I am making a clock with a temperature sensors and have some questions about powering the whole thing.
What is the best way to power this thing? I don't want cables running to my clock so it has to we "wireless". Since this is going to be inside a house solar power is probably out of the question? I only have two solar panels from a solar garden lights. Probably the best thing is is to power it with battery? I have a 12V nimh accumulator (I think it is around 800mAh - I am not sure - it is quite big :P). So what is the best way to power the Arduino + 4 7-segment displays + temperature sensor + a LED or two?
So the only option is batteries, however then the life time is limited if you have LEDs as they take quite a bit of powe. How long would be good time for the battery to last?
Is this an aesthetics thing? Is it going to be on the wall? Do you own the home? If the answers to all those are "yes", then install (or have installed - make sure plenum rated cable is used) a plug behind the clock (they make ones that are recessed into the wall for flat-panel TVs).
If you want to use DC power, then install a plug for DC where your clock will be (use a telephone wall plate/jack, perhaps), and run the line to where your wall-wart for DC is plugged in, and hook that up (note: use plenum rated cable, plus keep the amperage low, to 500ma or less).
Beyond those options (and batteries), you really don't have much choice for an electronic clock; I can think of a few novel ways to beam power to the clock (a microwave "beam", using a tesla coil, maybe a high-power laser), but the power has to come from somewhere.
If you knew a clockmaker (don't we all?), you could try a mainspring-driven generator to recharge batteries periodically as needed, but that is probably waay beyond a DIY project like this (in order to get the efficiency needed).
Is the clock going to control something, or is it just to look at? Are you going to be away for long periods? Does it have to always be "on"? If you are only looking at it occasionally, perhaps some kind of motion detector could be used to turn on the display, otherwise it stays off; if you clocked the Arduino using the internal resonator @ 1 MHz, and pulled off several other tricks to keep things low-powered (number one would be not to use the Arduino to keep the time, but just to show the display - use a Real Time Clock (RTC) chip like is used on a PC motherboard for the clock needs - they can stay running for years off a CR2032 cell) - you could run this thing from batteries for a long time, if you re-think your assumptions, ideas, and resources (oh, and battery choice, as well).
From these options the cable is probably the best one. I was lucky and found a nice way to hide the cable - so that is not a problem anymore. What about a backup power battery. Since this is now going to be powered using a normal DC power. But I still need something to power it when there is no power. A 9V battery would probably run it for an hour or two? The main question here is how to wire the battey and the power source. Detecting when there is no power with arduino and shutting off displays would be good too - any ideas how that can be done?
That would be good. But I don't have this chip and getting it around here is a little problem (closest store that maybe has it is 60km away >:( ). I would order it on farnell but they won't just send me one simple chip.
So what about detecting power shortage and shutting down displays with arduino? Or something like wiring a DC power and a battery parallel to arduino? This only needs to work if I accidentaly unplug the power, or there is a power loss. So it needs to work for an hour at most.