Wiring NPN NC sensors to DB25-1205 BOB

[millhouse_]

Hi, I'm a bit confused about how to wire the often used NPN proximity switches.


The setup:

1x 12v Power Supply
1x 5v Power Supply
DB25-1205 chinese BOB
6-36v Proximity Sensor NPN NC


The BOB ist connected to +5v and -5v of the 5v supply. The sensors are connected with +12v and -12v of the 12v power supply.

Only the signalwires from the sensors are connected to the Pins of the BOB. The signalwires are connected with diodes to block current flow towards the BOB.


The Pins only change from 5v to ~1v if I connect the BOBs GND to both: The -12v Terminal and the -5v Terminal of the power supplies.


Since the negative Terminals aren't really GND I'm wondering if this can damage anything?


If the BOBs GND is only connected to the -5v terminal the Pins state don't change.

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[robe_uk]

You are going to have to have the -ve connected.

here is a video showing some information I found

[breezy1849]

Hi, I'm a bit confused about how to wire the often used NPN proximity switches.

The BOB ist connected to +5v and -5v of the 5v supply. The sensors are connected with +12v and -12v of the 12v power supply.

Only the signalwires from the sensors are connected to the Pins of the BOB. The signalwires are connected with diodes to block current flow towards the BOB.

The Pins only change from 5v to ~1v if I connect the BOBs GND to both: The -12v Terminal and the -5v Terminal of the power supplies.

Since the negative Terminals aren't really GND I'm wondering if this can damage anything?

If the BOBs GND is only connected to the -5v terminal the Pins state don't change.

For the electrons from the 12v supply to get back to it from the BOB there has to be a common connection between them and this is through the -ve connections of the power supplies.

So it is standard practice to connect all power supply -ve's together. Sometimes this is called grounding the PS but you do not have to connect this to building ground (earth).

Regards,
Arie.

[john-100]

Hi millhouse_

How many NPN N/C sensors are you trying to connect to one BOB input ?

what's the part number of the sensor ?



A while ago I reverse engineered the BOB from photos found on line




You can not simply parallel NPN N/C sensors - the BOB would only see a change when all sensor outputs open


The BOB input is connected to pin 2 of an opto-isolator

when the input is grounded current flows from +5V via a 330 ohm resistor and the opto-isolators LED

( the resistor limits the LED current to about 10 mA )

John

[millhouse_]

Thanks for the infos. Only one sensor per Pin is connected.

If it is no problem to connect to both -ve then it is working as it should.


Sensors are LJ12A3-4-Z/BX


@john-100 You do really great work. Very helpful.

And thanks to breezy1849 too.

[john-100]

Hi millhouse_

this is what you should have



if you have the jumpers fitted on the BOB

Vss is connected to the printer port GND & - 5V
Vdd is connected to the +5V supply

without the jumpers Vss & Vdd need to be connected to an additional 5V supply
this has the advantage of the input terminals being totally isolated from the printerport

John

[millhouse_]

This is how it looks now:


The -ve from both power supplies are bridged now.

All + & - from the sensors (brown/blue) are connected to a 12v Terminal at the z-axis.

All black signal wires go to the control box in one 3x0.75mm^2 wire.

The signal wires go from the blue terminal to the simple build with WAGO clamps and a diode between each pair. From there the signals go to BOB.

[john-100]

Hi millhouse_

is this how you connected the diode

with the cathode connecting to the sensor output ?




John

[millhouse_]

Basically yes. But only the black wire is connected with the BOB. Blue and Brown are at the 12v Terminalblock behind the Z Axis and from there only one 2x 0.75mm^2 goes back to the controlbox and directly to the 12v PSU.


With the bridged -ve's it works. The Pins switch between HIGH&LOW.


Edit: You can see it at the blue terminal block. On the left there are the 3 signals and on the right 12v+- going to the 12v PSU.