The Giving Tree and An Arduino Clock

When we bought our house we hired an electrician to install ceiling fans and lights. He worked, worked, and worked. My illusions of electrician’s work dissolved. Their work is a great deal more like laying bricks than solving equations. Oddly, the truth for electricians is also the truth for electronics.

Nearly two years after starting, more than 18~months after finishing the electronics, source code, and cardboard mock-up I finally finished my clock. I have recreated—nay, improved upon—the greatest alarm clock I have ever owned.


In some ways my clock was started about thirty years ago, when some foresighted person planted a lovely little apple tree in what is now my back yard. The lovely little apple tree was a gorgeous mature giant shady apple tree until the spring of 2012. The spring came warm and early but also trisected by two severe cold spells. The frost killed several of my trees major limbs, and may eventually kill the tree.

One of the major limbs became firewood. Then one of the firewood logs became boards, and finally one of the boards became the main faces of the clock. Making a board from firewood is, no doubt, ancient. For me it was a new experience with old techniques. I used a hand powered bow saw, affixed the firewood in the vice, and ripped boards by hand. It is a tiresome process and the boards were not machined to the parallel faces you get in dimensioned lumber. No matter, hand planes helped me get one face smooth and flat. I marked a constant thickness, flipped the board over, and planed the uneven face. The finished board was smooth, clean, and beautifully figured.

The original vision for the box of the clock had hand-cut dovetails visible on the front. However, the apple wood was unsupportive. It was brittle, and prone to fracturing on detail peices. It was also almost too small. The cherry boards I selected for the sides were not long enough to dovetail join the front and back.


The final result is lovely to me. It has a lid so the interface is not visible on the bedside, and the lid showcases the figure of the apple wood. The display is a simple red that does not keep me awake at night. The plain box design weighs enough to keep from migrating around the nightstand, and the interface is actually good.


Design and Use

I set out to imitate the functions of my beloved but deceased clock, see Behold! A number pad. I started the design with a set of use cases, Set Time, Set Alarm, Turn on Alarm, and Turn off Alarm. The notes below are from the original design before I bought the first part, with no edits but formatting.

Set Time

User opens decorative lid. User sets mode switch “set time” mode. The display is turned to current time. User enters numbers starting with most significant digit, in 24 hour time format. After entering the first number, only that digit is displayed.

If user enters a mistake, he hits “clear (#),” and the process restarts at “user enters numbers”. Clear: display is blanked. To keep the setting, user leaves the “set time” mode. On error, the time remains unchanged. Error conditions are:

  • User exits "set time" mode without entering a new time at all.
  • User exits "set time" mode without entering a valid time.

Set Alarm

Same as “Set Time,” but using “set alarm” mode.

Turn on Alarm

User turns on alarm. Display flashes time the alarm is set for about 5 seconds. Display resumes normal operation. The “alarm on” LED lights. When the time reaches the set alarm time, a beeper will sound. The beeper will sound until the alarm is shut off.

Turn off Alarm

Alarm LED is turned off. If the alarm is currently beeping, the beeping ceases.

Electronics Design

I ended up with a Sparkfun keypad, and care of Andrew’s diligent work I was able to configure the Arduino microprocessor to use the keypad almost painlessly. I arranged the pinout as follows:

Pin Use
0 not available
1 not available
2 keypad 0
3 keypad 1
4 keypad 2
5 keypad 3
6 keypad 4
7 keypad 5
8 keypad 6
9 alarm on/off switch
10 alarm tone (PWM)
14 (A0)  
15 (A1) display brightness potentiometer
16 (A2)  
17 (A3) mode switch (3-state) alarm/time
18 (A4) i2c – display and realtime clock
19 (A5) i2c – display and realtime clock

Pin A3 was used for analog read to determine the state of a simple resistor network. Switching the mode switch changes the resistors in a voltage divider. I designed this before I learned about the Arduino’s internal pull-up resistor, so the analog electronics are more difficult than I would make them today.

In case anyone wants it, the code is here also.

#include <Wire.h>
#include "Adafruit_LEDBackpack.h"
#include "Adafruit_GFX.h"
#include <Keypad.h>
#include "RTClib.h"

DateTime now;

#define PIN_ALARMTONE 10

#define PIN_BRIGHTNESSPOT A2 // on the left

#define COLON 2
#define PM 8
#define ALARM 4
#define EXTRA 16

int brightness( void){
  // the analog reading is between 0 and 1024, so divide by 64 to get the
  // values between 0 and 15
  return analogRead( PIN_BRIGHTNESSPOT) >> 6; 

class ClockDisp : public Adafruit_7segment {
  uint8_t alarmOn;
  uint8_t pmOn;
  uint8_t colonOn;
  void writeState( void);
  void alarm( bool);
  void pm( bool);
  void colon( bool);
  void toggleColon( void);
  void printTime( DateTime now);
  void blank( void);

void ClockDisp::blank( void){
  print(10000, DEC);

void ClockDisp::writeState( void){
  writeDigitRaw( 2, alarmOn | pmOn | colonOn );

void ClockDisp::alarm( bool newState ){
  if( newState){
    alarmOn = 4;
    alarmOn = 0;

void ClockDisp::pm( bool newState){
  if( newState){
    pmOn = 8;
    pmOn = 0;

void ClockDisp::colon( bool newState){
  if( newState){
    colonOn = 2;
    colonOn = 0;

void ClockDisp::toggleColon( void){
  colonOn ^= 2;

void ClockDisp::printTime( DateTime now){
  //DateTime now =;
  int hour = now.hour();
  if( hour > 12){
    pmOn = 8;
    hour %= 12;
  } else{
   pmOn = 0;
   if( hour == 0) hour = 12;
  int decimalTime = hour * 100 + now.minute();
  print( decimalTime);
const byte ROWS = 4; //four rows
const byte COLS = 3; //four columns
//define the cymbols on the buttons of the keypads
char keys[ROWS][COLS] = {
byte rowPins[ROWS] = {7,2,3,5}; //connect to the row pinouts of the keypad
byte colPins[COLS] = {6,8,4}; //connect to the column pinouts of the keypad

//initialize an instance of class NewKeypad
Keypad customKeypad = Keypad( makeKeymap(keys), rowPins, colPins, ROWS, COLS); 

//Adafruit_7segment matrix = Adafruit_7segment();
ClockDisp clockDisp = ClockDisp();
int alarmNum = -1;
bool isAlarming = 0;
bool hasAlarmed = 0;

void setup(){


  if (! RTC.isrunning()) {
    Serial.println("RTC is NOT running!");
    // following line sets the RTC to the date & time this 
    // sketch was compiled, except on Mac.
    RTC.adjust(DateTime(__DATE__, __TIME__));
  clockDisp.setBrightness( 10);

void loop() {
  Serial.println("Top of loop");
  now =;
  Serial.print("Brightness pin: ");
  Serial.println( analogRead( PIN_BRIGHTNESSPOT));
  clockDisp.setBrightness( brightness());
  // Print the time, and update the colon
  Serial.print("Time decimal: "); Serial.println( now.hour() * 100 + now.minute());
  clockDisp.printTime( now);
  if( millis()%1000 < 500){
  delay( 20);
  /* The alarm function is actually a little bit complicated. It is
  easy to test if the current time is equal to the alarm time. We 
  want the alarm to start beeping and not to stop until the switch 
  is thrown. However, if we immediately turned the switch back on,
  the current time would still exceed the alarm time, and it would
  start beebing. Therefore, I created the hasAlarmed variable, which
  records that the alarm has gone off. hasAlarmed will reset to FALSE
  when the time is less than the alarm time, indicated that we've come
  around the clock again.
  if( digitalRead( PIN_ALARMSWITCH)){
    clockDisp.alarm( 1);
    if( isAlarming){
      if( millis()%1000 < 500){
        tone( PIN_ALARMTONE, 1000);
        noTone( PIN_ALARMTONE);
    } else if( hasAlarmed){
      if( (now.hour() * 100 + now.minute()) < alarmNum ){
        hasAlarmed = 0;
    } else {
      // See if we need to turn on the alarm
      if((now.hour() * 100 + now.minute()) == alarmNum){
        isAlarming = 1;
  }else if( isAlarming){
    clockDisp.alarm( 0);
    noTone( PIN_ALARMTONE);
    isAlarming = 0;
    hasAlarmed = 1;
   // Contol of setting state switch
   int mode = analogRead( A3);
   Serial.print("Mode reading: "); Serial.println( mode);
   if( mode > 900 ){ // set the time
    Serial.println("Setting the time");
    // blank the display  
    int numDigits = 0;
    int timeNum = 0;
    while( analogRead(A3) > 900){
      // wait for keys, display them, check state
      // wait for entry of time
      char customKey = customKeypad.getKey();
      if( customKey > 0){
        if( numDigits > 0){
          timeNum = timeNum*10 + (int)(customKey - '0');
          timeNum = (int)(customKey - '0');
        numDigits ++;  
        clockDisp.print( timeNum);
    // Test to see if we got a valid time:
    if( (timeNum < 2500) && (timeNum > 0)){
      RTC.adjust( DateTime( now.year(), now.month(),,
              timeNum / 100, timeNum%100, 0));
      // An error, flash bars at the user
      clockDisp.print( 10000);
      delay( 300);
  }else if( mode > 490){ // state 2
    clockDisp.print( alarmNum);
    int numDigits = 0;
    while( 1 ){
      int ar = analogRead( A3);
      if( (ar < 490) || (ar > 900)) break;

      // wait for keys, display them, check state
      // wait for entry of time
      char customKey = customKeypad.getKey();
      if( customKey > 0){
        if( numDigits > 0){
          alarmNum = alarmNum*10 + (int)(customKey - '0');
          alarmNum = (int)(customKey - '0');
        numDigits ++;  
        //clockDisp.print( (int)(customKey - '0'));
        clockDisp.print( alarmNum);
    // Test to see if we got a valid time:
    if( (alarmNum >= 2500) || (alarmNum < 0)){
      // Invalid time entered, flash error bars at user
      clockDisp.print( 10000);
      delay( 300);
    } else{
      hasAlarmed = 0;

int getDecimalTime(){
  DateTime now =;
  int decimalTime = now.hour() * 100 + now.minute();
  return decimalTime;

Wiring an Arduino to a Five-state Guitar Switch

I am rebuilding my Arduino-based data collection system. Named the RIMU for some long-forgotten and pointless acronym. My old one used a momentary switch—a button—to change which sensor is displayed on the LCD. It is awful because if the Arduino is busy when you push the switch then nothing happens. I’m fixing it with a 5-position switch sold for changing the pickup combinations on electric guitars.

I was thinking about some really good machine interfaces, and some really bad. I walked about the house with a camera. Below are some photo collections of good and bad interfaces around the house.


Good interfaces control devices unambiguously with touch. You should not have to look at the device to know what mode you have changed it to. Really great interfaces, like a light switch, reveal their state by touch.

My switch is almost as good as a light switch. With five states it is hard to tell which state it is in by feeling its position; however, it is easy to change state. Even better, its position is an absolute indication of its state. There is no issue of the Arduino being busy when the state changes, because the state is fixed by the switch. It is not toggled by some transient state.

I bought my switch from Sparkfun, and also lifted this product image from them.


If you can find old rotary switches with detents you can use the analog read function of the Arduino to get all the states with a single pin. In this case, though, I will use three pins to represent the states. I believe it is possible to configure this switch with a  few resistors to represent all its states with a single analog input, but it is more complex to figure out and I am not short on pins. My definition of pins. The ones with wires are the only ones I need to use.


With just three digital inputs, the Arduino can read all five switch states. The Arduino’s microprocessor includes built-in pull-up resistors. These are easy to use, but a little bit confusing. A pull-up resistor means that if the pin is not attached to anything, the Arduino will read the pin high.

In the following table I show a Y to indicate how the switch is connected internally. However the ABC column shows what you would read from the pins in the Arduino.

Pos 1 2 3 ABC
1 Y 110 6
2 Y Y 100 4
3 Y 101 5
4 Y Y 001 1
5 Y 011 3
// Define the spins to read the switch
#define pinSwitch1 2 
#define pinSwitch2 3 
#define pinSwitch3 4

unsigned char switchState( void){
  unsigned char state=0, in;
  in = digitalRead( pinSwitch1);
  state |= in << 2;
  in = digitalRead( pinSwitch2);
  state |= in << 1;
  in = digitalRead( pinSwitch3);
  state |= in;
  switch( state){
    case 6:
    return 1;
    case 4:
    return 2;
    case 5:
    return 3;
    case 1:
    return 4;
    case 3:
    return 5;
  return 0;
void setup( void){
  pinMode( pinSwitch1, INPUT);
  pinMode( pinSwitch2, INPUT);
  pinMode( pinSwitch3, INPUT);
  digitalWrite( pinSwitch1, HIGH);
  digitalWrite( pinSwitch2, HIGH);
  digitalWrite( pinSwitch3, HIGH);
  Serial.begin( 57600);
  Serial.println( 'Started');

void loop( void){
  Serial.print( "Switch: ");
  Serial.println( switchState());
  Serial.print( " ");
  Serial.print( digitalRead(pinSwitch3));
  Serial.print( " ");
  Serial.print( digitalRead( pinSwitch2));
  Serial.print( " ");
  Serial.print( digitalRead( pinSwitch1));
  Serial.print( " ");
  delay( 1000);

Arduino Flickering Candle

Update December 24, 2013: Mokus refined his code so that the distribution is now well-behaved (nearly normal) and the PSD no longer turns up at high frequencies). The plots and post have been updated to reflect this change. He will push code to the same link as available.

In my previous post on Candle Flame Flicker I describe the statistics of the optical intensity flicker from a candle in terms of the probability density of the brightness samples and in terms of the power spectral density of the brightness. In this article I discuss how to make an Arduino drive an LED to flicker in a way that is statistically similar to the measurements I took on a real candle.

In the measurements I observed the spectral roll-off of the candle to start between about 5 and 8 Hz, and to decline at a nominal rate of around 44 dB for each decade increase in frequency. The 2nd-order infinite impulse response filter is computationally efficient in terms of using a small amount of memory and requiring few calculations. However, the Arduino is not good at floating point arithmetic. On the Arduino, floating point is done in software, has relatively few bits of precision, and is about 4 to 40 times slower than fixed point (integer) math. It is quite difficult to find useful benchmarks. The basic process is to create white noise and then filter it to the correct spectral shape. Afterward, the signal is scaled to have the appropriate variance and offset with a constant to represent the average brightness of the “flame”.

The approach I used was to design the IIR in Python with scipy’s signal module. I specified a 2nd order lowpass Butterworth filter, specifying a cutoff frequency of 8 Hz, and a sample frequency of 60 Hz. I normalized the coefficients to work in a 16 bit integer space, following Randy Yates’ 2010 Practical Considerations in FIR Fixed Filter Implementations, mainly. From a synthesis perspective, there is some prior art. Philip Ching, a student at Cornell synthesized candle noise quite cleverly, though he neither reported nor replicated the correct statistics. A fellow with the handle Mokus did a very, very tiny implementation for a microcontroller with only 64 bytes of RAM. He helped me modify his code so I could compare his statistics, and after adjustment his spectrum and distribution match fairly well. The high-frequency of his PSD looks a little different from the other methods, but these may not be noticeable to the observer. Finally, there was Eric Evenchick’s coincidental post on hackaday. Mokus reported that Evanchick’s implementation has too slow an update rate; I noticed that Evanchick did not report on the statistics he was targeting nor what he achieved. I did not recreate his work to test.

Then, on to the tests. I really was interested in building and comparing statistics from a 16 bit implementation, a 32 bit implementation in both a Direct Form I and a Direct Form II implementation. Indeed, I had great difficulty getting the filters to perform because I kept misdesigning the integer coefficients and overflowing the registers. As I sought a solution to the 2nd-order filter approach, I also created a 4-stage digital equivalent of an RC filter. The 4-stage RC approach was always stable though it needed a higher sample rate and used much more processor power to create statistically consistent results. On the other hand, it has a more accurate spectrum. A comparison of three different 16-bit methods to Mokus’ and to the actual measurements is shown in the figure below. The legend shows the mean, standard deviation, and their ratio to the right of the label. The All my filters did a credible job of reconstructing the histogram.


The power spectral density (PSD) of the different methods tells a different story. The Direct Form II 16 bit filter is the most visually appealing of the methods I tried. It rolls off more like the physical data than the other methods, except compared to the 4-stage RC filter. The Direct Form II filter is more computationally efficient.


The results for the 32-bit versions show greater variance than the 16-bit versions, but the quality is not markedly better.




I wrote a proof code for the Arduino UNO both to see it flicker and to test the processor speed—separate parts of the code. The results are that compiling with 1.0.3 resulted in a 4,722 byte program that calculated 10,000 new values in 6,292 ms, or 629 microseconds per value. In theory this could produce a sample rate of nearly 1.6 KHz. Or another way of thinking about this is that the final code uses about 629 us/17 ms or about 4% of the processor capability of the Arduino UNO. That leaves a lot of resources available to do other Arduino work or maybe means it can fit in a cheaper processor.

I have released two pieces of code under the GNU Public License 3, you can get the Python I used for filter design and the Arduino test code at the links. If you want the data, please contact me through the comments and I am willing to provide it.

Candle Flame Flicker

For a project I wanted to make an LED flicker like a candle. I searched for the signal statistics of candle flicker, and I found no data. One student web site suggests that candle flame flicker is a 1/f-type random signal with roll-off of 20 dB per decade increase in frequency. Similar processes are typical for turbulence, so this student’s plan seemed reasonable. However, the student did not discuss whether the signal is Gaussian or not, and did not describe the low-frequency characteristics of the signal. 1/f noises may have a spectrum that follows the 1/f curve to very low frequencies, but because they would require infinite power at f=0 they always have some lower frequency change. I had no data.

Candles aren’t hard to get, and I already had a silicon photodiode for an absorptive smoke density measurement system I’m working on. I also had an analog-to-digital converter (ADC) in an ADS1015 on a breakout board from Adafruit. I made the very simple circuit shown below and attached it to a Raspberry Pi to sample the data. There are a lot more details, but those are later.


The voltage measured by the ADC is directly proportional to the current through the resistor. The current through the reverse-biased diode is directly proportional to the incident optical power. Very simple. I recorded a minute of data at a low 250 Hz sample rate, and subjected the data to analysis. The setup was a nominally dark room with the sensor a few inches from the flame. I flapped my hand at the candle to get it to flicker while recording.




The first graph below is a histogram of sample values.


The histogram is about normally distributed, but the right-hand tail is not Gaussian; it is too fat. In other words the candle is occasionally much brighter than normal. The time series (below) shows the same properties. There are clearly visible large excursions.


For human eyes the candle’s flicker will be well represented if the frequency spectrum of the flicker and the distribution of the flicker approximately match a natural source. The spectrum below can be thought of as an average brightness (the peak at the left), a flat spectral region out to about 4 Hz, and then a 1/f-style roll off at 44 dB per decade.


Numerically, we have the recipe for a flickering candle:

  • Samples should be normally distributed with a standard deviation equal to 0.25 of the mean.
  • The power spectral density of the signal should roll off at about 40 dB per decade with a 3 dB cutoff frequency around six cycles per second.

To make this work on the Arduino using the pulse-width modulated outputs, we can further constrain the problem:

  • The maximum value cannot exceed 255 counts—or make the limit that the mean plus two standard deviations is 255.
  • From this, we can derive that 2s+m = 255, use the fact that 0.25m=s to find that the mean m=170 and the standard deviation is about 42.
  • A sample rate of between 30 and 120 Hz should be more than adequate to satisfy the Nyquist criterion for human vision (see Wikipedia).
  • Values may not go below zero or above 255
  • A second-order infinite impulse response (IIR) filter has a roll-off of 40 dB per decade

If we can find a numerically efficient way to generate a time-series of Gaussian random variables inside the Arduino, filter them with a 2nd-order fixed-point IIR, scale them (if needed) then we should be able to make a flickering candle.

Unfortunately, I have failed repeatedly to get a stable fixed point IIR filter. The cookbook solution specification above should be easy to implement, but I have not found it so. A better solution will have to wait for another post.

Barbecued Ribs and Smoke Metrology

I made smoked ribs yesterday and tested out a new measurement system based on an Arduino, data logger shield, and a Sharp GP2Y1010 dust measurement unit. I didn’t have a long enough cable to get the smoke measurement unit inside the smoker, so I put it in my own contained pod above the smoker. By “pod” I mean the red re-purposed steel can on top.


The smoke extension pod has a port for sensor access and an adjustable damper. You can see the ribbon cable coming from inside the can to the data logger.


The sensor access port is on the bottom front of the picture below.


The smoke sensor, shown below, has some very small deposits around the sensor hole—the big hole in the center. In addition, I used the RIMU to log the temperature inside the smoker.


The temperature, oddly, was much too high. It is supposed to be 225F, but I appear to have set it at 265F. Perhaps there is a problem with the RIMU, but more likely there is a problem with the built-in sensor. The error in temperature control did not hurt the ribs, they were excellent. Further investigation is needed.temp_in_plot


The smoke voltage measured with the Sharp GP2Y1010 appears to distinguish between noise (between 0 and 1 volt) and total saturation. It is, nevertheless, a pretty good indicator of when smoke was applied. I loaded the smoker with chips at 10 am, and there was no smoke noticeable until 10:30 am. The full load of chips produced smoke for about 1/2 hour and then stopped almost completely. I put in a second, smaller, dose of chips at 11:30, which produced about 20 minutes of smoke from 11:45 until shortly after 12:00.

The period of time from about 14:30 until 15:30 is unexplained. I would guess that the ribs were slowly dripping, and each drip would burn and generate a short dose of smoke. It started at a time when I had moved the smoker onto the porch to avoid threatening rainstorms, and perhaps an angle changed. The smoker was shut off briefly, which explains the temperature drop around 14:20. The pattern of increased smoke does appear to match the temperature cycle.

The amount of smoke, or at least its duration, seems to relate well to the amount of chips. Smoke seems to start about 20 minutes after chips are applied, and lasts for up to 30 minutes.

That Noisy Fan–Calibrated Measurement

I am working with the freetronics microphone module, which is described as having a sensitivity of “-40 dB typical”—let us assume it is dB (SPL) referenced to 20 micropascals root mean square (rms) at 1 KHz. When I think of an electronic element’s sensitivity, though, I’m thinking volts per micropascal and this is not provided.

The microphone’s schematic suggests the SPL measurement is the rms average over 3 milliseconds, and that the signal is proportional to rms pressure level. This suggests a log scale for display (a change from previous work).

The upshot is that the fan generates about 62 dBA; but to get that conclusion, I had to perform a spectral correction to match measurements from a more calibrated sensor.


Remember that I made the measurements with the RIMU data logger I built. That data logger has a digital low-pass filter on the microphone SPL. The low-pass filter has much too slow a response, but the basic result is OK. The RIMU, shown in the next picture, is the instrument.


I borrowed an SPL meter from my father (thanks!). It is, unfortunately, a C-weighted measurement, measuring in dBC. The C-weighting is occasionally a very useful measurement of sound level. Most measurements are done A-weighted, which is similar to human hearing. My challenge is to convert a measurement made with an unweighted microphone in arbitrary units to A-weighted measurement in dBA. The answer is to take a measurement with the instrument in dBC, and record the sound a with a sound recorder, figure out what converts C-weighting to A-weighting for this signal.


In the spectra below you can see that the spectrum recorded without weighting, by the Zoom recorder. I then applied an A weighting and a C weighting. What’s important is the conversion between the rms value for A and the rms value for C, which is a 6 dB correction in one case, and a 13 dB correction in the other.




So, to rehash the steps

  • Record a sound level with the RIMU
  • Measure a reference condition in dBC with the SPL
  • Record a 15 second period with the Zoom
  • Apply the A weighting to the Zoom record
  • Apply the C weighting to the Zoom record
  • Find the difference in dB between the A and C weighted records
  • Assume the dBC weighting corresponds to the measurement made with the RIMU, and adjust the RIMU values so that they match the C-weighted measurement from the SPL
  • Apply the C-to-A correction to the RIMU measurement.

Pretty epic pain, but at least I have the measurements. In the future I will recode the RIMU to take short A-weighted and C-weighted snapshots, and then calibrate the RIMU on the dBA record. Look for a follow-up post far in the future.

Arduino Analog Sample Rate

The Arduino Uno is not the ultimate signal processing machine, but it can do some light duty work on burst data. I had trouble finding the maximum sample rate the Uno can support, which is totally critical for most kinds of work.

With native prescale

  • Samples, type casts into floats, and stores in an array at about 8929 Hz, or 112 microseconds per read-and-store
  • Samples and into stores into an array of unsigned integers (16 bit) at about 8930 Hz, or 112 microseconds per read-and-store

We can step this up quite dramatically by setting the prescale set to 16:

  • Samples, type casts into floats, and stores in an array at about 58600 Hz, or 17 microseconds per read-and-store
  • Samples and stores into an array of unsigned integers (16 bit) at about 58606 Hz, or 17 microseconds per read-and-store

Pretty close to a 60 KHz sample rate, more than adequate for audio sampling. Of course, the Arduino doesn’t have enough memory to be a serious audio processor, but it is pretty goood.

The forums discuss this, and arrive at a similar conclusion.

#define NSAMP 5000

void setup(){
  Serial.begin( 57600);
  float array_float[ NSAMP]; // float
  unsigned int array_int[ NSAMP]; // 16 bit
  unsigned long int micsbegf, micsendf, micsbegi, micsendi;
  for( int i = 0; i < 2; i++){
    if ( i == 1){
       // Set prescale to 16, and retry
    // Record floats (extra time for type conversion presumably)
    micsbegf = micros();
    for( int i = 1; i < NSAMP; i++){
      array_float[ i] = (float)analogRead( A1);
    micsendf = micros();  
    // Record floats (extra time for type conversion presumably)
    micsbegi = micros();
    for( int i = 1; i < NSAMP; i++){
      array_int[ i] = analogRead( A1);
    micsendi = micros();  
    if( i == 1){
     Serial.println("with prescale set to 16");
    Serial.print("recorded ");
    Serial.print( NSAMP);
    Serial.print(" float samples in ");
    Serial.print(micsendf - micsbegf);
    Serial.println(" usec");
    Serial.print("recorded ");
    Serial.print( NSAMP);
    Serial.print(" unsigned integer samples in ");
    Serial.print(micsendi - micsbegi);
    Serial.println(" usec");

void loop(){
  delay( 10);


Refigured Data Logger

The RIMU data logger I built last year received a major upgrade. I added a sound pressure level meter to learn how noisy it is and updated the case to make the switches easy to find, label, and operate.

It took a long time to get it working, mainly because I forgot that using the Adafruit Data Logger Shield occupies digital pins 10-13. Stupid mistakes were overcome and it is working now.

In the following picture you can see the sensor pod to the right and the main processor to the left. The reddish cable coil at the top right of the screen is the thermocouple, which I use to log high temperatures like the oven, barbecue grill, or smoker. The black cable running off-screen at the bottom right is for the thermistor. The thermistor is suitable for temperatures up to about 250 F, and is on thin enough wire that it can be placed outdoors while the electronics are sheltered indoors.


The display always shows the date and time—useful to confirm that logged data will be correctly time tagged. In the upper right of the display is a logging indicator (looks like L hugging a degree symbol). The degree symbol changes to a 1 when the logger is recording data.

The bottom row of the display shows the measurement from one sensor at a time. Hitting the mode button will step through all the modes. Unfortunately, this interface is awful. The Arduino is doing a lot, and it checks the button state as often as it can; however, it is common to press the mode button to no effect because the Arduino did not happen to check the pin at the time it was pressed.


There are 7 display states:

  • Thermocouple temperature (degrees C)
  • Barometric pressure (millibars) and temperature (C) from the BMP085
  • Visible light luminosity (lux) from the TSL2561
  • Infrared luminosity (lux) from the TSL2561
  • Sound pressure level (uncalibrated units, an integer between 0 and 1023)
  • Relative humidity (%) and temperature (C) from the DHT22
  • Thermistor temperature (F)

The relationship between the display states and the sensor pod are straightforward.


I’m happy with this design, but there are a few things I would change. First, I would like the state control to be a physical, mechanical switch. I could easily accomplish this with a 7-position (or more) rotary switch. I would wire resistors between each position, and then use an analog pin to read the state directly. A variable resistor with coarse detents would work too. So far I have not been able to find a variable resistor with coarse detents (but I can find one with very fine detents), and rotary switches are way too big for the case.

The other major thing I’d like to change is the physical interface between the Arduino case and the sensor pod. I would replace the terminal blocks with a finer pitch set that would fit on the board in a row and have enough room for labels. I would then use a bundle of 22 ga. stranded wires in different colors so that the sensor pod would be more flexible.

In software I need to make a change so that there is positive feedback to the user if there is an error writing to the log file. This should be easily accomplished, and is likely the next software change.

RIMU Calibration

I turned the freezer up to 7. I’m sure yours goes to 11, but I am not so lucky. After being turned up to 7, the resulting mean temperature is still too high at 18.8°F. And with that, we bid adieu to our refrigerator. It is around 20 years old—no way to be sure, and it has had a good life. I suppose the EPA would frown on a Viking funeral?


As a final check, I tested the thermometer by running it from a stirred ice bath up to a rolling boil. The ice bath temperature, averaged over about 1 minute, is about 33.1°F. A 1°F error does not present any problems.


Calibration in an ice bath is quite easy, but calibration at boiling is not. The pressure estimated from the RIMU predicts a temperature below the measurement, while the local airport’s barometric pressure predicts one above. In any case, the error is a few percent or less.

Freezer Swan Song

The last few weeks our ice cream sandwiches have been awful. I’m throwing away ice cream sandwiches, and I’m only marginally pickier than my dog. They’ve been gummy, almost squishy.

We have been suspicious of the refrigerator since we smelled a hot electric odor—like burning dust. I monitored the fridge with the RIMU, and it looked more or less reasonable. I monitored the freezer too, for one night. It is looking less good.


In addition to the short-lived odor, there is another reasons to be suspicious. The fridge gets warm on the outside, notably warmer than it used to.

Normal fluctuation in freezer temperature should be about 5 degrees, measured with a mass-damping technique…that I did not employ. I estimated the air temperature variation, not the food temperature variation. The air temperature variation is larger. The air temperature swings through a larger range than I would expect.

The average temperature, though, looks altogether too high: 16°F. The normal target temperature is 0°F. I’m assuming some of the spikes are because we were accessing the freezer, but they may all be defrost cycles.


We’ll continue fiddling with the temperature control, but likely the freezer is singing its swan song. That’s one note, about 0.25 millihertz.