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Author Topic: Help needed Connecting NPN Proximity sensors to BB`s  (Read 24816 times)

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Offline kcrouch

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Re: Help needed Connecting NPN Proximity sensors to BB`s
« Reply #10 on: May 11, 2008, 08:06:41 AM »
The BOB has an led with a 330 ohm current llimiting resistor in series with it. By adding the additional 330 ohms pull-up to 5 volt the current is reduced by 50%, but should still be enough. It would also be ok to not use my pull-up to 5 volts, but instead use a single 490 ohm pull-up to the 12 volt supply. This would give full current to the led. Your voltage divider would also work, but it involves more parts. The problem is that the sensors are open collector NPN, while the board is looking for PNP style outputs. This is why You need to change the active state and use the switches in parallel, rather than in series. A proper solution would involve connecting the switches to and gates, but then you would need another circuit board with the gates on it. Wired as I propose, the BOB led would be on when the switches are clear and off when any of the switches are near metal. However the circuit won't be fail safe in the event of a broken switch wire.
Kenny
Having way too much fun! Something must surely be wrong.

Offline jimpinder

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Re: Help needed Connecting NPN Proximity sensors to BB`s
« Reply #11 on: May 11, 2008, 02:45:46 PM »
Sorry - I didn't  have any details of the Brealout Board.

I would go back to one of my earlier posts - I would think a different sensor, preferably 5 volt PNP, would make life a lot easier, especially when you are all up and running - and all of a sudden, one of your sensors fails. The fewer parts, the easier is life.

Having said that - the BOB is a bit of a *****. The vast majority of sensors pull the signal to 0v. This is what the computer port is looking for. Why invert it.
Not me driving the engine - I'm better looking.
Re: Help needed Connecting NPN Proximity sensors to BB`s
« Reply #12 on: May 11, 2008, 06:18:41 PM »
Thanks Guys for you input,

All seems like lots of  trial and error (and lots of smoke,If I stuff it up),so I might give it a miss

Yes ,Jimpinder the BOB is abit of a #?##@,I haven't had much luck with other things on the board as well, and as I have wasted enough time now,I think its going to look real nice sitting next to the other paper weight,the G100.
Another excellent purchase,NOT!!!!

Rather than pull all the switches off and rewire new switches ,I think I will just go and get another BOB,the one I was looking at to start with that handles these switches,
A pitty I was told different prior to purchase?

Anyway, thanks guys for looking into this for me,

Regards
Whatda?


Offline SimonD

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Re: Help needed Connecting NPN Proximity sensors to BB`s
« Reply #13 on: May 12, 2008, 04:34:49 PM »
Am I missing something here...

The C23 wiring info indicates how to connect a 12v sensor...

Quote
NOTES:
This board has 18 inputs and 16 outputs for a total of 34 I/Os.
1.All inputs and outputs are provided with pull-down resistors.
2. If you need to connect a device that outputs more than 5vdc, you can use a resistor to limit the current.
Use the following resistor values:
10 vdc - 1 MOhm
12 vdc - 1.5 MOhm
24 vdc - 3.9 MOhm.
To use these resistor values your card must be powered with 5vdc.  If you are powering your board with a different voltage unexpected results can happen., including damage to the board.

http://cnc4pc.com/Tech_Docs/C23R2WG.pdf

Does this not work?

Offline kcrouch

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Re: Help needed Connecting NPN Proximity sensors to BB`s
« Reply #14 on: May 12, 2008, 07:09:58 PM »
I'm afraid so. Unless Ohm's law has changed today. I suspect a typo in the documentation.

The optoisolator LTV847 has a 1.4v forward voltage drop. In a 5 volt circuit the total lost voltage across the current limiting resistor is 3.6v. With a 330 ohm resistor already in series and using a 5 volt supply You get 3.6/330 or 11ma current. With a 12 volt supply You get 12-1.4 or 10.6v/11ma = 963 ohms. Since there is already a 330 ohm resistor present You have 963-330 = 633 ohms. I believe that the M is a typo and should be K Ohms. even then, the resistor will not allow for sufficient current transfer through the opto. You want to get at least the same 11ma that the board was designed for. That said a resistor of 633 ohms at 12v is correct. The LTV847 will easity accept 50 ma so a 560 ohm resistor at 12 volts will assure sufficient opto operation albeit at 13ma.

Regards,
Kenny
Having way too much fun! Something must surely be wrong.

Offline SimonD

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Re: Help needed Connecting NPN Proximity sensors to BB`s
« Reply #15 on: May 13, 2008, 03:39:00 PM »
Thanks for the explanation! I would have blindly followed the values given in the documentation!

Thanks
Simon