Wiring of npn inductive proximity switches.

[wackocrash5150]

So I am just finishing up my build of a C-Beam XLarge (expanded from a regular) and I decided to try proximity switches instead of mechanical ones like last time. I picked up some LJ12A3-4-Z/BX switches and they do not operate how I figured they would. Thinking in terms of a mechanical switch, they're described as a "normally open" yet still let voltage pass (in my case 24v) when not detecting. When metal is detected, that changes, but it only drops to 0.8v instead of the expected 0v. I would've normally thought this as "normally closed" behavior. Not a problem as "NC" seems to be the safer option anyway.
I have put a 2.8k resistor over the negative (blue) and signal (black) wires to drop the signal output to a better voltage for my BoB, but the think that on 2 of the 3 parallel cards i have for the PC, it's still sensed as "on" (hope I'm phrasing that correctly)
Am I correct in thinking that 0.8v is around the range where something senses the difference between on and off? Is it the BoB or the parallel card that makes the difference? Is it possible to lower this voltage to 0v?


Thanks in advance

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[john-100]

the LJ12A3-4-Z/BX is a N/O switch that functions like a machanical N/O that can l be connected between the common ground and either a PC's parallel printer port input or TTL input of a BOB like CNC4PC's C10



0.8V is a valid TTL logic 0 and over +4V a logic 1

what BOB are you using ?

to get +24V you could either have the PNP version that connects the positive supply to the output
or made an error wiring the NPN switch

John

[wackocrash5150]

The breakout board is a ST-V2 from SteppersOnline https://www.omc-stepperonline.com/st...kit-st-v2.html

I will double check the installed model of the sensor, as I have both AX and BX models on hand, but I am almost positive I installed the BX ones. Below is a quick MS Paint drawing of how i have it now. Please forgive the crudeness of it.

[john-100]

OK now I see what BOB you have , the inputs are opto-isolated

the LED anode in the "PC817" opto-isolator is connected to +10V from the regulator that powers the VFD circuit

the LED cathode is connected via a 1K resistor to the BOB's input terminal
PC817 data



when the input is open circuit I would expect to see about 8V (10V - the 2V drop across the LED)

when you connect the input to ground , I would expect the current to ground to be about 8mA

depending on the current transfer ratio of the PC817 , the photo transistor will be able to sink 4mA to ground
which is more than enough to pull your printerport input low (standard TTL load 1.6mA)

by adding the resistor across the switches output & ground its possible for enough current to flow for the printer port input to see a logic low even when the switch is open -
8V / 3.8K = 2.1mA with a minimum transfer ratio of 50% the opto isolators transistor can sink 1.05mA
for the original TTL IC's used in the printer port adaptors , a standard load = 1.6mA

its not clear from the manuals I have found what the current you need need to sink for a valid low for the super I/O in modern PC's

remove the 2k8 resistor and let the BOB input go up to +8V when the switch is open

John

PS

diagram for HALL switch & BOB input

[Al_The_Man]

This also may help as an explanation of sink/source as applied to PNP/NPN device inputs, prox etc.
Al.

[wackocrash5150]

Was home briefly for lunch and confirmed; LJ12A3-4-Z/BX sensors. Also, I will try changing the resistor value so it's closer to 8v. When I measure the voltage off the EStop pin (Pin 10 and Ground) it's around 8v as well ..... I wrongfully expected it to be around 5v, which is why I was trying to attain that voltage with the resistor I'd used.

[john-100]

just remove the 2.8K resistor its not needed

the BOB's input being open circuit is what you want until the closed switch grounds it

John

[skrubol]

I would get rid of that resistor altogether and leave it open. NPN's conduct to gnd, and your bob has a pull-up to 10v, so when it's open, the input should be around 10v, closed, around 0v.

[wackocrash5150]

I would get rid of that resistor altogether and leave it open. NPN's conduct to gnd, and your bob has a pull-up to 10v, so when it's open, the input should be around 10v, closed, around 0v.

... even using a 24v power supply? I'd assumed that running 24v from the black/signal wire to one of the inputs on the BoB would nuke it. (or is the importance of the resistor/5v more relevant to arduino and other applications?)

[john-100]

the + 6 to 36V connected to the LJ12A3-4-Z/B brown wire only powers the HALL detector

the black wire connects to the open collector of the NPN transistor as in my diagram in post 4 -
the NPN open collector can not supply current from the brown supply wire

the blue wire is the common connection to the NPN transistors emitter and the negative of the +6 to 36V supply

no power can flow from the positive supply to the BOB's input (unless the HALL effect switch is damaged or incorrectly wired)

only when the NPN transistor is switched on can current flow out of the BOB's input via the transistor to the common ground

John

PS

for any one that's paranoid adding a diode to the BOB input will prevent the direct connection of a positive supply damaging the BOB

(the diodes anode needs to be connected to the BOBs input terminal - the cathode with the white band goes to the black switch wire)

[wackocrash5150]

So when I got home and had some time, I thought to myself "What the heck. I've got 4 BoBs and only need two. Lets try it."
I removed the made sure that the brown and blue wires were hooked up to the positive and negative on the 24v PSU respectively and then hooked to pin10 on the BoB with the black signal wire. The BoB was also powered from the 24v PSU on the 12-24v input. BINGO! Worked like a charm. Strangely it still functions like a NC mechanical switch in that when the wire is disconnected, it then trips the BoB. (this is ideal for what I wanted). I think my main problem was going on the assumption that I could not run the inputs on the BoB at much more than 5v.
Thanks for the suggestions guys!

P.S. - It also worked on the newer parallel card as well that was not picking up signal change at all.

[zaxtone]