By Jeremy Cook
With rising energy prices and environmental concerns, many are turning to solar power. There’s a lot to love about this technology, as it literally pulls usable energy out of thin air. However, since the sun arcs across the sky, rarely is a static solar panel aimed directly at our solar system’s “energy orb,” meaning less-than-perfect efficiency.
This problem can be readily solved with automated panels, which turn to face the correct direction. Building a solar tracker is fairly straightforward and can be accomplished with light-dependent resistors and a microcontroller. Instructions for how to make a solar tracking circuit are laid out in this article, which could serve as inspiration–or even as a starting point–for a larger scale system.
Component setup
For this build, the breadboard setup moves via a gearmotor attached to a 3D-printed assembly. Components-wise, you’ll need the following:
- ATtiny85 microcontroller
- Small gearmotor
- L293D motor driver
- (2) Light-dependent resistor (LDRs)
- (2) 2kohm resistors
- Small breadboard
- Breadboard power supply
- Wire
- 3D-printed mechanical components
- Steel base (for weight)
This solar tracker circuit, shown below, uses a pair of voltage dividers to feed voltage values from the left and right LDRs to the ATtiny85’s analog-to-digital converter pins (ADCs). These convert 0-5V signal to 0-255 values. Based on this information, the microcontroller is able to decide whether to rotate left, right, or stay in the same place to point toward a light source.
Arduino IDE code for this operation is shown below and is available for download on GitHub, along with the 3D-printable base and LDR shielding design. The setup is simplified by having the breadboard act as both a mechanical component and as the electrical connection system. Note that values may need to be adjusted depending on your LDR and resistor setup.
ATtiny85: Low-power superstar
The ATtiny85 microcontroller used here is an extremely small all-in-one control package, set up to run at a 1 MHz internal clock. In active mode at 1 MHz with a 1.8V supply, this microcontroller draws only 300µA of current, and a fraction of that in power-down mode. In this build, the processor is supplied with around 5V, which means it uses about a single mA of current when active.
Compared to other devices, such as the ESP32 (which gives you wireless capabilities), this is a minuscule amount of power. Internal power drain is something that should be considered when designing energy harvesting devices, whether on a large or small scale.
Development considerations for solar bots
This device was put together in a short time to show how an autonomous sun tracking device can be simple to set up. In fact, it would even be possible to build a similar system based on discreet electronics, doing away with the microcontroller altogether.
At the same time, using a small computing device like the ATtiny85 provides lots of options as to the device’s behavior. For example, it may be advantageous to wait several minutes between checking the sun’s location, since the sun’s arc is slow and consistent, or even to take an average of LDR values to account for temporary cloud effects. You could even use a PID setup to smooth out motor operations, or move based on any number of other mathematical functions.
Once you move past the basics, a microcontroller quickly pays for itself in simplicity. You could even apply two more LDRs and a plus-shaped light blocking jig to construct a full up/down/left/right tracker. You’d need another motor, and likely a more capable microcontroller (although the L293D module can accommodate a second motor without extra hardware).
One difficulty with using an ATtiny85 by itself is that there’s no built-in serial output or onboard programming capability. This means that tuning may require a more difficult trial-and-error process than if you could continuously monitor things on a computer screen. Reprogramming means removing the chip and inserting it into a programmer, or using an in-circuit programming arrangement. Neither is an insurmountable problem, but a simple dev board, like an Arduino Uno or similar, would make things much easier.
The downside is that you’ll eventually want to replace the dev board with another option, as such devices can be comparatively power hungry. The Arduino Uno, for example, uses nearly 50mA in its standard mode, or around 50 times the ATtiny85’s requirements at 1MHz.
One final consideration: many USB power packs shut themselves off if current isn’t being drawn. It seems that the ATTiny85’s 1mA current needs, plus the breadboard power supply, aren’t always enough to keep power flowing. A cheaper model may actually work better here.
Solar panel robots: Beyond local control
As admirably small and inexpensive as the ATtiny85 is, for other sun tracking you might consider upgrading to an ESP32 or other wireless-capable device. Such a setup could track the sun’s position without using any onboard sensors at all, relying instead on calculations based on the location, time, and date. It could also provide power generation and status feedback to your phone or a central system.
Of course, if you were to design and build a solar tracker for practical usage, you would need to ask the question of whether this sort of “solar panel robot” is indeed the best solution. In some cases, adding a few more panels to a solar array would produce a more cost-effective setup. This project is an interesting technology demonstrator, but don’t be afraid to step back and consider what works best for you!
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