I am on my first Arduino project which will include 5 big capacitive sensors. I am using the CapSense library to continuously read them as the contact could be very brief. I need this to be portable so I am looking for ways to power my project. A single 9V battery? 5 AA batteries? I'm assuming I need to connect it through the voltage regulator, not through 5V since I need the power level to be fairly constant. Correct?
So assuming I connect the power through voltage regulator... do you think that the lowering of voltage in batteries through time will affect sensor readings? Or should the voltage regulator take care of that until the charge in batteries is too low for Arduino to use? In other words, does it guarantee 5V output at 6V (or more) input? And if not, is that a problem for capacitive sensors? Sorry for dumb questions, I'm new at this...
With capacitve sensing, you have to continiously poll, thus, probably no sleeping mode for your ATMEGA. That will drain your battery very fast. a 9V block battery is definitely not enough.
Can you go into detail what you are planning to do?
You need a step up / down buck boost regulator like this one.
Then run your system on AA batteries.
Proper batteries - alkaline AAs - are more than sufficiently stable to power the Arduino directly and you do not want to use a regulator with batteries if you can avoid it, certainly not a linear regulator. If you must use a regulator, a switchmode device such as Mike suggests would be preferred - unless you choose to save power by sleeping which you have said you do not.
Just use three AAs to give 4.5V. The CapSense system should be self-calibrating and in general you wish the system to self-calibrate over time to allow for changes in temperature and possibly humidity if your sensors are not well insulated (such as wooden components). If you use an 8 MHz clock system, it will allow for discharge of the batteries to under 3.3V.
Capacitive sensing uses negligible power, but other parts of the project would more likely be a concern.
Also, keep in mind that capSense is measuring the capacitance between the sensor plate and circuit common. Connected to a power supply, circuit common (aka "ground") is pretty large. But completely isolated by using a battery, not very large.
So it might be a good idea to either use a metal box and connect circuit ground to the box, or include a large metal plate inside the device. And don't finalize your code or resistor values until you've tried it running from batteries in something close to the final configuration.
Great, so I can bring power directly to 5V without using a regulator since periodic recalibration should make differences in battery charge irrelevant for readings.
Re: putting Atmel to sleep: what about using an external low power board to read sensors? Something like MPR121 Capacitive Touch Sensor Breakout Board (SparkFun Capacitive Touch Sensor Breakout - MPR121 - SEN-09695 - SparkFun Electronics)? Am I correct in thinking that I would get an interrupt when a contact would be sensed and that interrupt could wake up Atmel from sleep to process it? I could conserve power this way. Of course MPR121 uses 3.3V so that complicates things.
ElCaron I will place sensors under a rubber mat and beep when touch is sensed. Touches can be very brief - even just 40ms. The distance between sensor and mat surface will be about 5mm. System should work for at least 30 minutes, but more would be better
polymorph do you mean use metal box as housing for Arduino? I could do that. I am very curious what will happen once I disconnect Arduino from the computer. How do you debug software when you can't look at the serial monitor?
How do you debug without a computer? If it works, it is debugged. ;') You could temporarily connect an LCD display to show you the numbers it is getting from capSense when connected only to batteries.