The new AA Power board is a pretty darn flexible little board, if I may say so myself. Its switch regulator draws very little current itself, 7..30 µA depending on the input voltage. The input voltage range is approx 1.0 .. 5.5V, to be able to start up, but it will drain the power source all the way down to 0.4V if it can, pulling every last bit of juice out of regular AA batteries.
But the AA Power board is also capable of providing a pretty decent amount of current when needed. This is essential for the on-board wireless radio of the JeeNode, but it even works with heavier loads than that:
That’s an Arduino-compatible JeeNode with an LCD Plug and a 2×16 character LCD with backlight – all running off a single 1.2V AA NiMH battery!
The maximum current depends on the input voltage. It is guaranteed to be at least 60 mA from a 1V supply, going up to 140 mA from a 1.8V supply. Note that the input current can be much more than that – drawing 60 mA at 3.3V means the battery may have to deliver about 200 mA at 1V to make it happen. A clever little power regulator it is – a producer of energy it is not!
There a several ways to connect the AA Power board with a JeeNode:
- inline, feeding the PWR pin with 3.3V (this will be dropped some 30..50 mV by the on-board regulator of the JeeNode itself, with no ill effects)
- in piggy-back mode, with an AA cell inserted in the battery clips (alkaline or NiMh)
- as a shield on top of the JeeNode, again with an AA cell inserted
There’s also a fourth way to use this board: leave off the battery clips, and connect a battery between any PWR and GND pins on the JeeNode itself (or use the battery holes) – this requires a solder jumper on the AA Power board.
The flexibility of the regulator means that you can connect any power source between 1.0 and 5.5V to PWR and GND, just as you would with a stand-alone JeeNode. Whatever it is, the +3V pin will carry the essential 3.3V level.
There is one issue to beware of: when PWR is connected to BAT+ via the solder jumper, then do not hook up a second power source at the same time. The most common case is probably: when a battery is connected to PWR, do not connect an FTDI adapter such as the BUB, because it’ll put 5V on the PWR pin … and the battery (or the BUB!) probably won’t like it.
The PWR pin can in fact be used in four different modes:
- normal – it’s higher than 3.3V and the on-board regulator brings it down to 3.3V for the +3V pin – this would be the case with 3 to 8 AA’s, for example (no need for an AA Power board)
- boosted – it’s lower than 3.3V and it’s used to feed the AA Power board – in this case the on-board regulator does nothing (and could be omitted)
- parallel – the PWR pin is connected directly to the +3V pin – this can be used with the AA Power board to make sure the PWR pin also carries power (always 3.3V), in case some plugs expect a supply voltage on the PWR pin
- floating – the PWR carries no power – this is the case when the AA Power board is used without solder jumper (default case)
The important point here is that the PWR pins do not necessarily carry a higher voltage than the +3V pins. It might be more (normal), less (boosted), the same (parallel), or none (floating). Not every JeePlug can be used with each mode of operation, so be careful to check.
Tomorrow, I’m going to fool around with a bunch of batteries :)