LED Curve Tracer: Firmware

The main loop of the LED Curve Tracer firmware waits for a button push, then steps the LED current upward in 5 mA increments, measures a bunch of voltages, and prints the results in the tired old CSV format that feeds into Gnuplot and spreadsheets like nothing else.

The output looks thuslike:

# LED Curve Tracer
# Ed Nisley - KE4ZNU - July 2012
# VCC at LED: 4872 mV
# Bandgap reference voltage: 1039 mV

# Insert LED, press button 1 to start...
# INOM	ILED	VccLED	VD	VLED	VG	VS	VGS	VDS	<--- LED 1
0	0	4867	3697	1169	0	0	0	3697
5	4585	4867	3013	1853	1959	48	1911	2965
10	9628	4867	2970	1896	2075	101	1973	2869
15	14672	4867	2917	1949	2166	154	2012	2763
20	20174	4867	2898	1969	2282	211	2070	2686
25	25218	4867	2883	1983	2339	264	2075	2619
30	30262	4867	2864	2002	2407	317	2089	2546
35	34847	4867	2850	2017	2498	365	2132	2484
40	40349	4867	2835	2031	2546	423	2123	2412
45	45393	4867	2821	2046	2614	476	2137	2344
50	49978	4872	2826	2046	2657	524	2132	2301
55	54563	4872	2811	2060	2729	572	2156	2238
60	60066	4872	2802	2070	2792	630	2161	2171
65	64651	4867	2792	2075	2874	678	2195	2113
70	69694	4867	2792	2075	2922	731	2190	2060
75	75197	4867	2777	2089	3004	789	2214	1988

# Insert LED, press button 1 to start...

I suppose calling it a “curve tracer” isn’t quite accurate, because the graphs actually come from the plotting software. Feel free to add one of those gorgeous color LCD panels and draw curves on the fly.

The code includes a constant for the measured V_CC voltage from the regulator on the board, which is not, in general, ever the 5.000 V you expect. This will vary from board to board, so don’t kvetch if you get the wrong results without changing the constant. I assume that the analog reference voltage is just about exactly equal to the supply voltage, because I can’t measure it any more accurately.

Measuring V_CC, however, calibrates all the other voltages, including the LED supply regulator and the bandgap.

Just for fun, the code measures & reports the 1.1 V bandgap reference voltage, which works out to be a bit on the low side at 1.039 V. That’s well within the hardware’s 10% tolerance spec, but no better than the usual power supply tolerance, so you must measure one or the other to get the right answer.

The Pro Mini board includes a cap on the analog reference pin, making it as quiet as it’ll get. The datasheet recommends an elaborate LC filter upstream of AV_CC that nobody uses and there’s no room for, but it doesn’t really matter for this purpose.

The function that adjusts the gate voltage to produce a given LED current includes a bailout test to make sure the MOSFET drain-to-source voltage doesn’t drop too low. That happens with blue or violet LEDs (or, maybe white LEDs, which have a similar chemistry) at high currents, where the forward drop exceeds 4 V. There’s not enough headroom between the +5 V LED supply and the 10.5 Ω sense resistor, but the LED supply can’t exceed 5 V because we’re using the Arduino’s internal ADC and that’s limited to the supply voltage.

The Arduino source code:

// LED Curve Tracer
// Ed Nisley - KE4ANU - July 2012

#include <stdio.h>

//----------
// Pin assignments

const byte PIN_READ_LEDSUPPLY = 0;	// AI - LED supply voltage		blue
const byte PIN_READ_VDRAIN = 1;		// AI - drain voltage			red
const byte PIN_READ_VSOURCE = 2;	// AI - source voltage			orange
const byte PIN_READ_VGATE = 3;		// AI - VGS after filtering		violet

const byte PIN_SET_VGATE = 11;		// PWM - gate voltage			brown

const byte PIN_BUTTON1 = 8;			// DI - button to start tests	green
const byte PIN_BUTTON2 = 7;			// DI - button for options		yellow

const byte PIN_HEARTBEAT = 13;		// DO - Arduino LED
const byte PIN_SYNC = 2;			// DO - scope sync output

//----------
// Constants

const int MaxCurrent = 75;				// maximum LED current - mA

const float Vcc = 4.930;				// Arduino supply -- must be measured!

const float RSense = 10.500;			// current sense resistor

const float ITolerance = 0.0005;		// current setpoint tolerance

const float VGStep = 0.019;				// increment/decrement VGate = 5 V / 256

const byte PWM_Settle = 5;				// PWM settling time ms

#define TCCRxB 0x01						// Timer prescaler = 1:1 for 32 kHz PWM

#define MK_UL(fl,sc) ((unsigned long)((fl)*(sc)))
#define MK_U(fl,sc) ((unsigned int)((fl)*(sc)))

//----------
// Globals

float AVRef1V1;					// 1.1 V bandgap reference - calculated from Vcc

float VccLED;					// LED high-side supply

float VDrain;					// MOSFET terminal voltages
float VSource;
float VGate;

unsigned int TestNum = 1;

long unsigned long MillisNow;

//-- Read AI channel
//      averages several readings to improve noise performance
//		returns value in mV assuming VCC ref voltage

#define NUM_T_SAMPLES    10

float ReadAI(byte PinNum) {

  word RawAverage;

  digitalWrite(PIN_SYNC,HIGH);                // scope sync

  RawAverage = analogRead(PinNum);            // prime the averaging pump

  for (int i=2; i <= NUM_T_SAMPLES; i++) {
    RawAverage += (word)analogRead(PinNum);
  }

  digitalWrite(PIN_SYNC,LOW);

  RawAverage /= NUM_T_SAMPLES;

  return Vcc * (float)RawAverage / 1024.0;

}

//-- Set PWM output

void SetPWMVoltage(byte PinNum,float PWMVolt) {

byte PWM;

  PWM = (byte)(PWMVolt / Vcc * 255.0);

  analogWrite(PinNum,PWM);
  delay(PWM_Settle);

}

//-- Set VGS to produce desired LED current
//		bails out if VDS drops below a sensible value

void SetLEDCurrent(float ITarget) {

  float ISense;				// measured current
  float VGateSet;			// output voltage setpoint
  float IError;				// (actual - desired) current

  VGate = ReadAI(PIN_READ_VGATE);					// get gate voltage
  VGateSet = VGate;									//  because input may not match output

  do {

	VSource = ReadAI(PIN_READ_VSOURCE);
	ISense = VSource / RSense;						// get LED current

//	printf("\nITarget: %lu mA",MK_UL(ITarget,1000.0));
	IError = ISense - ITarget;

//	printf("\nISense: %d mA VGateSet: %d mV VGate %d IError %d mA",
//		   MK_U(ISense,1000.0),
//		   MK_U(VGateSet,1000.0),
//		   MK_U(VGate,1000.0),
//		   MK_U(IError,1000.0));

	if (IError < -ITolerance) {
	  VGateSet += VGStep;
//	  Serial.print('+');
	}
	else if (IError > ITolerance) {
	  VGateSet -= VGStep;
//	  Serial.print('-');
	}

	VGateSet = constrain(VGateSet,0.0,Vcc);
	SetPWMVoltage(PIN_SET_VGATE,VGateSet);

	VDrain = ReadAI(PIN_READ_VDRAIN);		// sample these for the main loop
	VGate = ReadAI(PIN_READ_VGATE);
	VccLED = ReadAI(PIN_READ_LEDSUPPLY);

	if ((VDrain - VSource) < 0.020) {			// bail if VDS gets too low
	  printf("# VDS=%d too low, bailing\n",MK_U(VDrain - VSource,1000.0));
	  break;
	}

  } while (abs(IError) > ITolerance);

//	Serial.println("# Done");
}

//-- compute actual 1.1 V bandgap reference based on known VCC = AVcc (more or less)
//		adapted from http://code.google.com/p/tinkerit/wiki/SecretVoltmeter

float ReadBandGap(void) {

  word ADCBits;
  float VBandGap;

  ADMUX = _BV(REFS0) | _BV(MUX3) | _BV(MUX2) | _BV(MUX1);	// select 1.1 V input
  delay(2); // Wait for Vref to settle

  ADCSRA |= _BV(ADSC);										// Convert
  while (bit_is_set(ADCSRA,ADSC));

  ADCBits = ADCL;
  ADCBits |= ADCH<<8;

  VBandGap = Vcc * (float)ADCBits / 1024.0;
  return VBandGap;
}

//-- Print message, wait for a given button press

void WaitButton(int Button,char *pMsg) {
  printf("# %s",pMsg);
  while(HIGH == digitalRead(Button)) {
    delay(100);
    digitalWrite(PIN_HEARTBEAT,!digitalRead(PIN_HEARTBEAT));
  }

  delay(50);				// wait for bounce to settle
  digitalWrite(PIN_HEARTBEAT,LOW);
}

//-- Helper routine for printf()

int s_putc(char c, FILE *t) {
  Serial.write(c);
}

//------------------
// Set things up

void setup() {
  pinMode(PIN_HEARTBEAT,OUTPUT);
  digitalWrite(PIN_HEARTBEAT,LOW);	// show we arrived

  pinMode(PIN_SYNC,OUTPUT);
  digitalWrite(PIN_SYNC,LOW);		// show we arrived

  TCCR1B = TCCRxB;					// set frequency for PWM 9 &amp; 10
  TCCR2B = TCCRxB;					// set frequency for PWM 3 &amp; 11

  pinMode(PIN_SET_VGATE,OUTPUT);
  analogWrite(PIN_SET_VGATE,0);		// force gate voltage = 0

  pinMode(PIN_BUTTON1,INPUT_PULLUP);	// use internal pullup for buttons
  pinMode(PIN_BUTTON2,INPUT_PULLUP);

  Serial.begin(9600);
  fdevopen(&s_putc,0);				// set up serial output for printf()

  printf("# LED Curve Tracer\n# Ed Nisley - KE4ZNU - July 2012\n");

  VccLED = ReadAI(PIN_READ_LEDSUPPLY);
  printf("# VCC at LED: %d mV\n",MK_U(VccLED,1000.0));

  AVRef1V1 = ReadBandGap();			// compute actual bandgap reference voltage
  printf("# Bandgap reference voltage: %lu mV\n",MK_UL(AVRef1V1,1000.0));

  Serial.println();

}

//------------------
// Run the test loop

void loop() {

  WaitButton(PIN_BUTTON1,"Insert LED, press button 1 to start...\n");
  printf("# INOM\tILED\tVccLED\tVD\tVLED\tVG\tVS\tVGS\tVDS\t<--- LED %d\n",TestNum++);
  digitalWrite(PIN_HEARTBEAT,LOW);

  for (int ILED=0; ILED <= MaxCurrent; ILED+=5) {
	SetLEDCurrent(((float)ILED)/1000.0);
	printf("%d\t%lu\t%d\t%d\t%d\t%d\t%d\t%d\t%d\n",
		    ILED,
			MK_UL(VSource / RSense,1.0e6),
			MK_U(VccLED,1000.0),
			MK_U(VDrain,1000.0),
			MK_U(VccLED - VDrain,1000.0),
			MK_U(VGate,1000.0),
			MK_U(VSource,1000.0),
			MK_U(VGate - VSource,1000),
			MK_U(VDrain - VSource,1000.0)
		  );
  }

  SetPWMVoltage(PIN_SET_VGATE,0.0);

}