I recently mentioned the problem of un-locking various devices in the dark while not being able to see the instructions provided on the screen. Here’s what I had to say about the topic last fall.

It’s an HCI pattern, written in the fashion of Cristopher Alexander and a part of a larger design pattern language, dealing with mobile phone interaction. As I don’t believe the language really works as a language [mainly because there are far too few patterns in general in the language of ours, it was a school project, anyway] I will not publish it as a whole just yet.

The role of design patterns in the field of HCI strives to be something more than just a collection of trickery and work-arounds, as often is the case with traditional design patterns dealing with programming. The idea is to go to the roots and in an alexandrian fashion capture the timeless qualities of working solutions in a way that is also understandable to the end user, so that they can be involved in the design process as well.

But I don’t claim that this pattern would necessarily do all that…
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Discoverable Keypad Lock**

You have decided on the candybar format for the phone Form Factor which leaves the Button Layout exposed to accidental pressing. Hence, you will need a way to lock the keypad.

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Keypad locks need to be to easy and intuitive to open – also in the dark. Then again, too simple locks open accidentally and unneeded screen usage wears out battery.

Most candybar shaped cell phones have a method of locking the keypad, preventing any unwanted key presses while the phone is carried. Each cell phone manufacturer has their own way of locking and opening keypad locks: the procedures most commonly used are abrupt key combinations or holding of one button for a certain time.

Due to their nature, keypad combinations are hardly discoverable and hence violate guidelines of making things visible. The requirement of long continuous key presses, on the other hand, only utilizes one button and can be made more visible with a special symbol on the key, but unfortunately introduces the dimension of time to the procedure – making a simple task more complex than necessary.

Cell phones try to bridge the gap by giving on-screen instructions. On some models the screen and keypad are lit whenever a button is pressed, on the others it will not be lit until the lock is deactivated. The latter makes it essentially impossible to open the keypad in the darkness unless you know the key combination and the location of the keys by heart, as you are not able to read the instructions from the dark screen. The former wastes valuable battery power by lighting up the screen and keypad on every accidental key press.

The solution is to create an intuitive way of locking and unlocking the keypad and using just enough light to communicate the operation to the user. One possibility is to include a mechanical hold switch that does not slide accidentally but still remains easily discoverable. When the switch is activated and any key is be pressed, only the switch itself will be lit – providing an affordance of opening without the need of textual instructions and also saving battery power. A physical switch also communicates its status as opposed to a toggle button and is hence a non-modal solution.

Good and bad examples

• The phones manufactured by Nokia serve as an example of non-discoverable keypad locks. The screen providing instructions is not lit until the keypad lock has been deactivated. They neither make it intuitive to activate the lock, as there is no way to know that one is supposed to press first [menu] button followed by a [*]. (Normally shortcuts are accessed by pressing first [menu] and then the [number] corresponding to the wanted menu item. Thus, the shortcut to keypad is [*] but there exists no other way to activate it than the shortcut).
• SonyEricsson phones light up even when the keypad lock is activated and provide onscreen instructions to the user. They also make the locking process visible by showing the locking option in the menu. The drawback is that this method may end up wasting battery power.
• Apple iPod music players use a mechanical hold switch. The screen is not lit whenever the hold switch is activated. As the switch is not illuminated, locating it in the dark can be awkward.

When now thinking of a clamshell phone, we notice that it actually has a mechanical, intuitive locking mechanism that works without any onscreen instructions – namely opening and closing the lid.

Therefore:

Prefer dedicated controls for operating the keypad lock in order to avoid the need for textual instructions and learning procedures by heart. Also, make sure the lock can be opened in the dark by lighting up the relevant parts of the phone.

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When deciding on a separate control for hold switch, consult Dedicated Buttons. If the addition of dedicated control is not possible, see Menu Shortcuts and Automatic Keypad Lock and consider including a Keypad Lid. See also Quick Information for letting users acces features quickly without opening the lid….