Skip to main content

Measuring current draw with LTC4150 + ESP-12E

My LTC4150 Coulomb counter has finally arrived!

For testing, I hooked up the unit to the spare ESP-12E I have lying around:


All the jumpers on the LTC4150 are soldered (SJ1 = interrupt-driven counting; SJ2, SJ3 = 3.3V circuit).

The code for driving the Coulomb counter is as follows:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
#include <Time.h>
#include <TimeLib.h>

const int BATTERY_CAPS = 2300;
const byte INT_PIN = D1;
const float INT_TO_COULUMB = 0.614439;

bool trigger = false, init_done = false;
unsigned long total_time = 0, total_interrupts = 0;
volatile unsigned long num_interrupts = 0;
volatile unsigned long time1 = 0, time2 = 0;

void debug(const char *format, ...) {
  char buf[256];
  va_list ap;
  va_start(ap, format);
  vsnprintf(buf, sizeof(buf), format, ap);
  va_end(ap);
  Serial.println(buf);
}

void handleInterrupt() {
  if (time1 == 0) {
    time1 = millis();
    init_done = true;
  } else {
    num_interrupts++;
    time2 = millis();
    trigger = true;
  }
}

void setup() {
  Serial.begin(115200);
  pinMode(INT_PIN, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(INT_PIN), handleInterrupt, FALLING);
} 
 
void loop() {
  if (init_done) {
    Serial.println();
    init_done = false;
  }

  if (trigger) {
  cli();
    trigger = false;
    unsigned long interval = time2 - time1;
    unsigned long ni = num_interrupts;
    time1 = time2;
  num_interrupts = 0;
  sei();
  total_time += interval;
  total_interrupts += ni;
    float ma = (ni * INT_TO_COULUMB) / (interval / 1000.0) * 1000.0;
    float ma_avg = (total_interrupts * INT_TO_COULUMB) / (total_time / 1000.0) * 1000.0;
    float lifetime = BATTERY_CAPS / ma_avg / 24.0;
    debug("\ninterval = %ldms; num_interrupts = %ld; ma = %fmA; ma_avg = %fmA; lifetime = %f days",
      interval, num_interrupts, ma, ma_avg, lifetime);
    time1 = time2; 
  }
  else {
    Serial.print(time1 == 0 ? '#' : '.');
  }
  delay(10*1000);
}

When the load is a 1K resistor, expected current draw is ~4V/1K = ~4mA. The following output was observed:

interval = 150862ms; num_interrupts = 1; ma = 4.072855mA; ma_avg = 4.072855mA; lifetime = 26.598871 days
interval = 150775ms; num_interrupts = 2; ma = 4.075205mA; ma_avg = 4.074029mA; lifetime = 26.591200 days
interval = 150829ms; num_interrupts = 3; ma = 4.073746mA; ma_avg = 4.073935mA; lifetime = 26.591818 days
interval = 150710ms; num_interrupts = 4; ma = 4.076962mA; ma_avg = 4.074691mA; lifetime = 26.586882 days
interval = 150850ms; num_interrupts = 5; ma = 4.073179mA; ma_avg = 4.074389mA; lifetime = 26.588854 days

When the load is a 47K resistor, expected current draw is ~4V/47K = ~0.08mA:

interval = 8380827ms; num_interrupts = 1; ma = 0.073315mA; ma_avg = 0.073315mA; lifetime = 1477.645142 days
interval = 8124328ms; num_interrupts = 2; ma = 0.075630mA; ma_avg = 0.074454mA; lifetime = 1455.033325 days
interval = 7995918ms; num_interrupts = 3; ma = 0.076844mA; ma_avg = 0.075234mA; lifetime = 1439.949219 days

I aborted the test after 3 readings because the current draw is so low it was taking too long to get 5 readings. But I think the readings are consistent enough to conclude that the power meter circuit gives reasonably accurate readings.

Comments

Popular posts from this blog

Adding "Stereo Mixer" to Windows 7 with Conexant sound card

This procedure worked for my laptop (Thinkpad E530) with a Conexant 20671 sound card, but I suspect it will work for other sound cards in the Conexant family. I was playing with CamStudio to do a video capture of a Flash-based cartoon so that I can put it on the WDTV media player and play it on the big screen in the living room for my kids. The video capture worked brilliantly, but to do a sound capture, I needed to do some hacking. Apparently, there was this recording device called "Stereo Mixer" that was pretty standard in the Windows XP days. This allowed you to capture whatever was played to the speaker in all its digital glory. Then under pressure from various organizations on the dark side of the force, Microsoft and soundcard makers starting disabling this wonderful feature from Windows Vista onwards. So after much Googling around, I found out that for most sound cards, the hardware feature is still there, just not enabled on the software side. Unfortunately, to

Attiny85 timer programming using Timer1

This Arduino sketch uses Timer1 to drive the LED blinker: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 /* * Program ATTiny85 to blink LED connected to PB1 at 1s interval. * Assumes ATTiny85 is running at 1MHz internal clock speed. */ #include <avr/io.h> #include <avr/wdt.h> #include <avr/sleep.h> #include <avr/interrupt.h> bool timer1 = false , led = true ; // Interrupt service routine for timer1 ISR(TIMER1_COMPA_vect) { timer1 = true ; } void setup() { // Setup output pins pinMode( 1 , OUTPUT); digitalWrite( 1 , led); set_sleep_mode(SLEEP_MODE_IDLE); // Setup timer1 to interrupt every second TCCR1 = 0 ; // Stop timer TCNT1 = 0 ; // Zero timer GTCCR = _BV(PSR1); // Reset prescaler OCR1A = 243 ; // T = prescaler / 1MHz = 0.004096s; OCR1A = (1s/T) - 1 = 243 OCR1C = 243 ; // Set to same value to reset timer1 to

Hacking an analog clock to sync with NTP - Part 5

This is how it looks after I have put everything together. The Arduino sketch is available here . The 2 jumper wires soldered to the clock mechanism are connected to pins D0 and D1 on the ESP-12 (in any order). When the device first boots up, it presents an access point which can be connected to via the PC or smartphone. Once connected, the captive portal redirects the web browser to the configuration page:     A custom field has been added to the WiFi configuration page to enter the current clock time in HHMMSS format. Try to set the clock time to as close to the current time as possible using the radial dial at the back of the clock so the clock will have less work to do catching up. In the config page, the HTML5 Geolocation API is also used to obtain your current location (so if your web browser asks if you would like to share your location, answer "yes"). This is then passed to the Google Time Zone API to obtain the time and DST offset of your time z