Touch probe woe's

[Smokie555]

Hi, I hope someone can help me,.. I was trying to setup a cheap touch probe with my 6040/Gecko 540 CNC.

I have a height touch plate connected and working great through a 2.1mm connector jack on the front of my control box so I wanted to use the same plug in connection to work with the touch probe.

Problem is the touch plate simply grounds the centre input pin when the cutter contacts an earthed metal surface, were as the touch probes output wire switches its high 5v signal down to 0v when it touches something.

I thought I had it sorted by building a circuit using a 4011 NAND gate to invert this action (low to high) and then switch a mosfet module like a fast relay to act like the touch plate by simply grounding the Gecko’s input pin across the 0V in and -V out on the mosfet module.

Worked great on the bench with a meter set to buzzer but unfortunately when I tried it on the machine the Gecko pin stays in the earthed state (mosfet relay connected) regardless of the touch probes state (high or low). Maybe a mosfet leaks a little? But I’m no expert on the electronics.

Can anyone suggest what's wrong or think of a better solution. Cheers Ross

[m_c]

That probe shouldn't need any inverting.

As it stands, the G540 input pin must float/be pulled high (I couldn't find the exact details from a quick scan over the gecko site).
Having the probe pull that input to 5V should be fine, as even if it's not opto-isolated a PP should be 5V tolerant.
The probe will then simple switch it to ground when activated.

[Smokie555]

Thanks for the help m_c and I hope I can follow this.

My Gecko input pin currently reads 12v, when I earth this pin to the Gecko it triggers Mach3’s LED correctly for my touch plate, so do I connect the touch probe output to the 12v input pin? And does the touch probe need to be on the same power supply as the Gecko for a common earth?

Sorry if I sound a bit dumb but I’m better with the mechical stuff

Cheers
Ross

[m_c]

I did wonder what voltage the G540 pulled the input pin up to.

In that case I think you'd get away with only connecting two wires to the probe, assuming it is as simple as that circuit diagram shows for it (i.e. only a pull up resistor)
Assuming there is no fancy electronics in the probe itself (I doubt it), the only purpose of the 5-24V, is to pull the probe output high.

So if you connect the probe ground, and output in a similar way you connected your touch plate, it should work.

The only thing to be aware of, is what is the rated current of the probe output?
I'd hope it'll be good for at least 20mA, but I'd also just double check how much current the G540 takes to pull the input low.

[Smokie555]

Cheers m_c, I think you've got it right, the probe has a tiny circuit board inside with two LEDs (green open and red for a touch indication) so I think the power + wire is mainly to supply these so maybe the earth and signal hooked up to the Gecko pin is all I need. You won't believe what I have done this passed week to try a get this working, here's a video but don't laugh. I'll give it a try tomorrow.

https://youtu.be/Ph3E028JaNc

[m_c]

That complicates things somewhat.
The diagram for the probe is just a basic functional diagram then, and there's a bit more going on in it.

Have you measured the output voltage on the probe pin?
It might work by simply connecting the probe output (and GND) to the G540, but there's a potential the two might conflict with each other.
I would think given the probe is designed to accept 5-24V and with a NO output, it's output circuit should handle the 12V from the G540.
My concern would be if the probe voltage is grossly higher or lower than the 12V, but it all depends on the exact circuit in the probe.

This is the problem when there's a lack of documentation.

If needed, I'd implement a basic NPN transistor setup.

[JohnHaine]

These probes are basically to the original Renishaw design, which in effect have a pair of contacts that are closed before triggered and open when the probe moves the slightest amount. So this is the opposite logic to a touch plate that is grounded on touch. The probe incorporates an inverter so the output is active low.

I have the same probe that I use with Mach3 on a 5V BoB supplied by 5V from a USB wall wart - the probe connector is basically USB-C and I assume the electronics is designed for a 5V supply so I'd be wary about connecting the output to a higher voltage. If you can change the sense of the probe signal in your config, the easiest thing would be to just drive the gate of your MOSFET from the probe output and have a pullup to the Gecko +ve supply (which may be internal anyway). I'd just use a standard logic level MOSFET - or just an NPN transistor. The probes work great by the way.

[Smokie555]

The output voltage from the probe seems to match the input voltage so it can be 5v to 24v. I opened my CNC control box today to try the probe output and earth idea but had forgot my Gecko power supply is 48v which complacated things again. I did try dropping the voltage to the probe to 12v with a buck convertor inline with the 48v supply but no joy.

I’m just doing something wrong and as John Haine says his works great.

I just need to get this probe output that drops to less than 1v when triggered to earth the gecko pin but my Mosfet setup didn’t work on the machine. Can you give me a guide on this NPN transistor setup as it sounds like the answer. Thank again for the great advice.

[m_c]

Something like this -

[JohnHaine]

That's the sort of thing, but you need to know more about the Gecko inputs. Their manual is silent on the input config except to say that they expect to be grounded by a switch which implies there's a pullup resistor. Two things are needed, the internal voltage to which they are pulled up (which determines the max Vcc of the transistor needed) and the value of the resistor (which sets the transistor on current), It may well be that the voltage is 48v as you mention which would be OK for the 2n3904 m_c suggests. You could measure the on current by just connecting your multimeter on the current range between the input and Gnd. The maximum collector current for the 2n3904 is 200mA.

Then really the transistor needs a series resistor between the probe output and its base to limit the base current (as we don't know what the internal pullup in the probe is). In practice probably a 4k7 (4.7 k, 4700 ohms) would probably be fine to make sure the transistor saturates to pull down the input to ground.

Alternatively you could use a logic level MOSFET such as the IRLZ34N. This is reasonably beefy and would easily handle the voltage and any reasonable current. You just need to connect the probe output to the gate, the drain to the Gecko input, and the source to ground. Good practice would be a 100k resistor from gate to Gnd just to make sure the device is switched off when disconnected.