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Time for a bonfire

Ian Morgan



No, I am not scrapping the layout yet. A few weeks ago, a club junior member said he fancied having a model bonfire on his OO layout. I thought I could have a go at that, using an Arduino Nano and a couple of LEDs and resistors. I even found a ready made Arduino sketch on the Internet that randomly varied the brightness of three LEDs independently (two red and one yellow) at random, short intervals. I had it all complete and working ready to hand over at the next meeting.


Having made a success of that, I thought I should have a go at a smaller scale one to add to Freshwater. I decided to use two red and two yellow LEDs, small 0603 Surface Mount Devices (SMD). I started by soldering them to a couple of 2mm Scale Association PCB sleepers, one with suitable cuts in the copper. The two yellow ones are in the middle, with the red ones at each end. The cathodes are all electrically connected together, and will go to the 0V of the Arduino. The anodes will each connect to a resistor, and then to outputs of the Arduino, which will be Pulse Width Modulated (PWM) with a maximum voltage of +5Volt. I used 470 ohm resistors, which need to drop about 3 volts (as the LEDs use about 2V, depending on colour), so Ohm's Law indicates the current through the resistor and LED will be V/R or 3/470, or about 6mA, which will produce quite a lot of light from these high brightness LEDs. Note that small SMD LEDs come with a variety of markings on the underside to show which end is which.




I then added some fine enamelled copper wire, salvaged from an old Nokia charger transformer. The finer wire was very fine indeed, and almost invisible. The LEDs were tested at each stage with a 9V battery and 1K ohm resistor. I did kill a couple of the red LEDs from thermal shock, taking too long with the soldering iron, but they are cheap and come in strips of reasonable quantities. Provided they still work after assembly, they should last indefinitely.




The sleepers were then trimmed and the LEDs tested again. I mounted the Arduino Nano on a piece of strip board so I could mount the 4 resistors and a connector for the power supply. The Arduino will accept between 5 and 12VDC, and I have a 9VDC power bus underneath Freshwater to supply the Servo controllers.




A small play with the Arduino sketch resulted in the following:


// LED Fire Effect

int ledPin1 = 10;
int ledPin2 = 9;
int ledPin3 = 11;
int ledPin4 = 6;

void setup()
pinMode(ledPin1, OUTPUT);
pinMode(ledPin2, OUTPUT);
pinMode(ledPin3, OUTPUT);
pinMode(ledPin4, OUTPUT);

void loop() {
analogWrite(ledPin1, random(150)+105);
analogWrite(ledPin2, random(150)+85);
analogWrite(ledPin3, random(150)+85);
analogWrite(ledPin4, random(150)+105);

Those that understand Arduino code will note I reduced the maximum brightness of the two red LEDs as they tended to dominate the yellows.


Here is the result under test, using some thin white paper to diffuse the light. The phone camera does not cope with the bright light source very well:




So,  a small hole was made in the baseboard to poke the LEDs through, and the Arduino was secured, and connected to the 9VDC bus. All that remained was to disguise the LEDs and build the bonfire. I used thin white paper, formed into a sort-of tent shape to cover the LEDs and diffuse the light. This was not easy to make and fit at this size, but eventually I got it done. Some odd bits of coffee stirrer, with black charring painted on, were glued on, and some small pieces of sea foam finished it off.




and a short video of it in operation:






  • Like 4
  • Craftsmanship/clever 4


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I had a play with a Seuthe smoke generator I have had knocking about for a long time. It produced too much smoke, and the smoke moves too fast for this scale, so I will not be bothering with it. Perhaps I should try one of those 'mist' generators?


Edited by Ian Morgan
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