Inverting probe logic

[bvandiepenbos]

The auto-focus probe on one of my laser engravers keeps failing. The worst part is when it does it does not 'fail safe'
The probe is a simple mechanical switch that is N.C. you start the focus routine and the table moves up until it opens switch.
Problem is if the wire to probe breaks (it has many times) the table smashes up into the lens and X carriage ! BAD $$$
Poor design.

How could I invert logic of probe to N.O. but still have the machines control board "see" N.C. ?

That way if wire to probe fails again it will just stop (or not move) to prevent crash into lens.

[BeeAmaker]

We can talk about this tonight. Something doesn't sound right - Are you sure the probe is N.C.? I ask because if a wire breaks off a N.C. switch it will become an open, just as if the probe touched and the machine should stop. However, if the probe is N.O., then yes there would be no difference if a wire breaks off. You can fool the control board all you want but there still is nothing to tell it that the wire fell off vs a probe touch. How many wires are connected to the probe and is there something you can do to prevent the wires from breaking off?

[Larry]

Random thought... If the sensing circuit is NC (the circuit opens when at the focus point) you could add a safety limit switch. The 2nd switch opens if/when the tables gets too close to the lens. This signals the controller to zero at this too close but still safe location.

Pick a reliable switch for the 2nd stop / safety switch. Use the NC contact. Wire both switches in series: controller to switch 1, switch 1 to switch 2, switch 2 to controller. Now the controller stops if either switch or the wiring goes open.

Not quite what you're asking for, but it might save a few lens crashes.

[bvandiepenbos]

Thanks Larry for the idea. There is really no physical room for another switch in a effective location.

Glenn came up with a way to wire it that should work.