This little 'device' streamlines the process by pressing the right keys for you, and can be hidden in your palm so you simply mash your hand against the keypad and - apparently miraculously to anyone watching - unlock the door in one go.
Time to make: Less than 10 minutes
Time saved: About 30 seconds per day in my case; your mileage may vary.
Payback time: 20 days, in this case
Background: The research group I work in was moved to a new office. In the old place, I'd got used to an RFID card in my pocket allowing me to open the door just by leaning against the sensor, but the new office had a keypad on the door. It was going to use an extra 5-6 seconds every time I needed to get in the office!
There had to be a better way...
Step 1: Combinations rather than permutations
First off, we realised that it was unnecessary to type the "C" (clear) at the start unless the previous person to try had entered the code wrongly. That cut maybe half a second off the time taken to enter the code.
But then we noticed something: the code had a sequence of digits ("132") that weren't in numerical order - yet when they were typed in numerical order ("123"), the door still opened. That is, this model of keypad is a combination lock rather than a permutation lock. It didn't matter what order you typed in the code, as long as you typed in all the digits that made it up. You could type "1-3-2-5-9" or "3-2-1-9-5" or "9-2-3-5-1" or any other arrangement.
The other crucial point was that these keys didn't have to be pressed separately - i.e. not only was there no requirement for the digits to be in the 'right' sequence, there was no requirement for there to be a sequence at all. Thinking about how a simple mechanical lock like this works, that ought to be obvious.
So, if we could press multiple keys at once, we could save a lot of time. If you angled your hand just right, it was just about possible to press all the keys required at once, but it was pretty awkward.
We really needed to make some kind of device which did that. But we couldn't just press all the keys, or press any wrong ones: whatever device we made needed to press only the right keys.
The simplest thing to do seemed to be to make something which, when held onto the keypad, pressed the right buttons.