Npn NO proximity sensor question.

[silvs]

I have a concern regarding my npn NO sensors. I have them wired up in parallel with a resistor to drop 12v input to 5v. I have the resistor from -12v (blue wire) jumped to output (black wire). All works good, drops voltage to 4.9v. However, when sensor is triggered it drops to 0.75v instead of 0.0v as I would expect it to. Is that normal? Will that 0.75v fry my c10 BOB?

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[rodw]

You may need to add a pull down resistor

[jo_ky]

You shouldn't need any negative voltages. Connect blue to ground and connect brown to your positive voltage supply.

Connect a load resistor between brown and black. The value should be high enough that the current, from I = V/R, is less than the rated load current for the sensor. In this way, you can use any positive voltage that is within spec for the sensor, as the supply. Now, the black will be either at V+, with impedance R, or shorted to ground. I'm not sure why you're measuring 4.9v. That doesn't sound right.

If you were to then convert that output to 5V levels using just a resistor, you would get something like you describe. Perhaps that's the case, as your sensor has a built-in load resistor, and you added a resistor between black and the BOB?

The correct way is to use a second transistor to shift the level. However, if you've got something that works, you're good. Digital inputs have very high input impedance. It means that, no matter if it's 0.75 V or 5V or whatever, very little current is flowing.

[john-100]

what is the part number of the Npn NO proximity sensor ?

usually the sensor has an open collector output


so you will only need to set the jumper on the C10 BOB's input pullup / pull down resistor to pull up

in practice the C10 BOB's input should be between 0V & the +5V supply
(the absolute limits - 0.6V & +5.6V any more or less will forward bias the 74ACT245 input protection diodes )

circuit details of the C10 taken from the manual and traced from online photos



sensor to C10


John

[jo_ky]

Ah yes, with no load resistor it is open collector output. That should have been obvious, doh!

Yes, in that case it can be done right with no extra transistor. You activate pull-up on the C10, and it gets switched to ground when activated.

[silvs]

Hi. The is the proximity sensor I'm using.

LJ12A3-4-Z/BX Inductive Proximity Sensor Detection Switch NPN DC6-36V 4mm Normally Open Proximity Switch.

Im using a resistor to lower voltage to 5v but its jumped across blue and black wire. Should it be from brown wire instead? Do I even need the use of a resistor? I'm a noob when it comes to these switches.

Thanks for the help thus far.

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[jo_ky]

Lowering voltages is not what resistors do. They set the voltage to current relationship. Spend some time with ohm's law and the voltage divider equation, and it may start to make sense.

You can take it out, and use the pull-up on the BOB. In this case, a "load resistor" gives an output at the level of the sensor power supply, which is too much. A "pull-up resistor" is essentially the same thing, but on the BOB side, which has a different voltage powering all the logic, so that way our sensor output only goes up to 5 volts.

The only thing the sensor does is defeat the pull-up by grounding the output.

[silvs]

Ok. That makes sense now. That's for the help. I'll remove the resistor and wire directly to BOB. Thanks for the help.

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[john-100]

while I have been looking for a detailed data sheet for the LJ12A3-4-Z/BX Inductive Proximity Sensor
(so far I have failed to find the details I was looking for )

jo_ky has said in post 7 what I indicated in the 3rd diagram in post 4

sensors with NPN open collector outputs will not provide any voltage or current and can only sink current
from the likes of an onboard pull up resistor on your C10 BOB

John

[silvs]

This is the wiring diagram for two NPN NO sensors in parallel from the c10 BOB manual. I confused as to why it says to use a resistor if its integrated into c10 BOB.

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[coherent]

Just a little info for what it's worth.. I went through some of the same issues as you are regarding proximity sensors. Mine are 10-30 volts. I'm ok with electronics, but not a wiz. I was finally able to figure it out using resistors etc, but changed the setup by simply using an 8 channel 24 volt relay module that's optocoupler isolated and allows you to pull high or low on each relay as needed. You can also get 12v (or other voltages based on your proximity sensors and power supply available). They are only about $10 and make the sensors much easier to wire and connect to any breakout board and eliminates all the resistor mess and lowering voltages. You likely don't need an 8 relay one, but having a spare/extras if needed down the road for something else may be well worth the tiny difference in price.

[john-100]

the diagram in post 10 is correct when the proximity sensors have internal pull up resistors and are being powered from a supply that's higher than the 5V supply to the C10 BOB



John

PS
since I have not found a detailed data sheet for the proximity sensor

it may be worth testing to see if the sensor has a true open collector output

I would connect your 12V supply 0V to the blue common / 0V and
+12v to the brown + supply

if the sensor has an NPN open collector output then you should not have any voltage
between the blue common and black output when the switch is either off or on

if the sensor has an internal pull up resistor or LED indicator between the brown + supply
and black output
then you will measure a voltage between the blue common and black output when its switched off

[silvs]

Not surprising you can't find any data on them. They are those cheap Chinese ones from Amazon.

I did the suggested test. I measure +12v across blue and black wires. When sensor is triggered I get +0.75v. My test is with two sensors I'm parallel.

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[john-100]

for the 74ACT245 used on the C10 BOB

+0.75V is low enough to be a valid logic 0 (LOW)
so that's OK when the proximity switch is on


when all switches are off
+3.5V or more will be a valid logic 1 (HIGH)

what doe you measure when all proximity switch outputs are off ?

John

[silvs]

It measures +12.38v when off.

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[silvs]

A little off topic but will all the different voltage is have, 48v, 12v, and 5v, should I be using the -48v ground for all negative connections?

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[john-100]

+12.38V implies either you have
a proximity switch with a pull up resistor between the positive 12V supply and the open collector NPN output
so the output is high (+12V) when switched off

which is what a LJ12A3-4-Z/BX proximity sensor is

or

you have a proximity switch with a PNP output that connects the output to the positive supply when switched on

John

PS
to answer post 16

assuming the 48V supply is for your stepper drivers , 12V for your proximity switches and 5V supply for the C10 BOB

then

you can connect all the negative terminals together

note -
when the C10 is connected to a desktop PC printer port

the negative supply terminal will be connected to the mains supply earth / ground
via the PC power supply

the PC AT (or ATX) black 0V connections will connect the motherboard common including all the I/O connectors to earth

[jo_ky]

To be clear, the test has to be done with everything else disconnected. That's the case?

I would also check if there's any change in the voltage levels depending on whether it's one or both closed.

[john-100]

with two or more switches in parallel
as soon as one switch closes the voltage will drop to near 0V

with your results I can see why the connection diagram for the C10 BOB ( posts 10 & 12 )
a resistor is connected between the proximity sensor output (black wire) and C10 input

with a 12V supply the manual gives the total resistance for the pull up resistor between the +12V supply and output plus the resistance connecting the sensor output to the C10 input as 10K

with the C10 input jumper set for a pull down 4K7 resistor
the volt drop across the 4K7 resistor & BOB input would be

4.7 divided by 14.7 times 12V = 3.8V which is more than the minimum of 3.5V for a logic 1 input

with proximity switches in parallel the resistance between the +12V supply and the paralleled outputs will depend on how many switches are in parallel

with no details of the proximity switch available
you could connect the sensor outputs to ground / 0V via a 1K resistor
and use the voltage across the 1K resistor to work out what resistance you have between the +12V supply and the sensor outputs
subtracting the result from 10K to find the value of the resistance needed between the sensor outputs and G10 input

John

[silvs]

Ok. I got the two proximity sensors wired in parallel to output 4.47v when triggered. However, mach 3 is not recognizing the sensor when I trigger it. I tried setting the pin jumper on c10 BOB to pull up and to pull down... nothing. I tested the BOB inputs for the proximity sensor and they read 5.5v whether I have pin jumper to pull up or pull down. I followed this diagram. I'm confused.

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