Hi - this question may be far afield but I thought I'd ask.
I'm hacking an old HP plotter salvaged from a dumpster. The paper-gripper component has a 3 wire switch that is tripped by a cam when the gripper motor rotates - the cam has two positions. I haven't been able to figure out why they used a 3 wire switch.
With a multimeter I've sorted out
----blue_----|
----purple----|
----white-----|
--blue & white are connected
--when switch is depressed blue & purple are connected
So why is white there? It was definitely in use. Just wondering if I am missing something.
It could be that they're using the SPDT switch to get a "three-state" reading: one where the switch is fully-depressed, one where it's fully-released, and one where it's in-between.
Or maybe they're using it to verify that the switch module hasn't become disconnected, because there will always be some sort of signal coming from it.
Wiring a SPDT switch to a cross coupled nand gate or a direct RS flip flop gives a bounce free switch signal transition without any time delays, caps or software routines needed to eliminate contact bounce.
We would have to see the schematic of the plotter to know how and why the switch is being used, so we can only speculate on why a SPDT switch is being used.
I don't have any of the plotter electronics or a schematic to try to detective that out - I just have the barebones plotter mechanism - but one (e.g. me would think that the HP engineers used a 3 wire switch for a reason - so maybe they were indeed incorporating a debounce solution in hardware. That's all beyond me though, so I'll have to do it the more simple way that I know - but I thought I would put the question out.
Sometimes looking at a piece of production hardware is instructive (when it is not completely unintelligible...)
would think that the HP engineers used a 3 wire switch for a reason - so maybe they were indeed incorporating a debounce solution in hardware. That's all beyond me though, so I'll have to do it the more simple way that I know - but I thought I would put the question out.