '.'; // ... unless there are N digits before decimal

}

*c++ = '\0';

}

void DisplayResistance(float x)

// ---------------------------------------------------------------------------------------------

// Adds the resistance value, x, to the display buffer without refreshing the display

// - converts to kohm, milliohm or microohm if necessary

// ---------------------------------------------------------------------------------------------

{

static const unsigned char PROGMEM omega_bmp[] = // omega (ohm) symbol

{ B00000011, B11000000,

B00001100, B00110000,

B00110000, B00001100,

B01000000, B00000010,

B10000000, B00000001,

B01000000, B00000010,

B00100000, B00000100,

B00010000, B00001000,

B11111000, B00011111 };

char s[8];

char prefix;

if (x>=1000.0) // display in killo ohms

{

x /= 1000.0;

prefix = 'k';

}

else if (x<0.001) // display in micro ohms

{

x *= 1000000.0;

prefix = 0xe5; // mu

}

else if (x<1.0) // display in milli ohms

{

x *= 1000.0;

prefix = 'm';

}

else

prefix = ' '; // display in ohms

f2str(x, 5, s);

// display computed resistance

display.setTextSize(2);

display.setTextColor(WHITE);

display.setCursor(0,20);

display.print(s);

// display prefix

display.setCursor(85,20);

display.print(prefix);

// display omega (ohms) symbol

display.drawBitmap(103, 18, omega_bmp, 16, 16, WHITE);

}

void DisplayDebug(int a, int b, float x, float y, float Vbat)

// ---------------------------------------------------------------------------------------------

// Adds debug info to the display buffer without showing the updated display

// - Adds 2 ints (a, b) and a float(Vbat) to the top line and 2 floats (x, y)

// to the bottom line+, all in small (size 1) text

// ---------------------------------------------------------------------------------------------

{

// display x, y in lower left, small font

display.setTextSize(1);

display.setCursor(0,45);

display.print(x,3);

display.print(" ");

display.print(y,3);

// display a, b in upper left, small font

display.setTextSize(1);

display.setCursor(0,0);

display.print(a);

display.print(" ");

display.print(b);

// display Vbat in upper middle, small font

display.setTextSize(1);

display.setCursor(60,0);

display.print(Vbat,1);

}

void DisplayStr(char *s)

// ---------------------------------------------------------------------------------------------

// Adds a string, s, to the display buffer without refreshing the display @ (0,20)

// ---------------------------------------------------------------------------------------------

{

display.setTextSize(2);

display.setTextColor(WHITE);

display.setCursor(8,20);

display.print(s);

}

#ifdef TESTMODE

void loop()

{

while (digitalRead(P_PushToTest))

;

DriveP();

display.clearDisplay();

DisplayStr("Drive: +");

display.display();

delay(250);

while (digitalRead(P_PushToTest))

;

DriveN();

display.clearDisplay();

DisplayStr("Drive: -");

display.display();

delay(250);

while (digitalRead(P_PushToTest))

;

DriveOff();

display.clearDisplay();

DisplayStr("Drive: Off");

display.display();

delay(250);

}

#endif

void setup()

// ---------------------------------------------------------------------------------------------

// - initializae display and I/O ports

// ---------------------------------------------------------------------------------------------

{

DriveOff(); // disable current drive

Wire.begin(); // join I2C bus

display.begin(SSD1306_SWITCHCAPVCC, 0x3c, 0); // initialize display @ address 0x3c, no reset

pinMode(P_PushToTest, INPUT_PULLUP); // measure push button switch, active low

debug_mode = !digitalRead(P_PushToTest); // if pushed during power on, then debug mode

pinMode(P_Debug, OUTPUT); // debug port

}

void loop()

// ---------------------------------------------------------------------------------------------

// main measurement loop

// ---------------------------------------------------------------------------------------------

{

const float Rr = 10.0; // reference resistor value, ohms

const float Rcal = 1.002419; // calibration factor

const int N = 2; // number of cycles to average

const int M = 50; // samples per half cycle

static long Toff;

double Rx; // calculated resistor under test, ohms

byte PGAr, PGAx; // PGA gains (r = reference, x = test resistors)

float Vr, Vx, Wx, Wr; // voltages in V

float Rn; // calculated resistor under test, ohms, single sample

double Avgr, Avgx; // average ADC readings in mV

int j, k, n;

float Vbat; // battery voltage in V (from 2:1 divider)

char serialbuff[10]; // Buffer for sending the reading over I²C

display.clearDisplay();

DisplayStr("measuring");

display.display();

// determine PGA gains

DriveP();

Wr = ADS1115read(ADS1115_CH01, ADS1115_6p144);

Wx = ADS1115read(ADS1115_CH23, ADS1115_6p144);

DriveN();

Vr = -ADS1115read(ADS1115_CH01, ADS1115_6p144);

Vx = -ADS1115read(ADS1115_CH23, ADS1115_6p144);

// measure battery voltage ... while drive is on so there is a load

Vbat = analogRead(A3)*5.0/1024.0; // 2:1 divider (5V FS) on 4.2v lithium battery

DriveOff();

PGAr = CalcPGA(max(Vr, Wr)); // determine optimum PGA gains

PGAx = CalcPGA(max(Vx, Wx));

// measure resistance using synchronous detection

Avgr = Avgx = 0.0; // clear averages

Rx = 0.0;

n = 0;

for (j=0; j<N; j++) // for each cycle

{

DriveP(); // turn on drive, positive

for (k=0; k<M; k++)

{

digitalWrite(P_Debug, 1);

Vx = ADS1115read(ADS1115_CH23, PGAx);

digitalWrite(P_Debug, 0);

Vr = ADS1115read(ADS1115_CH01, PGAr);

Avgx += Vx;

Avgr += Vr;

Rn = Vx/Vr;

if (Rn>0.0 && Rn<10000.0)

{

Rx += Rn;

n++;

}

DriveN(); // turn on drive, negative

for (k=0; k<M; k++)

{

digitalWrite(P_Debug, 1);

Vx = ADS1115read(ADS1115_CH23, PGAx);

digitalWrite(P_Debug, 0);

Vr = ADS1115read(ADS1115_CH01, PGAr);

Avgx -= Vx;

Avgr -= Vr;

Rn = Vx/Vr;

if (Rn>0.0 && Rn<10000.0)

{

Rx += Rn;

n++;

}

DriveOff();

Rx *= Rr * Rcal / n; // apply calibration factor and compute average

Avgr *= 1000.0 / (2.0*N*M); // average in mV

Avgx *= 1000.0 / (2.0*N*M);

// display the results ... battery icon, Rx measurement, debug info if requested

display.clearDisplay(); // ... and display result

BatteryIcon((Vbat-3.0)/(4.2-3.0)); // 7.5V = 0%, 9V = 100%

//display.drawLine(0, 8, 127, 8, WHITE); //Draw separator line under icons

if (n==0){ // no measurement taken ...

display.setTextSize(2);

display.setCursor(51,20);

display.print(F("OL"));

}

//DisplayStr("-----");

else

DisplayResistance(Rx);

//Send Reading via I²C

Wire.beginTransmission(0x50);

Wire.write(dtostrf(Rx, 5, 5, serialbuff));

Wire.endTransmission();

if (debug_mode)

DisplayDebug(PGAr, PGAx, Avgr, Avgx, Vbat);

display.display(); // show the display

// and then wait for next measurement request

Toff = millis()+60000L;

while(digitalRead(P_PushToTest)) // loop until measure button pressed

{

// Enter power down state for 120ms with ADC and BOD module disabled

//LowPower.powerDown(SLEEP_120MS, ADC_OFF, BOD_OFF);

if (millis()>Toff) // after 7 seconds ...

{

display.clearDisplay(); // clear display

display.display();

}

Posted on August 8, 2014 by The Engineer — Leave a comment

NCR Receipt Printer Script

Just a short script to directly print files to the NCR 7197 Series printers under Linux as there is no direct driver. Certainly not finished or pretty, but functional.

#/bin/sh
###Depends on fmt & iconv for file formatting for printer to operate correctly. Also expects UTF-8 encoded text files.

if [ "$1" == "--help" ];
    then
		echo "NCR 7197 file printing Script"
		echo "Usage:"
		echo "./ReceiptPrinter.sh <FILE> <SERIAL PORT NUMBER>"
		echo "To trim paper off, pass --cut parameter with port number instead of file name."
		echo "This script is currently set up for USB Connection of the printer"
		echo "Make sure your user has access to the serial port you intend to use,"
		echo "otherwise an error will occur"
		echo "This script uses fmt & iconv to format the provided text file for the printer."
		echo "This is 44 characters wide, word wrap enabled & format conversion to US English"

	else
		if [ "$1" == "--cut" ];
			then
				echo -e "\x1B\x07\x1D\x56\x41\x06" > /dev/ttyUSB$2
				echo "Scissor Operated"
				exit 0
	else
		if [ ! -z "$1" ];
			then
				cat $1 | fmt -w 44 | iconv -f UTF-8 -t ISO-8859-1//TRANSLIT > /dev/ttyUSB$2
				echo -e "\x1D\x56\x41\x06" > /dev/ttyUSB$2
				echo "Print Complete"
				exit 0
			else
			echo "No File provided!!! Returning to shell!"
			fi	
		fi
	fi
exit 0

Posted on February 16, 2013 by The Engineer — Leave a comment

Amano PIX 3000x Timeclock

This is a late 90’s business timeclock, used for maintaining records of staff working times, by printing the time when used on a sheet of card.

Here is the top cover removed, which is normally locked in place to stop tampering. The unit is programmed with the 3 buttons & the row of DIP switches along the top edge.

Closeup of the settings panel, with all the various DIP switch options.

Cover plate removed from the top, showing the LCD & CPU board, the backup battery normally fits behind this. The CPU is a 4-bit microcontroller from NEC, with built in LCD driver.

Power Supply & prinhead drivers. This board is fitted with several NPN Darlington transistor arrays for driving the dox matrix printhead.

Printhead assembly itself. The print ribbon fits over the top of the head & over the pins at the bottom. The drive hammers & solenoids are housed in the circular top of the unit.

Bottom of the print head showing the row of impact pins used to create the printout.

Bottom of the solenoid assembly with the ribbon cable for power. There are 9 solenoids, to operate the 9 pins in the head.

Top layer of the printhead assembly, showing the leaf spring used to hold the hammers in the correct positions.

Hammer assembly. The fingers on the ends of the arms push on the pins to strike through the ribbon onto the card.

The ring of solenoids at the centre of the assembly. These are driven with 3A darlington power arrays on the PSU board.

There is only a single drive motor in the entire unit, that both clamps the card for printing & moves the printhead laterally across the card. Through a rack & pinion this also advances the ribbon with each print.

Posted on February 2, 2012 by The Engineer — Leave a comment

Zebra P330i Card Printer

This is the teardown of a Zebra P330i plastic card printer, used for creating ID cards, membership cards, employee cards, etc. I got this as a faulty unit, which I will detail later on.
This printer supports printing on plastic cards from 1-30mils thick, using dye sublimation & thermal transfer type printing methods. Interfaces supplied are USB & Ethernet. The unit also has the capability to be fitted with a mag stripe encoder & a smart card encoder, for extra cost.

On the left here is the print engine open, the blue cartridge on the right is a cleaning unit, using an adhesive roller to remove any dirt from the incoming card stock.
This is extremely important on a dye sublimation based printing engine as any dirt on the cards will cause printing problems.

Here on the right is the card feeder unit, stocked with cards. This can take up to 100 cards from the factory.
The blue lever on the left is used to set the card thickness being used, to prevent misfeeds. There is a rubber gate in the intake port of the printer which is moved by this lever to stop any more than a single card from being fed into the print engine at any one time.

Here is the empty card feeder, showing the rubber conveyor belt. This unit was in fact the problem with the printer, the drive belt from the DC motor under this unit was stripped, preventing the cards from feeding into the printer.

Here is a closeup of the print head assembly. The brown/black stripe along the edge is the row of thin-film heating elements. This is a 300DPI head.

This is under the print head, the black roller on the left is the platen roller, which supports the card during printing. The spool in the center of the picture is the supply spool for the dye ribbon.
In the front of the black bar in the bottom center, is a two-colour sensor, used to locate the ribbon at the start of the Yellow panel to begin printing.

Inside the top cover is the indicator LCD, the back of which is pictured right.
This is a 16×1 character LCD from Hantronix. This unit has a parallel interface.

Front of the LCD, this is white characters on a blue background.

Here is the cover removed from the printer, showing the drive belts powering the drive rollers. There is an identical arrangement on the other side of the print engine running the other rollers at the input side of the engine.

Here the back panel has been removed from the entire print engine, complete with the mains input wiring & RFI filtering.
This unit has excellent build quality, just what is to be expected from a £1,200+ piece of industrial equipment.

The bottom of the print engine, with all the main wiring & PCB removed, showing the main drive motors. The left hand geared motor operates the head lift, the centre motor is a stepper, which operates the main transmission for the cards. The right motor drives the ribbon take up spindle through an O-Ring belt.

Card feeder drive motor, this connects to the belt assembly through a timing belt identical to the roller drive system.
All these DC geared motors are 18v DC, of varying torque ratings.

Here is the main power supply, a universal input switch-mode unit, outputting 24v DC at 3.3A.

PSU info. This is obviously an off the shelf unit, manufactured by Hitek. Model number FUEA240.

The PSU has been removed from the back of the print engine, here is shown the remaining mechanical systems of the printer.

A further closeup of the print engine mechanical bay, the main stepper motor is bottom centre, driving the brass flywheel through another timing belt drive. The O-Ring drive on the right is for the ribbon take up reel, with the final motor driving the plastic cam on the left to raise/lower the print head assembly.
The brass disc at the top is connected through a friction clutch to the ribbon supply reel, which provides tension to keep it taut. The slots in the disc are to sense the speed of the ribbon during printing, which allows the printer to tell if there is no ribbon present or if it has broken.

Here is a further closeup, showing the RFID PCB behind the main transmission. This allows the printer to identify the ribbon fitted as a colour or monochrome.
The antenna is under the brass interrupter disc on the left.

The I/O daughterboard connects to the main CPU board & interfaces all the motors & sensors in the printer.

Here is the main CPU board, which contains all the logic & processing power in the printer.

Main CPU. This is a Freescale Semiconductor part, model number MCF5206FT33A, a ColdFire based 32-bit CPU. Also the system ROM & RAM can be seen on the right hand side of this picture.

Bottom of the Ethernet interface card, this clearly has it’s own RAM, ROM & FPGA. This is due to this component being a full Parallel interface print server.

Top of the PCB, showing the main processor of the print server. This has a ferrite sheet glued to the top, for interference protection.

Posted on March 12, 2011September 17, 2011 by The Engineer — Leave a comment

Brother P-Touch 80 Label Maker

Here is a label maker, bought on offer at Maplin Electronics. Full Qwerty keyboard with 1 line dot matrix LCD display visible here. Power is 4 AAA cells or a 6v DC Adaptor.

Rear cover removed. Battery compartment is on the left hand side, space for the tape cartridge on the right. Ribbon cable leading to the thermal print head is on the far right, with rubber tape drive roller.

PCB under the top cover with the main CPU, a MN101C77CBM from Panasonic. This CPU features 48K Mask ROM & 3K of RAM. Max clock frequency is 20MHz. 32kHz clock crystal visible underneath a Rohm BA6220 Electronic speed controller IC.
This is used to drive the printer motor at a constant accurate speed, to feed the tape past the thermal head. Miniature potentiometer adjusts speed.
Ribbon cable at the bottom of the board connects to the print head, various wiring at the left connects to the battery & DC Jack.

Printer drive mechanism. Small DC motor drives the pinch roller though a gear train. DC Jack & reverse polarity protection diode is on the right.
This unit uses a centre negative DC jack, which is unusual.

Thermal tape cartridge, black text on white background.