LED Tea Light

With Xmas not far away I’d ordered some LED tea Lights for the wife to make some decorations to give away to the family.  I was interested to find out what was driving the LED, but because all the candles were required I was not able to autopsy one to find out.  This got me thinking, I have an Adafruit Trinket that would be great to drive an LED via PWM and create the desired flame effect.


Now after searching the web for examples of what it was I wanted to do, I finally settled on this one from .ledhacker This is pretty much what I was after! Here’s a copy of that original code…

#define FLICKER_LED_PIN 10
// the start of the flicker (low)
static int flicker_low_min = 200;
static int flicker_low_max = 240;
// the end value of the flicker (high)
static int flicker_high_min = 230;
static int flicker_high_max = 256;
// delay between each low-high-low cycle
// low->high |flicker_hold| high->low
static int flicker_hold_min = 40; // milliseconds
static int flicker_hold_max = 80; // milliseconds
// delay after each low-high-low cycle
// low->high->low |flicker_pause| low->high...
static int flicker_pause_min = 100; // milliseconds
static int flicker_pause_max = 200;  // milliseconds
// delay low to high and high to low cycle
static int flicker_speed_min = 900; // microseconds
static int flicker_speed_max = 1000; // microseconds
void setup() {
int flicker_random_low_start = 0;
int flicker_random_low_end = 0;
int flicker_random_high = 0;
int flicker_random_speed_start = 0;
int flicker_random_speed_end = 0;
void loop() {
  // random time for low
  flicker_random_low_start = random(flicker_low_min, flicker_low_max);
  flicker_random_low_end = random(flicker_low_min, flicker_low_max);
  // random time for high
  flicker_random_high = random(flicker_high_min, flicker_high_max);
  // random time for speed
  flicker_random_speed_start = random(flicker_speed_min, flicker_speed_max);
  flicker_random_speed_end = random(flicker_speed_min, flicker_speed_max);
  // low -> high
  for (int i = flicker_random_low_start; i<flicker_random_high; <span="" class="hiddenGrammarError" pre="" data-mce-bogus="1">i++) {
    analogWrite(FLICKER_LED_PIN, i);
  // hold
  delay(random(flicker_hold_min, flicker_hold_max));
  // high -> low
  for (int i = flicker_random_high; i>=flicker_random_low_end; i--) {
    analogWrite(FLICKER_LED_PIN, i);
  // pause
  delay(random(flicker_pause_min, flicker_pause_max));

So I loaded this on the Trinket and at first I didn’t include a current limiting resistor and the LED showed a slight flicker, but not anything like a flickering tea light! After adding the current limiting resistor (220Ω)and that didn’t really change much except for dim the LED ever so slightly.  So obviously I needed to tweak the code and see what I end up with..

Now my first thoughts on why it didn’t work out as expected is that the code was obviously written for a 5v logic, however this trinket is 3.3v logic. I made the following changes (after a little fiddling)and got more of what I was after  🙂

< #define FLICKER_LED_PIN 10
> #define FLICKER_LED_PIN 0
< static int flicker_low_min = 200;
< static int flicker_low_max = 240;
> static int flicker_low_min = 15;
> static int flicker_low_max = 175;
< static int flicker_high_min = 230;
> static int flicker_high_min = 180;
< static int flicker_hold_min = 40; // milliseconds
> static int flicker_hold_min = 20; // milliseconds
< static int flicker_speed_min = 900; // microseconds
> static int flicker_speed_min = 800; // microseconds

As you can see I only made small changes to some timings and they seemed to work better for the 3.3v logic

The next thing was how long would this run on a 1000mah lipo battery? well, somewhere between 4 to 5 days!

I’ve placed the code and supplement files on github should you wish to make one. 🙂

© 2016, Scott Evans. Creative Commons License
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