Easy Electrical Question - Before I fry this board.

Hi Rick,

What sort of proximity sensor do you have? :o

That seems like a straight short circuit. If you feed 2A to your pins, yes you will fry them and probably do a lot of damage to the rest of the MCU.
Without knowing which sensor you have it's hard to help you (proximity could be a ton of things).

Edit:
For future reference on the MCU proper operating conditions can be seen in your part datasheet under 24.3 Recommended Operating Conditions. There are other tables for absolute maximum, the absolute maximum values although should not damage instantly your MCU, they will degrade it quickly and should be avoided, instead you should respect the limits imposed in chapter 24.3 on the datasheet.
For the GPIO Table 24-6. should give you the info about the maximum input voltage.
Although I didn't find anything about maximum input current, you should not force much current, anything like 1mA is already plenty enough for most cases (high speed readings would need to consider input capacitance and impedance).

If you are to connect the sensors output to an input on the TIVA, then the current draw will only depend on the TIVA's input impedence, so no problem(as long the voltage does not exceed the TIVA limits).

Now if the(this) sensors output draws 2 amps, then something else must be(is currently) connected to it...

Suspect you'd fare better by providing a link to your sensor - thus enabling those here to "better" understand & contribute...

Your reported "seeing" of 3V27 @ 2.0 amp "output" begs (some) description of "how" you made such observation.   I'd place tall stack of chips against those numbers being the "real/proper" output rating...

As always - when client, boss, teacher or (poster) here describes something as, "easy" alarms sound & iron (or plastic) dome deploys...

Capabilities of the device aside, connecting it directly to the microprocessor pins is probably "a BAD idea".

It probably is 'a bad idea', to connect it directly, but it shouldn't harm the input either, as long the voltage is between the limit's.

The TIVA input will never draw more current than it needs, the worst case what will happen is that the sensors output will not switch if the impedance is to high.

Thank you - appreciated - my suggestions follow:    (for clarity I present the key drawing - your device)

I've tried now three times to add verbage to the above drawing - "Post" button spins & spins - never completes.   I've had to copy/past to salvage that writing - it appears next post....

Here's a schematic - there are many ways to achieve your goal - this is just one.    Its merit is safety for your MCU - only 3V3 or ground will be presented to your MCU.   Comparators are available in many forms (some in duals/quads) this is just one example...   Note that the Prox ground - your 12V power supply ground and 3V3 ground all must be made common (tied together).   Only 3V3 powers the comparator - thus it should buffer/protect your MCU.

In this implementation - when your Prox "turns on" (senses target) the black-blue voltage should approach ground - which causes the comparator's output to turn on - and provide 3V3 into your MCU.   (and that's current limited by the pull up resistor (shown).   When the target is "out of range/removed" the blue-black voltage should exceed 3V3 - and the comparator's output voltage will approach ground...

Note: I've edited to change comparator's power to ~12V (to exceed any voltage applied to either input pin).    Such comparator - to protect the MCU should employ "Open Collector Outputs" so that the output voltage fed to the MCU is limited to the 3V3 (as shown) provided by the output's pull-up resistor.

Robert Adsett72 said:

The comparator is good but a number of comparators will not withstand 12v on an input pin when supplied from 3V3.

You are (again) indeed, "Correct Sir."    Such was my screw-up during last night's rush.   To resolve I've edited the schematic to show the comparator powered from 12V (or the same voltage fed to the "top" prox. pin.)    To best protect the MCU I've specified an "open collector, output" style comparator - which greatly facilitates output voltage selection.   (the pull-up shown returns to 3V3 - perfectly mating to the MCU)

Driving to work this morn I realized that either a low voltage zener diode (< 3V3) - or a series connection of several silicon diodes - either connected to that black prox lead or terminal - likely eliminates the need for the comparator.   Zener may prove risky - as most zeners do not exhibit a sharp voltage "knee" when the zener voltage is below 4V.   Four series connected silicon diodes w/current limiting resistor should protect the MCU while enabling adequate signal swing @ the MCU.

Robert Adsett said:

There is a resistor pull-up scheme that's probably a little more robust.

Now we disagree!    Return your gaze to the drawing I cut/pasted/presented - you'll note that a 10K R (rather admirably) serves that "pull-up" purpose!    And its already there!    (w/in the prox switch)

That 10K R w/in the prox forces my series R to be much higher in value to enable the signal to exceed the combined "drop" of those cascaded diodes.   (schematic already amended...)   [edit: this poster pleads, "temporary insanity" for prior sentence.    So long as that prox 10K remains "pulled up" to ~12V - the resistor in my schematic is not truly needed.     Series diodes will "see" ~2V8 until the NPN turns on - which reduces that diode voltage to near zero - thus complying w/MCU's desired logic level, "signal swing."]

I did say more robust, not that it didn't work.

Provided the insert worked.  Here is a simplification of what I have used in the past

R1 forms the pullup to 12V sized to be noise resistant (by providing a good strong current flow) and provide sufficient operating current.

R1, R2, and R3 for a divider chain to reduce the voltage to the range of the micro's high input.

C1 provides noise filtering and a little surge capacity

D1  Provides power to the Prox while guarding against high voltage connections

D2 (Optionally) provides protection against shorts to positive supply. When the Prox pulls this low there will be 1V5 to 2V between R1 and R2, The R2/R3 divider will reduce this to be under the micro's low voltage threshold

R2,R3,D3,C1 provide protection against pulses (and if D1 is not present high voltage connections).

C2 is optional but will provide wetting current  and some filtering if present

D4,R4 provides a 'ground' connection with protection against connection to a negative source with R4 protecting against shorts to positive.

Not shown are protection against shorts on the power feed and EMC protection.

An option Schmidt trigger inverter is shown that would precondition the input before it reaches the micro.  Also would act as a last ditch sacrificial lamb to protect the micro.

This will also work with switch inputs and two wire prox switches.

Robert

That took multiple tries but it looks like the image is finally there.

OK an ugly drawing of the relay version.  I will try to clean it up later

I think this is simpler the understand. 

D1 provides polarity protection,

D3 (diode) and D2 (zener diode) provide coil suppression.

R1 is a pullup to the micro's power supply.

As before a Schmidt Inverter can be used to clean up the signal.

One of the advantages of this configuration is that the coil effectively isolates the micro from off board influences. The relay could be replaced by an optocoupler fairly obviously.

Again this will also work with switches and two wire proxes

Robert

Note: your 2nd diagrammed post "crossed w/this response."   This writing references your 1st diagram & definitions.

Believe that you've provided a very complete, "MCU input level shifting & protection circuit" for this community - good job & thanks.    And - your use of that (potentially) "sacrificial" input gate/inverter makes tremendous sense.     (may save the repair/replacement of a 64 pin (or larger) MCU - do that once - sure to be (one hopes) your last!)

Good and robust as your circuit is - should not we "adjust" our design to the expressed "needs/wants" of the client?    And - thus far - poster (while expressing appreciation) has yet to demonstrate his requirement for so, "bullet-proof" a circuit.   My effort was "quick/dirty" and, "Good for (most) G'ovt Work!"   Over design may not always prove best - and as the number of components increases - the robustness/reliability (usually) degrades.

Poster did (ever so briefly) mention his desire to, "Run a motor" in this exercise.   And based upon the current demand of that motor - and the safe rating of the prox's NPN switch transistor - the size/selection of your "R4" (which is situated directly in the ground path of prox) may have to be carefully calculated & considered.    Minus that mention - and user awareness - robustness is likely to be sacrificed...

Tradeoffs - and "devil in the details" - all conspire to demand full/adequate communication between the user & solution provider...

May I note that poster (once again) titled this as, "Easy Electrical Question!"    (easy {perhaps} when all facts are known, organized & presented {in detail} w/clear budgetary, size, usage and delivery guidelines!)    Is this (still) easy? 

cb1- said:

Note: your 2nd diagrammed post "crossed w/this response."   This writing references your 1st diagram & definitions.

cb1- said:

Poster did (ever so briefly) mention his desire to, "Run a motor" in this exercise.   And based upon the current demand of that motor

That's the second reason for the relay version. You can easily run two relays in parallel on the single prox detector.  The second relay can run the motor.  This has the advantage of being able to run and test either the motor or logic circuit separately and more important removing the motor noise from the board (except as it is introduced through a common power supply).  It would also allow running the motor on a separate power supply further reducing noise problems.

The first reason for the relay version is I think it is easier to understand and build.

You're right about the simplicity of the first version.  I don't know about you but I usually see more mistakes in Q&D lashups then one built with a minimal amount of protection.  I still do them though. I do think it's worthwhile letting know there's more to building a reliable input than just wiring it naked to the micro. A hobbyist might not need to provide full EMC protection but protection against inadvertent miswiring might be even more important.

Robert

cb1- said:

Is this (still) easy?

Isn't that one of the rules?  "It's never easy"

;)

Robert

Truth in advertising - I never even considered a relay - or two - with a 2nd power supply to (greatly) reduce noise & related issues. (almost always a nightmare - best prevented rather than "cured.")

As for "quick/dirty" our small firm requires "2nd set of eyes" and written description - so that a signed/dated, "paper trail" exists. (proves so helpful - days/weeks downstream when we wonder - wtf?)

One of our methods in "preventing harm" from such Q&D efforts is the design/development (and strict use) of multiple, "standard" board & MCU protective circuits (small pcbs) which eliminate many/most of the errors caused by "undisciplined" Q&D... We reuse such small pcb standards time & again - tweaking them as new needs arise...

Rick Faszold said:

I'm using an old PC Power supply, feeding the sensor with the 12V wires.
To go from 12V to 3V (off of the sensor), I'm using a Keedox® DC/DC Converter 12V Step Down to 3.3V 3A Power Supply Module, Non-isolated.

Rick Faszold said:

As I read the specs on the Converter, I can clearly see where this thing will output 3A.
Wonder if my I2C chips are fried now.... :-)

That's the setup. Electrically, it "works", in that I can see 3V 2A when near metal and nothing when on paper.
I was hoping to use a pin on the Tiva to detect closed/open circuit.Rick

Rick Faszold said:

I am sure there are TONS of things to talk about here.
For instance, how do I go from 12V to 3V and only allow 50ma-100ma for instance?

While this latest arriving post offers the simplest (single added R) "solution" it may not be safest!     Look again at o.p.'s sensor diagram:

Note that a load is shown - imposed between "brown & black" - and if that load is even as high as 20K impedance or less - (not at all hard to imagine) the "too simple" voltage divide just suggested - will not adequately limit the voltage fed to the MCU - likely destroying it!    Not good - that!

Earlier efforts here may have far better anticipated such (likely) occurrence.    Too quick/simple - while tempting - does have pitfalls...