For the next version of the widget, I’m planning to have an on board boost regulator. This design will allows the widget to run from just about any voltage source as low as 0.7v (so NiMh will be fine) and up to 5.5v, this boost regulator in conjunction with a standard LDO regulator will give me a nice input voltage range of 0.7v – 12V. Before I finalise the design I wanted to test things to see how well it’ll work.
I ordered in a couple of battery holders from Polou with an integrated boost regulator, since the widgets are designed for 3.3v operation I ordered this model http://www.pololu.com/catalog/product/796, that provides a 3.3v 100mA supply from a single AA battery, I’m not too sure what the minimum voltage is, but is running fine on a single NiMh cell.
Well today they arrived and I now have one hooked up to my solar powered widget. Installation was simple as the small pcb on the rear of the battery holder allows me to direct access to the cell contacts, so I soldered the solar cell +ve and –ve on the respective contacts.
From some previous tests with my solar powered widget, I was finding that the batteries would start charging when the solar cell output reached around 2.8 –2.9v, this is fine for during the day with the solar cell in direct sunlight, now with the single cell I’m finding that inside under artificial light I’m charging at around 1.5v, with plenty of overhead left.
You can see below the voltage output of the boost regulator is ~3.3v (3.322 to be exact) and the voltage from my solar cell is ~1.52v, this is inside under some standard down lights while writing this post at the kitchen table (actually one of the down lights is dead so I could expect a little more output from the cell)
Just to try something different I removed the battery, seeing if the regulator could run direct from the solar cell, unfortunately it doesn’t, I doubt the cell in the current light conditions can provide enough start up current for the regulator, I will however try it in the natural sunlight tomorrow.
With the addition of the boost regulator, and the widget in max power savings between sensor reading (when asleep widget is drawing 1.5uA, which is most of the time) then the widget should run indefinitely from the single cell….or maybe I could just get away with a solar cell and a super cap now there’s a future project….