The click pen
How does it know to go in or out?
The question
Click a pen and the tip comes out. Click again, with the same push, and it goes back in. How does it know?
Have a guess, then show the answer
It doesn't. A little collar inside turns a bit further with every click, landing on a ridge, then in a groove, turn and turn about.
Play
Click it, and look inside
Press the button. The inside of the barrel is drawn unrolled, as if you cut it open and laid it flat, so you can follow the collar.
Tip in: slid up a groove
Clicks: 0. In this drawing each click turns the collar an eighth of a turn, so eight clicks bring it all the way round. Real pens vary.
What's happening
- 1
A spring pushes the ink tube up, against a collar that is free to turn.
- 2
Inside the barrel are short ridges, with long grooves between them.
- 3
When you push, slanted ramps on the button twist the collar a little way round.
- 4
Let go and it twists a little more. If it lands on a ridge, it parks there, holding the tip out.
- 5
Next push, it twists past the ridge into a groove, and the spring slides it all the way up. Tip in. The pen remembers nothing; it just keeps turning.
Simplified, and sources
The drawings use plain proportions, not one particular pen, and the collar turns slowly so you can follow it. Real pens differ in the details, but the ridge-and-groove idea is the same.