Can anyone tell me how to replace mechanical limit switches with prox switches and wire them to a breakout board?

Have you decided on the type of proximity switches you want to use (make and model) ?.

Tweakie.

I have some 6 - 24 volt PHD (source- PNP) prox switches that I want to use on some pneu. cylinders to indicate that they are extended or retracted. They have 3 wires :  brown (+DC), blue (-DC), and black (to load and then grd.) I am just not sure how to wire them in and I don't want to do trial and error and create smoke!

With this type of sensor you cannot wire it directly to your BOB.
The easiest solution would, I think, be to operate the device at 12 Volts and connect the output (black) wire to a 12 Volt relay coil with the other side of the coil connecting to ground (blue) then, just in case there isn't one already fitted within the sensor, connect a diode across these relay coil connections (polarity + to black / - to blue). You can then use the relay contacts N.O. or N.C. (as you choose) to connect to your BOB and ground.

Tweakie.

Thanks so much for your help. I assume that I can pick up a diode at my local Radio Shack store. Is there any specific value or size that I need to ask for?

The diode doesn't have to be anything special a 1Amp type 1N4004 or similar would do just fine. (It just stops the back emf from the coil damaging the PNP transistor when the load is disconnected).

Tweakie.

With this type of sensor you cannot wire it directly to your BOB.
The easiest solution would, I think, be to operate the device at 12 Volts and connect the output (black) wire to a 12 Volt relay coil with the other side of the coil connecting to ground (blue) then, just in case there isn't one already fitted within the sensor, connect a diode across these relay coil connections (polarity + to black / - to blue). You can then use the relay contacts N.O. or N.C. (as you choose) to connect to your BOB and ground.

Tweakie.

Why would he not be able to just use series current-limiting resistors and clamping diodes to the +5V on the BOB, or just a resistor divider?  Relays seem like overkill.

Regards,
Ray L.

Hi Ray:
Could you draw me a schematic of what you are referring to? Thanks.
Charlie

Hi Ray:
Could you draw me a schematic of what you are referring to? Thanks.
Charlie

This problem is typically handled by putting a resistor in series with the 12V signal, to limit current into the BOB or PC.  Just limit the current to a few milliamps.  The resistor limits current flow from the 12V supply to the 5V supply of the PC or BOB.  A diode connected from the BOB input pin to its 5V supply ensures the voltage on the input pin cannot exceed the supply voltage.

You might want to take a look here:  http://cnc4pc.com/Tech_Docs/C11R9_3_USER_MANUAL.pdf starting on page 9....

Regards,
Ray L.