BreadSat (satellite-looking, Solar-powered Blinking LED Circuit on Breadboards)

Solar-powered, fairly efficient blinking LED circuit built on breadboards and intended to look like a satellite. It should be able to go overnight after being charged during the day. Inspired by the work of Mohit Bhoite (https://www.bhoite.com/sculptures/) and Peter Müller (https://smolspaceprogram.com/project/smolsat_1/, https://smolspaceprogram.com/project/orion/). This Instructable assumes basic familiarity with Arduino and electronics.

Parts

1. 1 X ATTiny13/ATTiny13A/other low-power 1.8V microcontroller + programmer (e.g. https://www.digikey.com/en/products/detail/microchip-technology/ATTINY13A-PU/1914671 with an Arduino Uno)
2. 2 X solar cells (e.g. https://www.digikey.com/en/products/detail/anysolar-ltd/KXOB101K08F-TR/13999152, which is 4.5V, 40mA Pmpp)
3. 1 X Schottky diode (e.g. IN58xx) is preferred for its lower voltage drop, else a standard silicon/p-n junction/rectifier diode (e.g. 1N400x)
4. 1 X supercapacitor, preferably 3F, minimum 5V (e.g. https://www.amazon.com/dp/B0D9D43HLQ)
5. 2 X low-voltage LEDs (e.g. https://www.digikey.com/en/products/detail/tt-electronics-optek-technology/OVLLR8C7/827139 and https://www.digikey.com/en/products/detail/tt-electronics-optek-technology/OVLLY8C7/827140)
6. 2 X 68 ohm resistors or similar
7. 2 X 150 ohm resistors or similar
8. 2 X mini, 170-pin breadboards with adhesive backing (e.g. https://www.amazon.com/dp/B07VD8N727)
9. Solid-core, U-shaped breadboard wire kit, or 22 AWG solid-code wire
10. 1 X small rubber band or zip tie

Tools

1. Hot glue gun
2. Electrical tape
3. Soldering iron
4. Wire cutters, etc.
5. Multimeter

1. This will vary depending on your microcontroller and programmer. I used the instructions here to program an ATTiny13A using an Arduino Uno: https://www.instructables.com/Updated-Guide-on-How-to-Program-an-Attiny13-or-13a/, which has clear diagrams and instructions.

1. I tried using Microcore (https://github.com/MCUdude/MicroCore) to program my ATTiny13A, but found that I kept getting “Error: cannot get into sync” when trying to program it. The core recommended in the Instructable above (https://github.com/sleemanj/Arduino-ATtiny-Core) worked fine.

1. Again, this will vary depending on your microcontroller and programmer. On the ATTiny13A, we are really just setting fuse bits/configuring the microcontroller. The attached image shows the settings I used in my setup, with the aim being to reduce power consumption:
2. Lowered clock source and processor speed
3. Reduced “millis accuracy”
4. Kept brown-out detection enabled at 1.8V, which does consume some power but ensures processor does not lock up when voltage gets too low

1. Sketch is here, but again, you may need to change it depending on your microcontroller: https://gist.github.com/adkeswani/11acbc3b8901205f33455b56119493f3

1. My code is based on https://gist.github.com/petermllrr/4d1f32c103d030856d1df86088ab2983 from https://smolspaceprogram.com/project/orion/, except:
2. It always blinks instead of waiting for the solar cells to stop charging
3. It alternately double-flashes two LEDs instead of just one

1. NOTE: If you are using the “ArduinoISP” sketch / “Arduino as ISP” programmer and encounter an “invalid device signature” error, you will need to reduce the clock speed in the ArduinoISP sketch and reupload to the Arduino. See images.

1. If possible, this is a good time to test that the microcontroller is working and programmed correctly.

1. Assuming you are using the sketch provided above and an ATTiny13/ATTiny13A, LEDs should be connected to pins 2 and 3 (which should both be free) on the pinout. Connect the 150 ohm resistors in series with each LED to avoid damaging them. They should alternately double-flash, one every 8 seconds.

1. Our end goal is to create a circuit like the image.

1. The solar cells (represented as DC source symbols in the diagram) charge the supercapacitor. Once it reaches 1.8V, the microcontroller will turn on and start blinking the LEDs.

1. A blocking diode prevents leakage from the supercapacitor back to the solar cells.

1. If the LEDs have a 2V forward voltage drop and consume 20mA, then these would be the correct resistor values to use at different voltages. I guessed that with varying levels of sunlight, 68 ohm resistors would be good enough:
2. 5V = 150 ohms
3. 3V = 50 ohms

1. The following steps describe how I assembled the satellite, but you can assemble it however you want. The reason for having 2 breadboards is that my initial aim was to display the satellite in a window and have one LED on each side of the satellite. The version photographed here will have both LEDs on one side, but I kept both breadboards because it gives more space for wiring and I prefer how it looks. It is possible to make the whole circuit on one breadboard.

1. I hot-glued and soldered breadboard wires to the solar panels as shown.

1. Note that the one of the solar panels is flipped upside-down so that the positive and negative wires are in the same row on the breadboard.

1. My aim was to attach the solar panels to the breadboards using the wires alone, without needing additional supports. This is the reason for the additional loop in the wires attached to the negative terminals, and the reason for the blue, unconnected wire holding the solar cell wires down.

1. For aesthetic reasons, I wanted a small gap between the breadboards and the panels instead of being flush.

1. It was easier to solder the wires to the solar panel once they were glued down (I could have used clamps but didn’t want to risk damaging the panels). I covered the back of the panel with electrical tape afterwards to hide my terrible soldering job.

1. The attached images show how I wired the rest of the breadboards.

1. NOTE: There is a correction in the 3rd image. The 1st image does not connect the negative leads of LEDs to ground. In the 3rd image, the negative leads are joined by a barely-visible black wire in the top-right and then connected to ground by the diagonal green wire.

1. Note the "bridge" between the two solar panels so that they are wired in parallel.

1. Tip: Sometimes, perhaps with cheaper breadboards, it can be difficult inserting wires into the holes. You can first push in a stiffer wire, e.g. one with a DuPont connector on the end, to loosen it up.

1. Note that on the solar panel breadboard, the positive wires are on the left (with the blocking diode) and ground is on the right. We will connect these over the top to the other breadboard, and these wires also become useful for mounting.

1. The supercapacitor sits on top of the ATTiny and plugs into the bottom row of the breadboard. I used a rubber band to secure the supercapacitor better, since it is a relatively heavy component

1. This is a good point to test that everything is wired up correctly, before we stick the two breadboards together.

1. If you connect the breadboards together and then measure the voltage across the leads of the supercapacitor, you should see the voltage rising.

1. If you don't see this, try moving to a brighter location (though the capacitor should charge even under bright fluorescents). Ensure that your wires are connected to the correct rows and with correct polarities.

1. The rubber band can be stuck in between the boards or you can wrap it around both boards.

1. Some connections need to go between the boards. I used bent breadboard wires to form these connections. These wires also provide convenient points to tie fishing lines if you want to hang the satellite up.

1. Using a grommet/eyelet/rivet (not sure what it is) and a transistor with the middle lead cut out, I made a simple "radar dish", purely for aesthetics.

1. Plug the leads of the transistor into the breadboard through the grommet. Ensure that it is plugged in a way that does not interfere with the circuit.

1. Once the capacitor charges to about 2V, the LEDs should start blinking.