Quote Originally Posted by polyglot View Post
You know you can get a plug to go in the bottom of a droid that gives you serial, etc? You can then plug an AVR or whatever into your droid and use that to do hardware interfacing, i.e. switching the enlarger directly from the phone. In fact, I'd be surprised if you couldn't use the CTS line or something on an FTDI chip to toggle your relay, no extra logic/micro required.
The only robust approach to interface the Android to some hardware are bluetooth and usb. Until recently, bluetooth was the only way to go to interface a device. However, Google has now introduce the USB host support for android device 3.1 and higher. Given that your phone has the hardware requirement, that is having a USB host capable chip, relying on the usb to make a connection to some hardware would be possible.

As the bluetooth module is quite relatively expensive compare to the rest of the component, it would be possible to reduce the cost of the timer interface by using the usb interface.

Quote Originally Posted by polyglot View Post
How do you deal with the screen fogging your paper?
By using a user interface only using red and turning the brightness to a minimum does the job. The test I made with my phone didn't show any fogging.

The only concern with using the android as timer in the darkroom is if the application gets overridden unexpectedly. For instance, by receiving a call. It's impossible with android to block other applications or to make your application monopolize the screen. As a work around, you can go to the art store and buy some transparent red sheets that you put over the screen. Or, you shut every network and enjoy your disconnected time in the darkroom