Solar lamps

[GoldingArts]

Hi,
I've been asked to look into setting up a series of solar powered lamps. The lamps are not meant to illuminate but rather glow to show off the design of the lamp itself.

I'm looking at creating a lamp that can provide basic white light, but the changed to show a basic colour or slowly changing colour sequence. I'd like to use a trinket for this as am familiar with it.

I'll create a simple relay powered by a photo-voltaic cell that will turn on the trinket at dusk, turn off at sunrise and switch over to charging the batteries.

The lights would ideally be on all night, but in winter this can be over 12 hours so I'm looking at options for the solar panel and batteries.

Outside usage, we have some extremes in weather, so I'll have to waterproof the casing and making sure it doesn't overheat/freeze. I'll be 3D printing the casings for this project, as well as the lamps.

Any suggestions appreciated!

[adafruit_support_mike]

Solar charging is a tricky process because the output from solar panels varies so much. Our Solar LiPo Charger has a circuit that can do it efficiently:

https://www.adafruit.com/product/390

The main restriction is that it requires a 6V solar panel. The circuits for panels with other voltages are more complicated, especially if you want any efficiency from them.

The Solar Charger turns itself on and off to respond to the existing light conditions, and automatically swaps between solar power and battery power to the load as conditions demand. Trying to control it with a timer would create more problems than it would solve.

For the lights themselves, you'll probably want to use NeoPixels:

https://www.adafruit.com/category/168

They're pretty much the standard for controllable lighting, and a Trinket can generate the control signals.

Using a clock to control when the Trinket generates the control signals would work, and the tool for the job is a Real-Time Clock. RTCs come in different levels of accuracy, with simple ones like the PCF8523 and DS1307 being good to within a few seconds per day:

https://www.adafruit.com/product/3295
https://www.adafruit.com/product/3296

and the DS3231 being accurate to within a few seconds per year:

https://www.adafruit.com/product/3013

The RTC and NeoPixels will use all the easily available pins on a Trinket, so you might want to consider the Trinket M0, which has six freely-available GPIO pins:

https://www.adafruit.com/product/3500

The last major concern would be LiPo performance in cold weather. All batteries lose performance when they get cold, as a side effect of the chemical reactions slowing down at a lower temperature. For LiPos, a rule of thumb approximation is that you lose 50% of the battery's capacity for every 20C of temperature drop. Most LiPos aren't rated to operate at all below 0C, so that would be another item you'd have to consider.

[pdh]

There are some simpler circuits on this page:

https://www.evilmadscientist.com/2008/s ... -circuits/

It shows a sequence of projects starting from very simple and ending with a microcontroller-based project like you're considering.

I've built the circuit described under "Detecting Darkness." The control circuit uses just a transistor, a diode, and a couple of resistors (no microcontroller). It uses a rechargeable NiMH battery which recharges during the day, and it lights an LED only at night -- like the solar "garden lights" that you can buy for a couple of dollars at hardware stores. As a bonus, NiMH batteries aren't too bad at moderately low temperatures (around freezing and a bit below).

The last examples show microcontroller-based circuits that add flexibility to the LED-lighting part.

The battery-recharge circuitry in these projects is pretty basic, but it seems to work OK for NiMH batteries because those batteries aren't as finicky as LiPos when it comes to recharging. But LiPo's have higher capacity than NiMH's, so it's a tradeoff...