[wjfiles]

Thanks for the responses.
I should have explained that I am using the HobbyCNCPRO board and there are pullup resistors in the circuit. I also use Mach3.
I had thought of using some very small SMT tactile switches but the the life of these is only about 100000 cycles, not a lot when probing.
wjf

[gar]

071002-2101 EST USA

Martin:

The debounce circuit you suggest is good. but as structured requires a SPDT non-shorting switch.

Really it probably is not necessary because the software detecting that the probe touched effectively detects the leading edge and goes off and does whatever is required. It does not come back to do another test for a substantial time. What is important is to minimize the possibility of a trigger before probe contact. So a normally closed switch connecting the input to common (ground) until probe contact provides a very low impedance at the logic input until probe contact.


wjfiles:

I have no idea what a HobbyCNCPRO board is and thus what its input circuit looks like.

At low voltages and low currents there are only a small number of materials in air that provide long term reliable electrical conduction. Silver is not one, but gold is very good in general.

.

[martinw]

071002-2101 EST USA

Martin:

The debounce circuit you suggest is good. but as structured requires a SPDT non-shorting switch.

Really it probably is not necessary because the software detecting that the probe touched effectively detects the leading edge and goes off and does whatever is required. It does not come back to do another test for a substantial time. What is important is to minimize the possibility of a trigger before probe contact. So a normally closed switch connecting the input to common (ground) until probe contact provides a very low impedance at the logic input until probe contact.


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Dear gar,

You are absolutely right about the SPDT stuff as regard the circuit refered to in my post. Also, I was not aware that software debouncing was available "further downstream". Apologies to all.

Best wishes,

Martin

[gar]

071003-0623 EST USA

Martin:

The reason there is fundamentally a software debounce is because the typical computer is a sequential device, or operated with time delays.

When looking for the probe to make contact you are in a continuous loop testing that input, or you use an interrupt to detect it.

Assuming you do not want to do noise filtering, then the first detection of the contact will cause an exit from the test loop to go service (process the operation) whatever is to be done upon this contact. Probably save the current location of X, Y, and Z. There will be instructions to move to a new location (path) to again test for contact. All this will generally take more time than the bounce time of the switch. Had an interrupt method been used, then upon receiving the interrupt it would be disabled until you were done processing data and ready to test for another contact. Either way the software is doing the debounce.

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