SMD Retractable Pen

This Instructables is an experiment for me to get acquainted with the mechanism used in the ubiquitous retractable pen but on a different application. The retractable pen is basically just a bis-stable mechanical switch (2 states: retracted and protracted), what I am proposing here, is to use the same kind of mechanism to actuate a pump (via a monostable turned bi-stable switch) to hold and release an SMD device but the fun part here is that the lever of the switch of the pump is the sucking tube itself

Materials:

• 8mm ID springs
• 8/6x200 (OD/ID x length ) Aluminum tube
• 12mm M3 screws (x3) with their nuts
• SMD vaccum pump & tubing
• 3D printer endstop
• Solid state relay
• Wires
• Crimping pins

Tools:

• 3D printer
• Soldering iron

If you are interested, you find some documentation online on how this system works, especially in the patent of this mechanism (US-3652173-A filed in 1970!) but I think it might be easier to show you.

First, on the two animations, you can see how the system works to convert the up and down motion (orange part relative to the green part) into a linear movement (movement of the pink part). We consider the green part fixed and the orange part slides up and down the green part (it is important to note that there is a constant offset of the teeth of the orange part compared to the teeth of the green part), the orange part catches the pink part on its way up with a ramp interface making the pink part slide forward past the top of the green teeth. In a way, the pink part oscillate between 2 resting positions on the green part thanks to the up and down movement of the orange part. This is possible because the teeth of the pink part are twice as large as the teeth of either the orange or green part thus always interacting with at least the greens or oranges teeth. One of the animations shows that the green part can have resting positions of different heights meaning that in one full up and down movement of the orange part, the pink part gets to the next resting height and it takes another full up and down travel of the orange part to move on to the next height for the pink part.

To make sure the pink part is always in contact to either the green or orange part, a spring is needed to press down on the pink part, to make sure the pink part is in one of the green resting positions not bothered by the orange part when not in use, another spring is used to place the orange part at its low position when not pressed.

This was just for you to understand how the system works, but you can see on the animation that eventually, you “run out” of the pink part which will fall down unless… You make it a circle ! And that is exactly how the real system works. You can take a look at what my models look like, but if you understood how the “linear” model works, you should be able to find a way to design your own 😊

The mounting is quite straightforward as we do not have that many pieces (8 in my example) plus the off-the-shelf-components (LM8UU, screws and springs)

A word of caution, we are dealing with high voltage (110VAC or 220AC), do not work with these kind of voltages if you are not completely sure of what you are doing, double check every step of the way that you are disconnected to you house power whenever making modifications.

That being said, the circuit is very simple, we are just using an “endstop” (used extensively in CNC machines including 3D printers) to check the up or down position of the system to turn the solid state relay on and off (Simply put, a solid state relay, or SSR, is circuit that controls a high voltage signal with a low voltage, here 12VDC to pilot 110VAC or 220VAC)

In this Instructables, we are just converting a monostable switch into a bistable switch. This tool is not the most usfull, you can see on the video that it is quite tricky to be delicate using this tool? Furthermore, the original SMD-mounting-pens that were delivered with the pump I had to buy for this project are actually better than this but still, this was an interesting experiment to see how the system could work

I have talked about the fact that the system build here is a bistable switch out of monostable. For my application, I needed only 2 stable states but it is interesting to note that there is no reason to limit this kind of mechanism to a bistable mechanism, you could easily extend the concept to more stable states if you increase the number of teeth on each gear and increase on the outer “stator” gear the number of levels (note that in this case, the mechanism would follow sequentially the states with “resets” between each states).