Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
Category: Software
General-purpose computers doing something specific
The tiny Lenovo Q150 has become the dedicated Windows box for running TurboTax this season. In earlier years, I used the Token Windows Laptop through a remote desktop session that appears on a Xubuntu desktop, but the Q150 runs rings around the old laptop.
This time, no matter what I tried, I couldn’t connect to the Windows 7 desktop on the Q150 from my Xubuntu desktop. The usual search results suggested Windows configuration settings that didn’t quite match what the Q150 provided; a bit more searching revealed that Windows 7 Home flavors of the OS (this one is Home Premium) lack the Remote Desktop Protocol server required to export the desktop. The Q150 could act as a client that controlled another machine’s desktop server, but not the other way around.
The suggested solutions required applying patches, in the form of EXE files downloaded from sketchy websites, or dropping in replacement DLLs obtained from similar sites. All that seems like Bad Practice, particularly for a Windows box used to prepare our taxes, and I was unwilling to proceed along those lines.
Instead, I fetched UltraVNC, installed it on the Q150, and it works perfectly. Remmina occasionally requires a resize-window-to-match-server at startup and then it’s all good.
From what I hear, Windows 7 doesn’t display the classic Blue Screen of Death nearly so often as before, although I did manage to lock it up during the course of this adventure. That’s OK, I can still use my favorite Windows wallpaper image:
Our Larval Engineer received a logic probe / pulser set for Christmas:
RSR Logic Probe Pulser Set – with formed covers
They’re the low-cost RSR-611 and -620 from the usual eBay vendor, not my ancient HP10525/10526 set, but they should suffice. Perhaps nobody uses logic probes these days, what with most of the parts being too small for even a needle tip, but …
Anyhow, they didn’t have caps over the sharp probe tips, so I rummaged around until I found the stash of cigar tubes (some of which went into that air flow straightener) that were about the right size. I thought about 3D printing an adapter between tubes and probes:
RSR Probe Cap Adapter – solid model
It’s actually a subtractive kind of thing, with a model of the probe tip subtracted from a suitable cylindrical object:
RSR Logic Probe – solid model
But then I realized the tubes were thermoplastic, held each one over a stove burner until the open end went transparent and droopy, rammed it down over the probe tip, and trimmed off the ragged edge. Worked fine, fits securely, and even looks pretty good:
RSR Covers – detail
I’ll never print the adapters, but maybe one of us will tweak the model to do something else…
The OpenSCAD source code:
// RSR Logic Probe / Pulser Cap
// Ed Nisley KE4ZNU December 2012
// Adapts cigar tube to probe body
// Layout options
Layout = "Build";
// Overall layout: Show Build
// Parts: Probe
//- Extrusion parameters must match reality!
// Print with +1 shells and 3 solid layers
ThreadThick = 0.25;
ThreadWidth = 2.0 * ThreadThick;
HoleWindage = 0.2;
function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit);
Protrusion = 0.1; // make holes end cleanly
//----------------------
// Dimensions
ProbeDia = 18.0; // dia of main body
ProbeTipDia = 6.8; // dia at end of plastic cone
ProbeTipLen = 30.0; // length of metal ferrule + tip
ProbeConeLen = 17.5; // cone taper length
TubeOD = 17.25;
TubeWall = 0.50;
TubeID = TubeOD - 2*TubeWall;
TubeLen = 15; // slip fit over tube body
BodyLen = 20; // slip fit over probe body
WallThick = 3.5*ThreadWidth; // basic adapter wall thickness
AdapterLen = TubeLen + BodyLen;
AdapterOD = ProbeDia + 2*WallThick;
AdapterSides = 4*4;
//----------------------
// Useful routines
module PolyCyl(Dia,Height,ForceSides=0) { // based on nophead's polyholes
Sides = (ForceSides != 0) ? ForceSides : (ceil(Dia) + 2);
FixDia = Dia / cos(180/Sides);
cylinder(r=(FixDia + HoleWindage)/2,
h=Height,
$fn=Sides);
}
module ShowPegGrid(Space = 10.0,Size = 1.0) {
Range = floor(50 / Space);
for (x=[-Range:Range])
for (y=[-Range:Range])
translate([x*Space,y*Space,Size/2])
%cube(Size,center=true);
}
module Probe() {
union() {
cylinder(r=((ProbeDia + HoleWindage)/2),
h=(BodyLen + 1.2*Protrusion),$fn=2*AdapterSides);
translate([0,0,(BodyLen + Protrusion)])
cylinder(r1=(ProbeDia + HoleWindage)/2,
r2=ProbeTipDia/2,
h=ProbeConeLen,$fn=2*AdapterSides);
cylinder(r=ProbeTipDia/2,h=(BodyLen + ProbeConeLen + ProbeTipLen),$fn=2*AdapterSides);
}
}
module ProbeSleeve() {
difference() {
cylinder(r=AdapterOD/2,h=AdapterLen);
translate([0,0,-Protrusion])
Probe();
PolyCyl((TubeOD + HoleWindage),(AdapterLen + Protrusion),2*AdapterSides);
}
}
//----------------------
// Build it!
ShowPegGrid();
if (Layout == "Show")
ProbeSleeve();
if (Layout == "Build")
translate([0,0,AdapterLen])
rotate([180,0,0])
ProbeSleeve();
if (Layout == "Probe")
Probe();
A bit of fiddling with the Arduino PWM hardware can turn a white LED into a stroboscopic tachometer to chop smooth motion into chunks:
Strobe – Maze 1 – 50 Hz 100 us
I was moving that pendant by hand and slight speed changes were easily visible:
Strobe – Maze 2 – 50 Hz 100 us
IBMers of a certain era may recognize the test object; the rest of you can go there.
That’s a 10 mm warm-white LED with 5 parallel chips, running at about 100 mA from a 5 V supply, and driven from the same PWM channel and MOSFET that used to drive also drives the red channel of the RGB LED Mood Light:
White LED Strobe
The ZVNL110A MOSFET has a 3 Ω drain resistance, which becomes a significant part of the resistance; you’d want a bigger, better, lower resistance MOSFET to wring more light out of the LED. In fact, I ran the LED from 12 V with the same resistor at a few hundred mA.
The reason you need more light is to make up for the minuscule duty cycle. In order to “stop motion”, you want a very short pulse; I picked a 100 μs pulse. At 50 Hz, that works out to a 0.5% duty cycle: not much light at 100 mA, but OK for a demo.
You can’t do this with the standard Arduino PWM setup, because it produces a constant frequency (about 488 Hz) and varies the duty cycle; we need a variable frequency with a constant pulse length. Because a stroboscope needs fine-grained control over the frequency, in order to stop the motion of rotating objects, it should run from one of the 16 bit Timer1 PWM outputs, which means either PWM9 or PWM10. Note that simply changing the timer’s clock prescaler as described there won’t suffice, because that gives very coarse control of the PWM frequency.
It’s probably worth noting that trying to do precise timing purely in software with, say, the millis() and micros() functions, produces terrible results…
The Arduino timer hardware includes control over both the period and the duration of the output pulses. The Fine Manual describes all the timer configuration registers starting on page 109; see that post for a push-pull PWM driver that formed the basis of this one.
Fast PWM (Mode 14) has some useful characteristics:
Single-slope operation: timer counts only upward
Output PWM9 goes high when TCNT1 resets to 0
Output PWM9 goes low when TCNT1 = OCR1A
TCNT1 resets when TCNT1 = ICR1
The lowest possible output frequency occurs with ICR1 = 0xffff, so that Timer1 counts from 0x0000 to 0xffff before resetting (which, in that case, is indistinguishable from simply wrapping). The wrap period = ICR1 * tick period and the corresponding frequency = 1 / period.
The clock prescaler determines the overall range of Timer1 by setting the tick period. The Clock Select bit field can take on 6 useful, albeit widely separated, values (the other two select the external clock pin):
0 – stop timer
1 – prescale 1:1 = 62.5 ns tick → 244 Hz
2 – prescale 1:8 = 500 ns tick → 30 Hz
3 – prescale 1:64 = 4 μs tick → 3.8 Hz
4 – prescale 1:256 = 16 μs tick → 0.95 Hz
5 – prescale 1:1024 = 64 μs tick → 0.24 Hz
For my purposes, a lower limit around 4 Hz seemed about right. That means CS = 3, the prescaler runs at 1:64, and the timer ticks at 4 μs.
The frequency upper limit could be just under 1/(pulse width), which would produce a very high duty cycle. I arbitrarily set the limit to 1/(4 × pulse width), for a 25% duty cycle that works out to 1/(4 × 100 μs) = 2.5 kHz = 150 k flash/min. If you’re using very high current drive, then limit the duty cycle to prevent toasting the LED.
Because a strobe tach needs quick & easy adjustment, the encoder knob tweaks the pulse frequency in 1 Hz steps. Pushing the knob to close the shaft switch (if you have such a knob, of course, otherwise use another button; they all do the same thing here) reduces the step size to 0.01 Hz, which is more useful for fine tuning when you’re close to the goal. A real application requires better control over the numeric values (probably using integer values); I used floating point and simply ignored all the usual roundoff issues:
Shut off interrupts to prevent interference with the high byte storage register
Stop the timer: CS=0
Load the new upper limit in ICR1
Force TCNT1 to be just below IRC1 to terminate the current pulse
Start the timer: CS=3
Enable interrupts again
You’d probably plunk that into a separate function in a real program…
Printing the frequency becomes a hassle without floating point formatting in printf(). It should appear on the character LED display, too. Optionally / additionally showing the value in rev/min would be very nice.
You’d want to increment the frequency by some reasonable fraction of the current value, perhaps rounded to 1 / 2 / 5 / 10 percent steps. Larger steps by pushbutton? Truncate the current value to a multiple of the step size?
You would also want some way to adjust the flash duration, but that’s definitely in the nature of fine tuning.
As it stands, a 100 μs pulse really does stop motion:
Fan stopped at 2500 rpm
That’s a fan running at about 2500 rpm, with the LED flashing at 41.86 Hz. The camera exposure is 1/2 sec @ f/3.5, handheld, which means the camera integrated about 20 flashes. Ambient light accounts for the background blur: I boosted the grossly underexposed image right out of darkness. The square on the hub is retroreflective tape for a laser tachometer that verified the speed.
Yes, half a second handheld. The morning tea wears off during the day…
In round numbers, 41.86 Hz = 23.9 ms / rev. The fan diameter is 86 mm, so the blade tips travel 1.1 mm = (270 mm / 23.9 ms) × 100 μs during each flash. The tips seem slightly blurred when you (well, I) look very closely in real life, but I think this lashup worked pretty well right off the sketchpad.
The Arduino source code:
// Stroboscopic Tachometer
// Ed Nisley - KE4ANU - December 2012
//----------
// Pin assignments
const byte PIN_KNOB_A = 2; // knob A switch - must be on ext interrupt 2
const byte PIN_KNOB_B = 4; // .. B switch
const byte PIN_BUTTONS = A5; // .. push-close momentary switches
const byte PIN_STROBE = 9; // LED drive, must be PWM9 = OCR1A using Timer1
const byte PIN_PWM10 = 10; // drivers for LED strip, must turn these off...
const byte PIN_PWM11 = 11;
const byte PIN_SYNC = 13; // scope sync
//----------
// Constants
const int UPDATEMS = 10; // update LEDs only this many ms apart
#define TCCRxB_CS 0x03 // Timer prescaler CS=3 -> 1:64 division
const float TICKPD = 64.0 * 62.5e-9; // basic Timer1 tick rate: prescaler * clock
enum KNOB_STATES {KNOB_CLICK_0,KNOB_CLICK_1};
// ButtonThreshold must have N_BUTTONS elements, last = 1024
enum BUTTONS {SW_KNOB, B_1, B_2, B_3, B_4, N_BUTTONS};
const word ButtonThreshold[] = {265/2, (475+265)/2, (658+475)/2, (834+658)/2, (1023+834)/2, 1024};
//----------
// Globals
float FlashLength = 0.1e-3; // strobe flash duration in seconds
word FlashLengthCt = FlashLength / TICKPD; // ... in Timer1 ticks
float FlashFreq = 20.0; // strobe flash frequency in Hz
float FlashPd = 1.0 / FlashFreq; // ... period in sec
word FlashPdCt = FlashPd / TICKPD; // ... period in Timer1 ticks
float FreqIncr = 1.0; // default frequency increment
const float FreqMin = 4.0;
const float FreqMax = 1.0/(4.0*FlashLength);
volatile char KnobCounter = 0;
volatile char KnobState;
byte Button, PrevButton;
unsigned long MillisNow;
unsigned long MillisThen;
//-- Helper routine for printf()
int s_putc(char c, FILE *t) {
Serial.write(c);
}
//-- Knob interrupt handler
void KnobHandler(void)
{
byte Inputs;
Inputs = digitalRead(PIN_KNOB_B) << 1 | digitalRead(PIN_KNOB_A); // align raw inputs
// Inputs ^= 0x02; // fix direction
switch (KnobState << 2 | Inputs) {
case 0x00 : // 0 00 - glitch
break;
case 0x01 : // 0 01 - UP to 1
KnobCounter++;
KnobState = KNOB_CLICK_1;
break;
case 0x03 : // 0 11 - DOWN to 1
KnobCounter--;
KnobState = KNOB_CLICK_1;
break;
case 0x02 : // 0 10 - glitch
break;
case 0x04 : // 1 00 - DOWN to 0
KnobCounter--;
KnobState = KNOB_CLICK_0;
break;
case 0x05 : // 1 01 - glitch
break;
case 0x07 : // 1 11 - glitch
break;
case 0x06 : // 1 10 - UP to 0
KnobCounter++;
KnobState = KNOB_CLICK_0;
break;
default : // something is broken!
KnobCounter = 0;
KnobState = KNOB_CLICK_0;
}
}
//-- Read and decipher analog switch inputs
// returns N_BUTTONS if no buttons pressed
byte ReadButtons(int PinNumber) {
word RawButton;
byte ButtonNum;
RawButton = analogRead(PinNumber);
for (ButtonNum = 0; ButtonNum <= N_BUTTONS; ButtonNum++){
if (RawButton < ButtonThreshold[ButtonNum])
break;
}
return ButtonNum;
}
//------------------
// Set things up
void setup() {
pinMode(PIN_SYNC,OUTPUT);
digitalWrite(PIN_SYNC,LOW); // show we arrived
analogWrite(PIN_PWM10,0); // turn off other PWM outputs
analogWrite(PIN_PWM11,0);
analogWrite(PIN_STROBE,1); // let Arduino set up default Timer1 PWM
TCCR1B = 0; // turn off Timer1 for strobe setup
TCCR1A = 0x82; // clear OCR1A on match, Fast PWM, lower WGM1x = 14
ICR1 = FlashPdCt;
OCR1A = FlashLengthCt;
TCNT1 = FlashLengthCt - 1;
TCCR1B = 0x18 | TCCRxB_CS; // upper WGM1x = 14, Prescale 1:64, start Timer1
pinMode(PIN_KNOB_B,INPUT_PULLUP);
pinMode(PIN_KNOB_A,INPUT_PULLUP);
KnobState = digitalRead(PIN_KNOB_A);
Button = PrevButton = ReadButtons(PIN_BUTTONS);
attachInterrupt((PIN_KNOB_A - 2),KnobHandler,CHANGE);
Serial.begin(9600);
fdevopen(&s_putc,0); // set up serial output for printf()
printf("Stroboscope Tachometer\r\nEd Nisley - KE4ZNU - December 2012\r\n");
printf("Frequency: %d.%02d\nPulse duration: %d us\n",
(int)FlashFreq,(int)(100.0 * (FlashFreq - trunc(FlashFreq))),
(int)(1e6 * FlashLength));
MillisThen = millis();
}
//------------------
// Run the test loop
void loop() {
MillisNow = millis();
if ((MillisNow - MillisThen) > UPDATEMS) {
digitalWrite(PIN_SYNC,HIGH);
Button = ReadButtons(PIN_BUTTONS);
if (PrevButton != Button) {
if (Button == N_BUTTONS) {
// printf("Button %d released\n",PrevButton);
FreqIncr = 1.0;
}
else
// printf("Button %d pressed\n",Button);
// if (Button == SW_KNOB)
FreqIncr = 0.01;
PrevButton = Button;
}
if (KnobCounter) {
FlashFreq += (float)KnobCounter * FreqIncr;
KnobCounter = 0;
FlashFreq = constrain(FlashFreq,FreqMin,FreqMax);
FlashFreq = round(100.0 * FlashFreq) / 100.0;
FlashPd = 1.0 / FlashFreq;
FlashPdCt = FlashPd / TICKPD;
noInterrupts();
TCCR1B &= 0xf8; // stop Timer1
ICR1 = FlashPdCt; // set new period
TCNT1 = FlashPdCt - 1; // force immediate update
TCCR1B |= TCCRxB_CS; // start Timer1
interrupts();
printf("Frequency: %d.%02d\n",
(int)FlashFreq,(int)(100.0 * (FlashFreq - trunc(FlashFreq))));
}
digitalWrite(PIN_SYNC,LOW);
MillisThen = MillisNow;
}
}
That’s a grandiose name for a blinking LED, if I ever saw one…
For reasons best left to the imagination, we needed some large signs for the front yard. I must look this up every time I do it, so here’s the process…
Create document in LibreOffice (or whatever), save as PDF
Convert PDF to EPS = Encapsulated Postscript
Apply poster to enlarge & paginate
Convert PS to PDF for ease of printing
Bash does the heavy lifting, after you install whatever packages your Linux distro may not have included:
While wiring up the LED stress tester, I realized I should abuse a string of amber LEDs along with the three red strings. Herewith, four amber LEDs from the top of their bag, with LED 5 = LED 1 retested:
Amber LEDs – 100 mA
Apart from being an outlier, that red trace seems much prettier than the others, doesn’t it?
The Bash / Gnuplot routine that produced the graph has a few tweaks:
#!/bin/sh
numLEDs=4
#-- overhead
export GDFONTPATH="/usr/share/fonts/truetype/"
base="${1%.*}"
echo Base name: ${base}
ofile=${base}.png
echo Input file: $1
echo Output file: ${ofile}
#-- do it
gnuplot << EOF
#set term x11
set term png font "arialbd.ttf" 18 size 950,600
set output "${ofile}"
set title "${base}"
set key noautotitles
unset mouse
set bmargin 4
set grid xtics ytics
set xlabel "Forward Voltage - V"
set format x "%6.3f"
set xrange [1.8:2.2]
#set xtics 0,5
set mxtics 2
#set logscale y
#set ytics nomirror autofreq
set ylabel "Current - mA"
set format y "%4.0f"
set yrange [0:120]
set mytics 2
#set y2label "right side variable"
#set y2tics nomirror autofreq 2
#set format y2 "%3.0f"
#set y2range [0:200]
#set y2tics 32
#set rmargin 9
set datafile separator "\t"
set label 1 "LED 1 = LED $((numLEDs + 1))" at 2.100,110 right font "arialbd,18"
set arrow from 2.100,110 to 2.105,103 lt 1 lw 2 lc 0
plot \
"$1" index 0:$((numLEDs - 1)) using (\$5/1000):(\$2/1000):(column(-2)) with linespoints lw 2 lc variable,\
"$1" index $numLEDs using (\$5/1000):(\$2/1000) with linespoints lw 2 lc 0
EOF
Running ten random red LEDs (taken from the bag of 100 sent halfway around the planet) through the LED Curver Tracer produces this plot:
Red LEDs – 80 mA
The two gray traces both come from LED 1 to verify that the process produces the same answer for the same LED. It does, pretty much.
Repeating that with the same LEDs in the same order, but stepping 10 mA up to 100 mA produces a similar plot:
Red LEDs – 100 mA
The voltage quantization comes from the Arduino’s 5 mV ADC resolution (the readings are averaged, but there’s actually not much noise) and the current quantization comes from the step value in the measurement loop (5 mA in the first plot, 10 mA in the second). Seeing the LEDs line up mostly the same way at 80 mA in both graphs is comforting, as it suggests the measurement results aren’t completely random numbers.
Putting three red LEDs in series could produce a total forward drop anywhere between 6.309 V (3*2.103) and 6.555 V (3*2.185), a difference of nigh onto a quarter volt, if you assume this group spans the entire range of voltages and the whole collection has many duplicate values and you’re remarkably unlucky while picking LEDs. For this particular set, however, summing three successive groups of three produces 6.445, 6.372, and 6.469 V, for a spread of just under 100 mV. That suggests it’s probably not worthwhile to select LEDs for forward voltage within each series group of three, although matching parallel LEDs makes a lot of sense. I have no confidence the values will remain stable over power-on hours / thermal cycling / current stress.
The capacity plot for the Wouxun KG-UV3D lithium battery packs shows that there’s not a lot of capacity left after 7.0 V, so shutting down or scaling back to lower current wouldn’t be a major loss. However, it’s not clear a fixed resistor will do a sufficient job of current limiting with 6.5 V forward voltage across the LED string:
At 7.5 V, 100 mA calls for 10 Ω (drop 1 V at 100 mA)
At 8.2 V, 10 Ω produces 170 mA (1.7 V across 10 Ω)
At 7.0 V, 10 Ω produces 50 mA (0.5 V across 10 Ω)
Obviously, 170 mA is way too much, even by my lax standards.
A 100 mV variation in forward voltage between stacks, each with a 10 Ω resistor, translates into about 10 mA difference in current. This may actually call for current sensors and direct current control, although using a sensor per string, seems excessive. Low dropout regulators in current-source mode might suffice, but that still seems messy.
The test rig will run from a hard 7.5 V supply, which means I can use fixed resistors and be done with it.
The raw data behind those graphs, with LED 1 and LED 11 being the same LED:
#!/bin/sh
#-- overhead
export GDFONTPATH="/usr/share/fonts/truetype/"
base="${1%.*}"
echo Base name: ${base}
ofile=${base}.png
echo Input file: $1
echo Output file: ${ofile}
#-- do it
gnuplot << EOF
#set term x11
set term png font "arialbd.ttf" 18 size 950,600
set output "${ofile}"
set title "${base}"
set key noautotitles
unset mouse
set bmargin 4
set grid xtics ytics
set xlabel "Forward Voltage - V"
set format x "%6.3f"
set xrange [1.8:2.2]
#set xtics 0,5
set mxtics 2
#set logscale y
#set ytics nomirror autofreq
set ylabel "Current - mA"
set format y "%4.0f"
set yrange [0:120]
set mytics 2
#set y2label "right side variable"
#set y2tics nomirror autofreq 2
#set format y2 "%3.0f"
#set y2range [0:200]
#set y2tics 32
#set rmargin 9
set datafile separator "\t"
set label 1 "LED 1 = LED 11" at 2.100,110 right font "arialbd,18"
set arrow from 2.100,110 to 2.110,103 lt 1 lw 2 lc 0
plot \
"$1" index 0:9 using (\$5/1000):(\$2/1000):(column(-2)) with linespoints lw 2 lc variable,\
"$1" index 10 using (\$5/1000):(\$2/1000) with linespoints lw 2 lc 0
EOF
And the Arduino source code, which bears a remarkable resemblance to the original firmware:
// LED Curve Tracer
// Ed Nisley - KE4ANU - December 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 = 100; // maximum LED current - mA
const int ISTEP = 10; // LED current increment
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("\r\nITarget: %lu mA",MK_UL(ITarget,1000.0));
IError = ISense - ITarget;
// printf("\r\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\r\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 & 10
TCCR2B = TCCRxB; // set frequency for PWM 3 & 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\r\n# Ed Nisley - KE4ZNU - December 2012\r\n");
VccLED = ReadAI(PIN_READ_LEDSUPPLY);
printf("# VCC at LED: %d mV\r\n",MK_U(VccLED,1000.0));
AVRef1V1 = ReadBandGap(); // compute actual bandgap reference voltage
printf("# Bandgap reference voltage: %lu mV\r\n",MK_UL(AVRef1V1,1000.0));
}
//------------------
// Run the test loop
void loop() {
Serial.println('\n'); // blank line for Gnuplot indexing
WaitButton(PIN_BUTTON1,"Insert LED, press button 1 to start...\r\n");
printf("# INOM\tILED\tVccLED\tVD\tVLED\tVG\tVS\tVGS\tVDS\t<--- LED %d\r\n",TestNum++);
digitalWrite(PIN_HEARTBEAT,LOW);
for (int ILED=0; ILED <= MaxCurrent; ILED+=ISTEP) {
SetLEDCurrent(((float)ILED)/1000.0);
printf("%d\t%lu\t%d\t%d\t%d\t%d\t%d\t%d\t%d\r\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);
}
I want to stress-test some LEDs for the long-stalled bike taillight project with a high current / low duty cycle drive. The usual specs give something like 100 mA at 10% duty cycle in a 100 μs period, but maybe they’ll withstand more abuse than that; I don’t have any specs whatsoever for these LEDs. The usual DC rating is 20 mA, so 100 mA at 20%, say 2 ms in a 10 ms period, should give the same average power as the DC spec. I plan to run them continuously until some failures to pop up or it’s obvious they’re doing just fine.
Although this would be a dandy Arduino project, a classic 555 timer IC makes more sense for something that must run continuously without changing anything. The usual 555 circuit restricts the duty cycle to more than 50% for high-active pulses, a bit over the 20% this task calls for. The simplest workaround is a Schottky diode across the discharge resistor to separate the two current paths: charge uses the upper resistor, discharge the lower, with the diode forward drop thrown in to complicate the calculations.
Rather than putz around with calculation, a few minutes iterating with Linear Technologies’ LTSpice IV produces a reasonable result:
NE555 pulse generator
In round numbers, a 1 μF timing capacitor, 2.7 kΩ charge resistor, and 13 kΩ discharge resistor do the trick. Given the usual capacitor tolerances, each resistor should include a twiddlepot of about half the nominal value: 1 kΩ and 5 kΩ, respectively.
I’m thinking of repurposing those Wouxun KG-UV3D batteries for this task and found a 7.5 V 3.5 A wall wart in the heap that will be close enough for the test rig. The 555 output should drive a logic-level MOSFET just fine, although even an ordinary FET would probably be OK for the relatively low current required for LED toasting.
The Smell of Molten Projects in the Morning 