Low power solar. What is the "best" way?

Good evening.

Not posted here in a good 5 years (moved career...now a full time Java/Spring developer mainly thanks to Arduino!).

I have gotten back in to Arduino/electronics as a hobby and was just wanting to simply power a small MCU (ATTiny85 or 84) which just uses a DHT sensor, takes a reading every say 15/30/45/60 minutes, send the result to a "gaetway" via NRF24L01 and then goes back in to a deep sleep.

I know there are circuits etc. out there but I can't decide which is the best solution.

So, for example options:

  1. Solar panel and small LiPo.

Advantages are LiPo can hold a lot of energy for my requirements.
Disadvantages are its a LiPo can costs more (charging IC etc).

  1. Solar panel, supercap and some discrete components.

Advantages are its "safer" and cheaper.
Disadvantage is a super cap does hold less "charge".

Then my next issue is having to regulate the voltage AFTER then lipo/super cap.
I know you can get low quiescent current LDOs but would you have the entire side with the MCU and NRF on 3.3V after a 3.3V LDO or would you have the supercap/lipo basically connected to the MCU and only turn on via a transistor the 3.3V LDO for the NRF when "needed"? I guess this transistor could also turn on the DHT only when needed.

Which of those options would give the best performance or is it extra building and design for no real gain?

Thank you for any insight :).

Solar panel and LiPo is very popular and components are "off the shelf" at the usual suspects. SparkFun, Adafruit, Pololu, and Seeed to name a few - they have different panels, chargers, regulators, and Li cells so you may want to check more than one and mix and match.

I would suggest getting one size larger than you think you need. This will allow for mistakes in computing what you need or one level of feature add on later.

Nick Gammon posted an excellent tutorial on just such a remote solar powered Arduino sensor. He used a supercap to avoid the battery charging problem completely.

See: https://www.gammon.com.au/forum/?id=12821


Excellent. That is exactly the sort of thing I want. Thank you very much!

I got a really cheap solar garden light (one that mounts on a fence with motion detection) and put a small 328p board inside it that has an RFM69 433MHz transceiver on it. It currently just reports outside temperature from a DS18B20 back via a gateway to Domoticz. I'm planning on repurposing the built in PIR sensor to report back motion as well.

For around 3 GBP you get a fairly waterproof box with a solar panel, lipo battery, charging circuit and a motion sensor.

EDIT: It's not a LiPo, but an 18650 Li-Ion battery.

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Great tip, been looking to do something with a solar powered setup.

Ordered a couple ....................

Yeah these look rather good for outdoor.
I was hoping for "small" as I may place these around the house in various places and they would look a little off. But for the outdoor sensors they are essentially as you said, an off the shelf solution.

I am looking at Nick Gammons stuff @jremington linked and he uses:

  1. Atmega328P-PU (~44uA in power down)
  2. MCP1700 (~1.6uA Quiescent)
  3. 0.47F 5.5V Supercap.

This all seems to work for him and its minimal parts.
Using an ATTiny84 (4uA) or even the ATtiny841 (1.52uA) and both can go down to 1.8V at 4MHz I think I could end up with a very small form factor which would mean they are easily just left on a windowsill or a bright part of the room and barely noticeable and also without the Brownout Detection on and no need for the external IC to reset the board each day in case of a brownout.

But definitely will be buying one or two of those for outside! Thank you :).

These are some interesting 6V panels which may be perfect for what I need.
His larger solar panel would charge the cap in something like 15s...so I am fairly sure even these could be enough (40mA about 35mm across).

They are ok. The original solar units didn't have an on/off switch like these cheap ones do. From memory, I leave mine in the off position. That way the motion detection doesn't trigger the LEDs, but it keeps charging.

I use a Moteino clone bought from Ali Express (fits in - just) that uses a 328p, has a footprint for one of the RFMxx series of transceiver modules and a footprint for a flash device that can be used for OTA software updates - but i've not tried that bit.

I'm going to repurpose the built-in PIR sensor for my own needs.

The only downside that I can see coming is that the cheap solar panels they use eventually frost over and become useless.

If you want to pay for it, DFRobot has a nice solar selection.

You got LiPo in this? Mine had a normal 18650 Li-ion cell inside. I fitted couple more cells to make it a 3-pack :stuck_out_tongue:

If by "original solar units" you mean the one of your earlier post, that rubber thing next to the PIR sensor is an ON/OFF switch for the light. Charging works regardless of its position.

Ah, could have been a Li- Ion, now that you mention it.

The original units that first came out didn't have the on off switch, but the cheap clones do, which allows you to keep the unit switched off (but still using solar energy to charge the battery) without the PIR triggering the LEDs.

Well made the general shopping list. I still have small caps and resistors from the many years a go, some of the parts he used were not available so I have alternatives.

Think this should be fine?

0.47F 5.4V Supercap
1N752A Zener Diodes
MCP1702-LDO-Low Quiescent 250mA 3.3V
Diodes - 1N4148a
4MHz Crystals (Optional)

Thank you to all, the parts arrived so doing some testing.
Seems to be working.
I am going to top up the capacitor and see if it just holds its charge with the MCP1702 overnight.
If I get the ATTiny84A working and will just see if I can get it to stay alive all night on a charge :).

If not frowned upon, could I ask where you got this light? Thanks.


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No problem, I got mine from ebay in the UK. The seller I got mine from hasn't got any more.

The key words seem to be: solar PIR 20 LED motion wall light

Got it, thank you.

No problem. Depending on the size of your project, there are other larger wall mounted solar PIR motion sensing units - usually with dozens, or 100's of LEDs. They have a bigger solar panel and potentially a larger void inside the casing to house your project.

Chinese goods store, maybe dollar/euro/whatever-your-currency store.

Thanks, been meaning to get something like this for a while but couldn’t choose as there were so many. Your feedback helped me decide.