problem with proximity switches

[Harryman]

I've been using mechanical limit/home NC switches on my desktop mill, and while working fine, repeatability has been lacking. The manufacturer of my mill, (Minitech) was kind enough to send me 3 proximity switches to swap out for my home switches. I've got the new switches wired up and working, but I can't get my controller (Wincnc) to recognise them.

Bear in mind I'm an electronics noob, but I'll give you what info I can.

Wincnc uses an internal pci card with input pins for a switch on each axis high and low, each input limited to 5V. I wired up the mechanical switches as per the manual, one leg to ground via the card and the other leg to the specific input. There's no voltage to the input, Wincnc seems to see the change in resistence as the switch opens. With a meter, as the switch opens, I get momentary readings that bounce @ from -20K Ohms to 20 K Ohms.

The proximity switches are NO, not preferred I know, but if I can get them working, I'll swap them out. They're rated 10-30V with a Pos, Neg and one leg to the input. I'm running 5V through it, less than the 10V rated, but it seems to work just fine, the little led on them lights up and I get a drop in voltage from 41 mV to 10mV. Resistance goes from infinity when open to 160 K Ohms. I'm using the 5V pos and neg from the card to power them, with the remaining wire to the specific input.

Wincnc doesn't see these switches at all. The mechanical switches work with the stock debounce settings at 20, I've figured out what debounce is and although I have no idea what those units represent, I've tried changing it from 10 to 2000 without success. I can switch the polarity in Wincnc to account for the change from NC to NO, but that obviously doesn't help either.

Thanks in advance.....

[gar]

071105-1120 EST USA

Harryman:

Let us suppose your prox switch has an NPN output transistor with no internal pull up resistor.

You have three leads and apparently properly identified these. So we assume that because the LED changes state for you that part of the device is working. An NPN transistor would have its base internally connected to the common (-) power lead. When turned off an NPN transistor has relatively low leakage from the collector to the base. The collector is connected to the prox switch output. When turned on the collector to base resistance is fairly low.

If we assume that when your VOM (volt ohm millampere meter) is in any ohms range that the meter + terminal develops a + voltage (a few meters do not), then connect the + meter lead to the prox switch output, but not connected to anything else, and the meter - lead to the prox common, and you will see a low resistance when the prox output transistor is on, and a higher resistance when off.

Using a Simpson 270 meter on Rx100 and a 2N4400 transistor I read greater than 200,000 ohms (meter full scale) when the transistor is off and about 180 ohms when on.

I suspect you need a pull up resistor, about 390 ohms, from +5 to the prox output.

.

[Harryman]

Thanks for the help, I'll check it out.