Arduinos are typically switched off simply by disconnecting the power. However, I've worked on several projects where this isn't ideal, the system really needs to go through a shut-down routine where SD files get closed and connected devices are told the device is going off line.
One of my recent projects could be improved greatly if it would send one last SMS when it shuts down, the problem is that I've really got no control of when a user pulls the plug! - no matter what instructions are given, there's really nothing stopping someone switching a unit off at anytime. So, what I'm thinking, is adding one or more ultracapacitors to the project, the idea being that when external power is pulled, the ultracapacitor will have enough charge to power the arduino for a few extra seconds so it can send a final SMS.
Does this sound plausible so far?
Lets say my project normally operates at 12 volts, and the arduino requires 5volts minimum. Ultracapacitors normally operate at 2.7 volts (see datasheet http://www.maxwell.com/products/ultracapacitors/docs/maxwell_technologies_product_comparison_matrix.pdf ) So i have 5 ultracapacitors in series that charge to 12 volts, and as soon as the external power gets disconnected then these capacitors will discharge into the arduino and power it for few seconds.
Here's the problem as I see it, an SMS can take up to 10 seconds to send, and requires quite a bit of power (2amps). Battery capacity is measured in mA/hour, but capacitor capacity is measure in farads (not something I understand), so how many and how big a capacitor would I need to provide 2amps for at least 10 seconds?
On a related note, anyone know where I can purchase (retail) ultra capacitors? The only distributors I can find are wholesalers.
All help appreciated