power consumption suggestion

hello. i am designing an small device which uses an two ping sensor and i need it to be battery powered. and i need battery to last for atleast 3+months. now how do i put my arduino in sleep mode as idle or complete power down mode to save energy. ping sensor uses 15ma each. and i need to trigger the sensor for every ~3-5min and send it to hub each. which is connected to a low power xbee module(any other can u suggest i need a complete network of sensors. says 100nodes to send to master hub). also i will be using an custom etched board for this process any low power atmega chip which can run arduino in low power. how long a normal 2*aa battery can hold???

any ideas or links which will be usefull is welcomed

thank you

Check Nick Gammons "sleep" page - http://www.gammon.com.au/forum/?id=11497 -

Any Arduino with built in USB circuitry will never get low enough. You can't disable or sleep the USB side to get anywhere near that amount of time on any reasonably sized battery. When I tried this on my Uno R3 the best I could do was ~30ma

A 3.3v Arduino Pro or 3.3v Arduino Pro Mini could probably be made to go that long with a decent sized lipo. If the previous link looks overly complicated and all you want to do is go low power try the narcoleptic library.

I've got an Arduino Pro lasting about a month on a 1000mah lipo battery with an nrf24l01+ radio and a DHT22 temp/humidity sensor sending updates every 10 seconds over the radio.

It's unlikely 2 x AA batteries will be enough to get you the timeframe you need, with only 2 you'd need boost converter anyway. I would think something like a 2000mah or 3000mah lipo is more what you need to look at. If these nodes will be outside then a solar panel to charge the batteries would give you pretty much unlimited run times.

To give you an idea of where you need to get to, the Pro Mini when asleep with the above hardware attached but powered down uses 70ua. That's 0.07ma. If you can't power down the sensors you're using from the code then you'll need to either run them directly from a digital pin by connecting their VCC to it, or run them via a transister or relay so you turn them off while you sleep. Be aware some sensors takes some time to give valid results after powering up

I think you'd be better off with the above mentioned radios, they are about 1/5th the cost of xbee radios and are likely significantly easier to setup and run