I've been experimenting with the push type solenoids I obtained for this project to determine how to get them to operate with enough force to push the keypad buttons reliably.  The force produced increases significantly by increasing the voltage from 12V to more like 24V.   As I said in a previous log, I think this will work fine due to the low duty cycle that will be applied.

But I discovered another property of these push solenoids.  The force produced varies significantly based on the initial position of the armature.

These solenoids work differently than most - the pin pushes outward instead of pulling inward.  How do you get the pin to push outward when the armature is attracted to the coil, not repelled?  The main part of the armature is thicker than the pusher end pin and so even though the pin is pulled inward the much thicker opposite end is pulled in with much more force, so the net force is to push the pin outward, until the large part of the pin bottoms out against the small hole end.

You would think that the point of maximum push force would be right before the pin hits bottom, as this point would have as much pin as possible inside the coil and would result in the best magnetic coupling between the pin and coil.  So when building my device I should place the solenoids as high as possible so that the keypad button is pressed just before the pin bottoms out.

But experimenting at my workbench reveals that the just-before-bottom position is NOT where the solenoid produces the most force.  There seems to be a point with the pin farther back and the fat end of the armature farther out the back of the solenoid that produces significantly more force.  But finding this point of maximum force is hard to find just holding a solenoid by hand over the surface of my workbench.

I need a device to reliably hold the solenoid in a stable, adjustable, and repeatable distance over a surface and a way to measure the force produced, so that I may build my device with the optimal spacing between the solenoid body and the keypad button surface.

So I am now thinking about building some sort of frame I can clamp to my workbench.  A frame on which I can mount a solenoid over my digital scale so I can power the solenoid, read the force produced, and adjust the spacing of the solenoid to find the optimal distance.