Newbie question about endstops

In my research, I've seen two styles of mechanical endstop switch: Some designs have the switch mounted on a small circuit board; others mount bare microswitches wired directly to the controller board.

I thought the purpose of the small boards was to eliminate 'bounce' in the switch signal. Correct?

If so, how do the other designs get away with simple switches?

-Matt

Switch bounce isn't really a problem with an endstop. The first time it changes state the end has been reached. It doesn't matter if it then bounces a few times.

Switch bounce is only a problem with things like keyboards or mouse buttons where each press does something. It can then be fixed with hardware or firmware filters. Funnily enough Microsoft don't seem to know this as their mice are prone to switch bounce.

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[www.hydraraptor.blogspot.com]

nophead Wrote:
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> Switch bounce isn't really a problem with an
> endstop. The first time it changes state the end
> has been reached. It doesn't matter if it then
> bounces a few times.

Okay, then what's the point of the circuit on the board-mounted switches?

I found this:

[wiki.makerbot.com]

...which mentions an LED for debugging. The circuit also has a 100nf cap; perhaps to eliminate noise?

Is that it? ...perhaps it's overkill?

-Matt

It's pointless because noise is picked up in the wires, not the switch, so the capacitor needs to be at the controller end of the cable to be effective. Some boards, like Melzi have capacitors. Others you need to keep the endstop wires away from the motor wires or you get false triggering. A capacitor across the switch will do nothing because the switch is normally closed, short circuiting it.

The LED might be handy but adding a PCB is expensive and you can view the state of the enstops with M119.

Edited 1 time(s). Last edit at 07/06/2012 06:12PM by nophead.

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[www.hydraraptor.blogspot.com]

Quote

Switch bounce isn't really a problem with an endstop.

I have different experiences. Every once in a while an endstop whould pick up a signal out of the blue. This is why Teacup ignores endstops while printing and debounces them in software while homing.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

That is not switch bounce, it is electrical noise. Switch bounce is when the contacts physically bounce when the switch closes leading to a series of pulses.

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[www.hydraraptor.blogspot.com]

Still thinking that I must be missing something, I asked the same question of the Makerbot support staff. They gave me permission to copy the exchange to this forum (below).

Bottom line seems to be as nophead has suggested: there's no _electronic_ magic in the circuit boards; they're merely for convenience of mounting and managing the switches.

-Matt

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Matt, Jul 09 15:36 (EDT)

Hello Makerbot,

I came across your mechanical endstop: [wiki.makerbot.com]
Since you've graciously made it open-source, I've noticed a lot of rep-rap devices
use it too.

Other than the debugging LED, what advantage does this board offer over simply
wiring a mechanical switch directly to the controller board?

-Matt

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Nate Leichter, Jul 09 16:42 (EDT)

Hi Matt,

Thank you for contacting MakerBot Support!

Aside from the LED, I'd say that some other advantages include the port for the
endstop cable, as well as the mounting holes. These directly correlate with the
convenience of the design as well as the ease of getting it in place (exactly where
you'd like it to be).

Have a nice afternoon!


Best,
Nate
MakerBot Support

Edited 1 time(s). Last edit at 07/10/2012 04:16PM by mkeveney.