You might be thinking that the project is finished and that you should take what we have already, slap everything in a box and start using it. Indeed, you could, but we need to talk about the most important thing of all : POWER.
Until now I powered both the receiver and the transmitter via the USB cable used to program the Duemilanove. However, I want portable sensors, not sensors that have to be plugged in to a wall socket or computer.
How much current does our Arduino Duemilanove need? I’m glad you asked! Too much, that’s for sure. Take a look :
That’s right – 25.73mA doing nothing but reading the signal on the PIR data pin. Current goes up when the alarm trips to around 30mA. OK, so I originally wanted to power these sensors with a 9v battery for convenience sake but I can obviously forget that. Never mind that 9v is WAY more voltage than I need to power this thing but most 9v batteries are rated at something like 500mAH. That means that the battery will (in theory, at least), let you have 500mA for 1 hour before it’s “dead”. Lets see – if we need 30mA and we have a 500mAH battery that will last…. Err…. (500/30 = 16.666666666666 (etc)). Around 16 hours per battery? Uh. No. That won’t work at all. Not even remotely close. Keep in mind that the rating on any battery is essentially “best case”, too.
It’s time to rethink just putting this remote sensor in a box. That’s part 4 😉