Grades 3–5 · Machines
How does a click pen remember whether its tip is out?
One click brings a pen tip out. Another hides it. The button does not need a battery or a tiny person deciding what to do. In a common kind of click pen, shaped plastic parts make the two positions alternate.
In the diagramA tiny turning part remembers each clickA simplified latch cycle · not a manufacturing drawingReturnedTurnsSlot lets refill riseHeld outLedge supports refillAnother complete press and release changes the latch state.
Pressing the button pushes the refill forward against a spring. Sloping surfaces, called cams, also turn a small part by a step. When the button is released, that part can land on a ledge. The ledge supports the refill, so the tip stays out even after the finger leaves.
The next complete press and release turns the part to a position where it can slide farther back. Now the spring can push the refill into the barrel. The mechanism keeps its state through the position of its parts. The two-panel drawing explains this latch idea; actual pens use different arrangements of cam teeth, slots and springs.
About this explanation
Starting knowledge: Recognize spring compression, sliding and turning.
Sources
- Francis Levoin / patent record — US3289638A: Push button operating device for ball point pens