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In the previous post we showed you how to make small Christmas shapes using an RGB LED and a small circuit based on PIC.
The designs were obtained working with the CNC some acrylic sheets.
But our CNC can do much more … Therefore we decided to make the greatest figure and design a new driver that mounts more LEDs.

Just because we do not like things simple we recreated all using a system based on Arduino.
This allows you to create an open source system easy to modify.
The microcontroller is an Atmega328 preprogrammed with the bootloader of Arduino UNO. The programming can be done via a USB / TTL (eg FTDI5V of SparkFun).
The circuit operation is very similar to that of the smallest model: here we find the photocell that allows to verify the amount of light present in the environment, but in this case, you can adjust the sensitivity of the circuit by a trimmer.

BOM

R1: 10 kohm
R2: 820 ohm
R3: 820 ohm
R4: 1 kohm
R5: 820 ohm
R6: 820 ohm
R7: 1 kohm
R8: 820 ohm
R9: 820 ohm
R10: 1 kohm
R11: 820 ohm
R12: 820 ohm
R13: 1 kohm
R14: 820 ohm
R15: 820 ohm
R16: 1 kohm
R17: 820 ohm
R18: 820 ohm
R19: 1 kohm
R20: 820 ohm
R21: 820 ohm
R22: 1 kohm
R23: 820 ohm
R24: 820 ohm
R25: 1 kohm
R26: 820 ohm
R27: 820 ohm
R28: 1 kohm
R29: 10 kohm
R30: 4,7 kohm
R31: 10 kohm
R32: 4,7 kohm
R33: 4,7 kohm
R34: 10 kohm
R35: 4,7 kohm
R36: 10 kohm
R37: 4,7 kohm
R38: 10 kohm
R39: Trimmer 4,7 kohm MV

C1: 100 nF
C2: 470 µF 25 VL
C3: 470 µF 25 VL
C4: 100 nF
C5: 15 pF
C6: 15 pF
C7: 100 nF
C8: 100 µF 25 VL

T1: BC547
T2: BC547
T3: BC547

LD1: LED 5 mm RGB c.a.
LD2: LED 5 mm RGB c.a.
LD3: LED 5 mm RGB c.a.
LD4: LED 5 mm RGB c.a.
LD5: LED 5 mm RGB c.a.
LD6: LED 5 mm RGB c.a.
LD7: LED 5 mm RGB c.a.
LD8: LED 5 mm RGB c.a.
LD9: LED 5 mm RGB c.a.

U1: 7805
U2: ATMEGA328P-PU (with bootloader)

Q1: 16 MHz

LDR1: photoresistor 2÷20 kohm

- Terminal 2 via (3 pz.)
- Socket 14+14
- Battery 12V/2A
- Strip male 6 via
- Plug
- Switch

Comparing the value read from the A/D converter connected to the photoresistor with that connected to trimmer R39, the micro decides whether to start the sequence of fading, or whether turn off the LEDs. RGB LEDs are driven by the transistor; this choice permit to control with a single line of microcontroller, more LEDs in order to create large luminous figures.

As you can see, we use a line dell’ATmega for each of the primary colors of red, green and blue, so it is clear that all the diodes will do the same play of light. Of course isn’t required to mount all the LEDs provided in the circuit: you mount those who need to obtain a good visual effect on the size of the pattern in the Plexiglass. Note that there are three terminals on the PCB: one to connect the power switch (ON / OFF) a second (BATT) to apply to the circuit power supply and a third (CHARGE) for a possible battery charger lead or a small 12-volt solar panel, which already incorporates the charging circuit. These solutions allow you to use the figures lack the power grid. In this regard, an analog input of the microcontroller is dedicated to control the battery voltage: when it drops below 10 V, the circuit will emit flashes to warn that the energy is about to end.

This circuit is evidently intended to be used externally (provided it is properly isolated) and will turn on and off independently, thanks to the ambient light sensor, which will illuminate the figures in the evening to let off in the morning. In short, is a solution to decorate the garden or backyard.

Building shapes
The materials chosen for this application are clear polycarbonate (or methacrylate) or Plexiglas, which can give the shape you want. Then we need to affect, deep enough (half or two thirds the thickness of the plate) the drawing or writing that you want to appear bright.

The Sketch

//****************************************************************
//*  Name    : RGB controller for common anode led               *
//*  Author  : Landoni Boris                                     *
//*  www.open-electronics.org                                    *
//*  blog.elettronicain.it                                       *
//*  www.futurashop.it                                           *
//****************************************************************

int red = 9;    // RED LED connected to PWM pin 3
int green = 10;    // GREEN LED connected to PWM pin 5
int blue = 11;    // BLUE LED connected to PWM pin 6
int photo = A4;    // BLUE LED connected to PWM pin 6
int trim = A5;    // BLUE LED connected to PWM pin 6
int volt = A2;    // BLUE LED connected to PWM pin 6
int r=50; int g=100; int b=150;
int rup; int gup; int bup;
int fader=1;
int inc=10;
void setup()
{
      Serial.begin(9600);
      Serial.println("Serial READY");
      rgb(r, g, b);
      r = random(0,255);
      g = random(0,255);
      b = random(0,255);

} 

void loop()  {

  Serial.print("trim  ");
  Serial.println(analogRead(trim)*2);  

  Serial.print("photo ");
  Serial.println(analogRead(photo)); 

  Serial.print("volt ");
  Serial.println(analogRead(volt));
  if (analogRead(volt)<600){
      Serial.println("low battery");
      rgb(0, 0, 0);
      delay(500);
      rgb(255, 255, 255);
  }

  if ((analogRead(trim)*2)>analogRead(photo)){
      Serial.println("trim > photo  -  off");
      rgb(0, 0, 0);
      fader=0;
  }
  else
  {
    if (fader==0){
      r = random(0,255);
      g = random(0,255);
      b = random(0,255);
    }
    fader=1;

  }
  //delay(2000);

  if (fader==1){
    funcfader();
  }
}

void funcfader(){
    Serial.println("fader");
    if (rup==1){r+=1;}
    else{r-=1;}
    if (r>=255){rup=0;}
    if (r<=0){rup=1;}

    if (gup==1){g+=1;}
    else{g-=1;}
    if (g>=255){gup=0;}
    if (g<=0){gup=1;}

    if (bup==1){b+=1;}
    else{b-=1;}
    if (b>=255){bup=0;}
    if (b<=0){bup=1;}

    delay(inc*2);
    rgb(r, g, b);
}

void rgb(int r, int g, int b)
{
  Serial.print("RGB: ");
  Serial.print(r);
  Serial.print(" ");
  Serial.print(g);
  Serial.print(" ");
  Serial.println(b);
  if (r>255) r=255;
  if (g>255) g=255;
  if (b>255) b=255;
  if (r<0) r=0;
  if (g<0) g=0;
  if (b<0) b=0;
  analogWrite(red, r);
  analogWrite(green, g);
  analogWrite(blue, b);
}

	Star
	Reindeer
	Santa Claus
	Tree

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When I was a little kid, my dad worked at Bell Labs. Every year around Christmas, we’d go visit him at work. One memory which has always stuck with me from my holiday visits was seeing a Christmas tree on an oscilloscope. I was pretty amazed by it. Engineers are a funny bunch — they tend to celebrate holidays in the most uniquely nerdy and wonderful ways, just like kids. When I recently acquired a new ‘scope and wanted to familiarize myself with it, I knew exactly what my test circuit was going to be.

In honor of the nameless BTL engineer whose scope scribbling captivated me as a child, here we are. Maybe the same thing will happen for some other kid. There are a lot of holiday parties coming up. You could put this on one of your scopes at work or at your hackerspace, and some other kid will see it, and it’ll fire their imagination too. It looks pretty neat at any rate, and it’s downright fascinating after a few fortified egg nogs.

While I suspect the original was built with 74xx logic circuits, or maybe a couple 555′s (you could easily make a Christmas tree shape by combining triangle (x) and sawtooth (y) waveforms), this one uses an AVR ATTiny2313. (I used my handy BB313 board). You could do this with an Arduino too (that is covered below) — pretty much anything that has an 8-bit timer with dual PWM outputs will do the job. The discussion will apply regardless of the implementation (though the numbers might change).

Before you continue, bear in mind that I assume you already know how to use an oscilloscope: how to attach the probes, adjust the voltage ranges, put it in XY mode, etc.

The ATTiny code is here, and this is the schematic for the circuit:

How it works:

The PWM outputs feed two RC low-pass filter circuits, with a time constant chosen to effectively suppress the jitter from the rectangular waveforms. The PWM outputs (and hence the DC components of the signals) jump from one point in the drawing to the next; in order to actually draw lines to make the tree shape, you have to interpolate between those points. The delay between one ‘drawing’ point and the next has to be chosen to allow the charge/discharge curve of the RC circuit to do the interpolation. You’ll notice in the photo above that the ‘straight’ lines are slightly curved. This is the because the jumps in x don’t quite equal the jumps in y. You’ll also notice that the tree appears a bit crooked. This is the result of the RC curve not settling out completely before the next transition — the voltages never quite get to where they’re supposed to go before they switch to a new value. Personally, I rather like these imperfections — the tree has a hand-drawn quality to it, like a homemade ornament. You could make the lines straighter and the drawing more uniform by using a longer delay, but it will lower the overall refresh rate, resulting in noticeable flicker. How you draw your oscilloscope Christmas tree says a lot about your personality.

The ATTiny’s clock is running off of a ceramic resonator at 16MHz. The PWM frequency is 16MHz/256 or 62.5kHz. The corner frequency of the filter is 160Hz, which is about 8.5 octaves (2.5 decades) below that. I used 10k resistors and 0.1uF caps here — you could also use any other “equivalent” RC combo (100k/0.01uF or 1k/1uF, etc.) If you choose to use a chip with a different clock frequency, then you’ll have to adjust the RC circuit accordingly.

Reducing the capacitor values (or increasing the resistor values) will shift the corner frequency of the filter up, and you’ll start to see the lines get crooked, like resistors in a schematic. These are triangle waves resulting from filtering the rectangular PWM signal. This can look pretty cool too, actually, as below where I switched to 0.022uF capacitors.

Now the tree has needles, and all it took was changing the coefficients of a partial differential equation — easy! (this might be a great way to introduce a kid to the concept of filtering — just sayin’).

Arduino Implementation:

Doing this on an Arduino isn’t much different than using the ATTiny, except that it’s more expensive and instead of a real schematic, you have to read this ridiculous handy cartoon (above).*

The Arduino code is here. It uses no libraries, so all you have to do is extract the folder into your Arduino directory, set up your oscilloscope and load the sketch.

Possible alternative methods:

• If you really, really want a perfect, orthogonal tree, you can use an R2R ladder or dedicated DAC/soundcard to generate the control voltages.
• You can get straighter lines if you use a current mirror to feed the RC circuit, thus making it a “true” integrator with a straight-line slope.
• If you want to increase the refresh rate, you could set an upper limit on the 8-bit timer to a lower value, say 64, and set all your points therein. Note on the ATTiny you’ll probably have to do this using an interrupt from the other timer, but it would give you faster PWM output, which means you can use a smaller filter time constant and thus a higher refresh rate.
• …or, you could overclock the AVR (you know you’ve always wanted to).
• As I mentioned in the beginning, you might be able to do something like this with a couple of 555′s generating triangle and sawtooth waveforms. If you get their phases matched correctly and with the right frequency ratio, you could have a pretty nice looking tree!

Be as creative as you like with it — I hope you enjoy making it work for you.

Merry Christmas!

*just kidding — I love Arduino! Buon Natale!

UPDATE – Several folks have done this themselves and written in:

EmbeddedEric did an awesome Frosty the Snowman with his daughters.

aussie_nick did one using a DSO.

Goran did one for the Calgary On-Air Engineers.

Gordon Pearce took it to the next level with animation using sine functions:

Nice!

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