This is the second post about a cell phone design project that started here.
The phone interface
What is the minimum required to place or receive a phone call ?
· A power button to turn on the device 
· A standard keypad 
· A send/end call button 
· A speaker and a mic, of course 
It's a mobile device so it needs :
· A hold button so you don't dial a number while the phone is in your pocket.
· A way to know your battery charge level 
· A way to know the reception you're getting at the place you're at 
A screen ? No. Why ? As said, the aim is to stay very basic and simple. We could use phones without screens for years, why couldn't we anymore ?
But we need visual feedback for some actions like :
· Dialing : to confirm a key press gesture 
· Connection established : yes you are slowly & surely eating your monthly plan
· Battery and reception levels as mentioned above.
The no screen manifesto prevents the use of a contact list, but we'll deal with it buy adding a memory key that can be used to fast dial up to 10 numbers.
Knowing that you'll lose only 10 numbers if the phone dies instead of the never backed up 76 ones you could have on a regular cell might help to accept this limitation. Or, consider that you won't need to play Dr. Kawashima to train your memory because you'll need to know all your numbers by heart.
Finally, the USB plug and its protective cap.
Schematically, here's what we end up dealing with :
Just to confirm that we didn't forget anything, let's run a scenario of placing a call from the turned off device.
1 - Check that the hold button is unlocked
2 - Press 2 seconds on the power button
3 - Continuous blinking signal from the reception level display to invite you to enter your pin number. If nothing happens, charge the battery by plugging the device to the nearest usb port around.
- - - - Starting from now, every key press will be confirmed by a single blink of a LED.
4 - Press the four digits of the pin number and dash symbol to validate.
5 - Three long blinks from the reception level display then information on the reception level confirm you accessed the network.
6 - Dial the number you wish to call and press the send/end call button.
7 - Once the number has been reached, the visual feedback LED is continuously on.
8 - Have a nice conversation.
9 - End call by pressing the send/end call button.
10 - Lock the phone with the hold button.
If the number you wish to call is one of the 10 memorized number of the phone, then the 6th stage becomes :
6alt - Press the memory key followed by the number of your choice.
When someone tries to reach you :
1 - The phone rings
2 - Check that the hold button is unlocked
3 - Answer call by pressing the send/end call button.
4 - The visual feedback LED is continuously on.
5 - Have a nice conversation.
6 - End call by pressing the send/end call button.
7 - Lock the phone with the hold button.
If you can't or don't want to answer the call, you can stop it from rigging by pressing the power button once.
Turning off the phone is as easy as pressing 2 seconds on the power button.