bed level sensor trouble

If a voltage divider is too complex, use the sensor (at rated voltage) to switch a relay. It works, is very simple and will interface easily to things that need a switch input.

Yes that's a very practical way to do it, and would allow NC or NO operation independent of the sensor switching mode. Does the delay in the relay switching cause issues with homing successfully if the homing feedrate is too high?

So this type of module would work well yes [www.ebay.co.uk]

I'm thinking of a kit with a sensor pre-wired to the relay with +12v and 0v wires, then endstop 3 pin connector pre-wired. All the end user would have to do is install the sensor and and connect the power to the module and endstop connection which would be setup for NC operation. The select pullup enabled for z min and logic false.

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DjDemonD
Yes that's a very practical way to do it, and would allow NC or NO operation independent of the sensor switching mode. Does the delay in the relay switching cause issues with homing successfully if the homing feedrate is too high?

That's dependent on sensor frequency and desired resolution. Delay is trivial to account for as long as it's consistent. Homing feed rate should not be high regardless.

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DjDemonD

Npn NO LJ12A3-4-Z/bx inductive
Nothing measured from 0v to signal wire
open or closed
Measured from +12v to signal wire
Supply 12.4v
Open voltage 0.12v
Closed voltage 12.41v

Npn NO ljc18a3-h-z/bx capacitative
Supply 12.4v
Measured from 0v to signal wire
Open voltage 10.13v
Closed voltage 0v
Measured from +12v to sign wire
Open 0v
Closed 12.40v

I also have an LJ18A3-8-Z/BX in my SmartrapCore Alu at the moment which should be an NPN NO sensor but which gives 12v open and 0v triggered (which means it is actually a NC sensor, despite it saying NPN NO on the side (in Chinese but which I had Google translate)!



So we can't rely on the sensors themselves to actually work the way they are meant to. Or to be sold on ebay correctly described, one listing was for an NPN NC sensor which said NO in the detailed description below.

Dude. Seriously. Take a step back from your current way of thinking.

All those results are consistent with NPN sensors that are actually working. You CAN rely on the sensors to work the way they are meant to. You just need to understand how they work.

Think of it like a switch inside, a switch that connects the signal wire to ground. Try measuring the voltage across a switch, when it's open and then when it's closed. You'll get 0 volts both times. Before you go around claiming that the switch isn't working, step back for a second, and think about how your multimeter works. All it's doing is measuring voltage. If there's nothing to supply that voltage, then you're not going to measure anything. Those sensors might have a very weak leakage inside them in some cases, but it won't supply any current worth speaking of. Your multimeter might pick up some of that leakage, but only because its internal resistance is very high.

Try this: Put your multimeter on continuity test mode (some models have this as a diode test mode). It should beep when you put your probes together. Ground one probe, and attach the other probe to your signal wire. This is emulating a controller board with a pullup resistor.

Now put the sensor near a piece of metal. If your multimeter beeps, then you've proven that your NPN sensor is working exactly as it should.


Good luck.

If you measure an open collector output with nothing connected to it except a meter the voltage will float and you will get misleading results. It requires a load connected to +V in order to behave as specified.

Okay I take your point that perhaps I'm getting too worked up about this. But I know I am not the only person baffled by these sensors.

But can anyone suggest, as this will help me to understand these things, why the second sensor mentioned Npn NO ljc18a3-h-z/bx would give 10.13v presumably connecting signal to the input voltage when the sensor is open when it is meant to be NPN and only connect the signal to ground? Might this behave differently with a load attached?

Also why the last sensor mentioned LJ18A3-8-Z/BX is operating as NC when it is meant to be a NO sensor in its description?

I will try testing some using the continuity check method as above and I'm grateful for the information.

It is these apparent inconsistencies which are causing me consternation as it makes getting a handle on the problem so difficult, and which prompt so many forum questions about sensors not working the way people expect them to.

Do you feel there is any merit in offering a foolproof sensor kit that always works? How do those reading this who are not electronics guru's feel about this idea?

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DjDemonD
Okay I take your point that perhaps I'm getting too worked up about this. But I know I am not the only person baffled by these sensors.

But can anyone suggest, as this will help me to understand these things, why the second sensor mentioned Npn NO ljc18a3-h-z/bx would give 10.13v presumably connecting signal to the input voltage when the sensor is open when it is meant to be NPN and only connect the signal to ground? Might this behave differently with a load attached?

Also why the last sensor mentioned LJ18A3-8-Z/BX is operating as NC when it is meant to be a NO sensor in its description?

I will try testing some using the continuity check method as above and I'm grateful for the information.

It is these apparent inconsistencies which are causing me consternation as it makes getting a handle on the problem so difficult, and which prompt so many forum questions about sensors not working the way people expect them to.

Do you feel there is any merit in offering a foolproof sensor kit that always works? How do those reading this who are not electronics guru's feel about this idea?

It IS an NO sensor. According to you the signal reads at supply voltage when not triggered and ground when triggered. That IS normally open for an NPN. You are switching ground, so "open" in this sense refers to the load being switched off. The signal leg is at zero potential with respect to the other leg of the load, which is connected to v+.

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DjDemonD

But can anyone suggest, as this will help me to understand these things, why the second sensor mentioned Npn NO ljc18a3-h-z/bx would give 10.13v presumably connecting signal to the input voltage when the sensor is open when it is meant to be NPN and only connect the signal to ground? Might this behave differently with a load attached?

As mentioned previously by myself and others, the 10.13 volts you measured is just leakage, and only visible due to the very high impedance of your multimeter. Try doing anything with that 10.13 volts and you'll realise that it's just an artifact. Put a 1k resistor between the signal leg and ground, and that 10.13 volts will instantly drop to almost zero.

Yes, it will behave differently with a load attached.

Try it.

holy crap thanks DJ for all this information I am going to have to absorb it all over the next few days as I wont be able to use my machine due to a large order I am doing on 3d hubs. But after it is done I will certainly try all of this and read through all your information thanks! you've be really helpful

koko I will check that stuff out as well