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Charges during the day, blings (blinks) during the night.

Outdoor Christmas lights? year around? no batteries? Gigantic autonomous art installations? Yes it can be all yours.... add your own light or perhaps piezo buzzer for audio pranks.


Specifications:
- 5V solar cell
- 2.5V Super Capacitor with 10F provides energy storage
- Attiny85 runs the charge circuit and blinks the LEDs
- Completely sealed/waterproof in epoxy
- 3 pairs of LEDs blink depending on available storage voltage
   (2) green  blink at 2.4-2.5 volts
   (2) red blink at 2.1-2.5 volts
   (2) red using Joule Thief blink 1.1-2.1 volts
- Attiny sleeps between blinks and during charging consuming 2 micro amps
    2uA is really small; for instance, a single AAA battery can bleed 2uA for 42 years

Other Thoughts:
- Printed circuit board vs Drawn circuit board
    A sharpie pen is used to draw the circuits
- Super Capacitors… 100,000+ discharge cycles is 270 years of daytime charging
   Designing wondrous, chintzy gadgets into the future
-  perhpas design some type of real time clock so a number of lights can blink in unison at say midnight or only during the month of December.


Step 1: Circuit and Parts

The circuit is pretty simple. Attiny85 Microcontroller, Super Capacitor, a few LEDs. At first I just had a zenner diode protecting the super capacitor from over charging (past 2.5Volts) but the zenner bleeds!  Next I used the attiny85 to control charging and turn on one or more LEDs as the charge approaches 2.5 volts.

In the next couple of steps you can see how I etched the board. You may choose to order a printed circuit board OSHpark or you may draw your own circuit board as shown in the next steps.

Circuit Board from OSHPark. ~$8 per board

or make your own board using:
- copper clad board from Radio Shack
- PCB etchant solution from Radio Shack

Here are the electronic parts. I made a BOM through Mouser.  The total cost is $6.88
  • qty(1) Super Capacitor 2.5V, 10F. mouser
  • qty(1) Attiny85, DIP. mouser
  • qty(1) 8pin dip socket. mouser or Radio Shack
  • qty(2) TO-92 NPN transistors. mouser
  • qty(1) 1K resistors 0805. mouser
  • qty(1) 10K resistors 0805. mouser
  • qty(2) 100K resistors 0805. mouser
  • qty(1) 22pF capacitor 0805. mouser
  • qty(2) n-channel mosfets SMD. mouser
  • qty(1) zenner diode 2.4v smd. mouser
  • qty(1) diode smd. mouser

Next you may need these parts:
Adafruit Round Solar Panel, $2.95
Epoxy Mold , from TAP plastics, $11.95


Other Schematic Notes
                      +---\/----+
       RESET  1|            |5  VCC
       A3 PB3  2|            |6  PB2 A1 (SCK)
       A2 PB4  3|            |7  PB1 (MISO) PWM
            GND  4|            |8  PB0 (MOSI) PWM
                       +--------+
 
The capVoltage (PB2) is sensing the voltage between the zener diode and 100K resistor. I manually measured the voltage across the super capacitor and across the 100K resistor and calculated the ADC (@ pin PB2, capVoltage) 
****this is required for testing as the attiny85 does not have a Serial Monitor to tell you what it is reading*****

Here is an abbreviated table of measurements:
Capacitor(V)      100k(V)       estimated ADC
1.1v                    0.26v           0.26/1.1 * 1023 => 242
2.03v                  1.01v           1.01/2.03 * 1023 => 509
2.49v                  1.42v           1.42/2.49 * 1023 => 583
   
this works because the zenner voltage leaks current depending on the super capacitor voltage where as the 100k resistor draws current at a more predictable rate.  In lieu of using the traditional breakdown voltage of the zenner (2.4V specified) I'm using the fact that the diode drains a lot of current (even below 2.4V!)... this happens in somewhat a predictable manner (per the table above).
You must meditate on this for awhile. 

Digital input PB3 drains to ground when the daylight is not strong enough to open the mosfet gate (Q2). This works really well as solar cells will maintain relatively high voltage with no current and little daylight. It has to be pitch black for the mosfet to drain PB3!


Step 2: Circuit Layout

As I mentioned, you may prefer to purchase a PCB from OSHPark although I had a ton of fun drawing and etching my own board.  The copper etch really blings once the epoxy is added.

PCB ordered through OSHPark. They are $8 per board but you have to order three.  See also the Eagle files below.

1) Clean your board well with an abrassive pad and a light soap/detergent.
2) Draw your board design using a sharpie.
     - you'll want to make the design one-sided; that is, minimize traces that cross where you'll have to add jumper wires
3) Etch your board using an etchant solution.

Radio shack sells an etchant here.
Radio Shack also sells the copper cladding here.

I recommend reviewing some etching procedures as you are working with ACID and it can be dangerous and produce some fumes.

Here's an instructables using the Radio Shack product Ferric Chloride.





Step 3: Soldering on the Parts.

The only interesting trick I used was to bend the IC socket leads to solder as an SMD.  See the picture below.

1) solder your parts
2) add jumper wires where traces cross
3) add the positive and negative jumper wires to the solar cell connections


Here are the electronic parts. I made a BOM through Mouser.  The total cost is $6.88
qty(1) Super Capacitor 2.5V, 10F. mouser
qty(1) Attiny85, DIP. mouser
qty(1) 8pin dip socket. mouser or Radio Shack
qty(2) TO-92 NPN transistors. mouser
qty(1) 1K resistors 0805. mouser
qty(1) 10K resistors 0805. mouser
qty(2) 100K resistors 0805. mouser
qty(1) 22pF capacitor 0805. mouser
qty(2) n-channel mosfets SMD. mouser
qty(1) zenner diode 2.4v smd. mouser
qty(1) diode smd. mouser


Step 4: Programming the Attiny85

You need to use an Arduino to program an Attiny85 microcontroller. There are quite a few instructuables on how to do this.  Here's a great one: https://www.instructables.com/id/Program-an-ATtiny-with-Arduino/

I've attached my code below.

1)  Program your attiny using the code below and the instructions above
2)  Test your code before casting

Step 5: Casting



Casting is easy. I use the 2 part epoxy from TAP Plastics a lot. If you cast in Polypropylene (most yogurt/sour cream containers) the epoxy does not adhere to the surfaces.

1) place part in container
2) mix equal two parts epoxy
3) apply epoxy to ornament

Parts:
Epoxy Mold , from TAP plastics, $11.95

Notes: prior to epoxy I added a copper wire as a hanger (12 gauge grounding wire). Next I used a bit of hot glue to keep the epoxy from encroaching into the copper hanger.
Solar Bling is on display at the AFRU Gallery ByteMe 3.0 exhibit for the month of January 2014.<br> <br> AFRU Gallery<br> 534 Southeast Oak Street, Portland, 97214<br> <a href="http://www.afrugallery.com/event/byteme3/" rel="nofollow">http://www.afrugallery.com/event/byteme3/</a><br> <br> <img alt="ByteMe" height="400" src="http://www.afrugallery.com/wp-content/uploads/2013/12/03_Small.jpg" width="600"> /&gt;<br>
<p>Great project!</p><p>How did you make the attiny85 check if the capacitor is under 1.1v volts, if the attiny85 itself requires over 1.8v to operate?</p>

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