I’ve been playing with idea of making tiny free-formed circuit sculptures in the shape of a satellite for a while now. I build a few LED blinker before, but this time I wanted to build something that did more than just blink an LED. I recently came across a small low power LCD module with a built-in driver and controller that you talk to over I2C. Most LCD modules out there are bare LCDs without the driver or controller, and the ones that have built-in drivers are too big. This particular one seems to be a perfect odd duck. So I decided to build a “satellite” around it using an ATtiny85 microcontroller and SHT31 sensor. The four solar cells connected in parallel charge a small super capacitor. The MCU periodically wakes up, takes a sensor measurement, displays the reading, and then goes to sleep to conserve power.
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Parts list- ATtiny85
- Four KXOB25-02X8F-TR solar cells
- A diode with low forward voltage drop.
- Adafruit SHT31 sensor breakout board or you can also use this one.
- 4 digit LCD module
- 1.0F 5.5V supercapacitor (you can also use a 3.8V 50F hybrid supercapacitor instead, for longer run times)
- 20 AWG brass rods
The schematic is simple. The LCD and the sensor are connected over I2C. No pull up resistors are required since the sensor board already has them built-in, but if yours doesn’t, then you’ll have to connect them separately. The four solar cells are connected in parallel and they charge the supercapacitor via a reverse polarity blocking diode. You can use any small diode with low forward voltage drop.
I would recommend first assembling the circuit on a breadboard. For programming the ATtiny85, I used the Pololu’s AVR programmer. You can also use an Arduino UNO to act as a programmer instead.
(I failed to take photos during the build process with my DSLR, so here are some crappy photos from my phone instead.)
I used a small proto PCB to align the joints. Once I soldered the sensor, I removed the proto PCB and replaced it with the LCD module.
The device can also directly power up from the solar cells alone in direct sunlight.
The code requires a couple of device libraries. An SHT31 sensor library from Adafruit and a CDM4101 library for the LCD. Ensure that you have them installed via the Arduino IDE.
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