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Thread: Can someone check my plan for digital inputs (V3 brain)

  1. #1
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    Sonicxtacy02's Avatar
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    Can someone check my plan for digital inputs (V3 brain)

    i've taken a look at the Digital Input FAQ and i believe i'm ready to tackle what i'm trying to do. I just want to confirm before frying my FB. My goal is to connect my rear backup camera's power to my reverse light's 12v. Then i'm going to take a step-down 12-5v voltage converter and connect the reverse lights 12v to the FB digital input. Doing this will allow me to detect when the cars in reverse and send a command to my front end to automatically switch to the camera screen.

    As i understand it my plan will work provided i use one of these to keep the input pin grounded until 5v is sent to it from the voltage converter, but does the 5v pin require constant 5v? I guess i dont get why i have to send 5v down the input pin and the 5v pin.
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    Constant Bitrate ACCLR8N's Avatar
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    No, when I'm checking for on/off I have the car power connected to a 2 resistor voltage divider. I use vehicle power, 10K Ohm resistor, 3.3K Ohm resistor to ground. Connect a Fusion Brain analog in to measure between the two resistors. It will normally see about 3.5V with car running. The 5v out and ground on that FB analog input port are not connected to anything. Set your software to turn on output for anything over 2.5V.

    Keep in mind most alternators put out just over 14V when the car is running. The 10K & 3.3K divider will protect the brain up to 20V. I've seen as high as 18V when having charging system problems.

    If you have your heart set on using the digital input, connect the back-up light power to a relay. The relay can then close a circuit between the digital 5v out and input pin. You will then need your 1K ohm resistor to pull the input pin low to a ground.

    Get the $13, 500 piece resistor pack.

  3. #3
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    Quote Originally Posted by ACCLR8N View Post
    No, when I'm checking for on/off I have the car power connected to a 2 resistor voltage divider. I use vehicle power, 10K Ohm resistor, 3.3K Ohm resistor to ground. Connect a Fusion Brain analog in to measure between the two resistors. It will normally see about 3.5V with car running. The 5v out and ground on that FB analog input port are not connected to anything. Set your software to turn on output for anything over 2.5V.

    Keep in mind most alternators put out just over 14V when the car is running. The 10K & 3.3K divider will protect the brain up to 20V. I've seen as high as 18V when having charging system problems.

    If you have your heart set on using the digital input, connect the back-up light power to a relay. The relay can then close a circuit between the digital 5v out and input pin. You will then need your 1K ohm resistor to pull the input pin low to a ground.

    Get the $13, 500 piece resistor pack.

  4. #4
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    whew.. just a TAD over my head lol. I've never worked with resistors before. So you're saying connect a 10k resistor to the 12v coming from the reverse light, then a 3.3k to ground, then connect those to the analog input pins on the FB? Sounds easy enough i spose.
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  5. #5
    Constant Bitrate ACCLR8N's Avatar
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    12v-----10K--3.3K-----GND The two resistors are connected to each other. The FB AIN is "T" in between them.

    http://en.wikipedia.org/wiki/Voltage_divider

  6. #6
    Raw Wave
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    I reckon JustChat's reply sums my thoughts exactly - albeit with somewhat less verbiage!

    But... (oh sht, here we go again...) For more info & better else simpler solutions....

    Many "digital" inputs are high impedance - ie, they will not provide nor accept (source or sink) much current.
    "Digital" inputs tend to "read" high or low, where low might mean "less than 1 volt" and high "more than 3 volts". (That may be a TTL spec; others may use about 0.5V above and below half the voltage supply - ie, 5V x 1/2 - 2.5V. +/- 0.5V means <2.0V or >3.0V.)

    So many 5V digital inputs may tolerate +16V - the common max design voltage used for 12V systems.
    Chips/IntegratedCircuits/ICs themselves may be different - especially the old "TTL" chips that started the 5V standard and were the technology when "computers" became common.
    But most "systems" should function that way.
    And I really should look into this Fusion Brain - I have merely looked at a pic of it!

    And whilst the voltage divider is a good and "safe" way to reduce signals, there are 2 exceptions:
    1. When the "divider" isn't needed - just a "pull up" resistor to hold the input "not low" - typically a 1k or 10k Ohm resistor to +12V etc. (That's for those with hi-impedance inputs).
    2. When the input cannot exceed +5V else the input will blow - eg, TTL. Then a 4.7V or 5.1V zenor-diode replaces the lower voltage-divider resistor. That means even if the source signal goes higher than 5, 12, 16, 50 Volts etc, the zenor "clamps" or limits the voltage to 4.7V etc. See http://en.wikipedia.org/wiki/Zener_diode.

    Whilst the "zener-clamp" circuit design may seem complex, once its circuit is known, a simple circuit with a resistor or zenor range can be used for all applications.
    EG - for the fusion, a xx-Ohm resistor with 4.7V (400mW) zenor for up to 50V input, else yy-Ohm z-Watt resistor with 4.7V 1W zenor up to 1,000V etc.
    And the zenor protects against negative spikes.
    And as with the voltage divider, a small capacitor can be added to suppress noise etc.

  7. #7
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    Got it. Thanks guys. One last question, is the 5v pin on the FB not used in this case?
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  8. #8
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    And now I read the PIC specs etc.

    So no digi inputs, but use the anal inputs instead.
    From the Digi i/p FAQ link above, that requires a 1k resistor between the input's +ve input and ground to hold the input at 0V (call that low).
    Then a switch between Digi +5V and the input pin gives it 5V (call that high).

    Ok - me has now d'oh'd the above.

    But yes - if the voltage divider is used, the +5V pin is not.

    But I worry that the analog (digi) input's ground might not be the same as vehicle ground. (The dreaded differences between signal grounds and power grounds.)

    The best solution is probably opto-couplers - eg, the 6-pin 4N28 or similar 4N2x or 4N3x packages.
    They are around $2 each.
    They are a 10mA LED on the input side, and transistor on the other.
    The advantage is that it's a mere LED circuit which is fairly voltage tolerant with a suitable resistor.
    Furthermore, if some bad input does blow the LED, it won't blow the FB input. (Opto couplers have an isolation of hundreds or more volts.)

    But surely GreenMan else others have a suggested interface circuit?
    (I'd have to sit and work out ground tolerance etc. the FB low is <0.8V & hi is >2.0V with an internal pull-up current of nearly 0.5mA..... not now though!)

  9. #9
    Constant Bitrate ACCLR8N's Avatar
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    Quote Originally Posted by Sonicxtacy02 View Post
    Got it. Thanks guys. One last question, is the 5v pin on the FB not used in this case?
    I just use the analog in pin (AIN). My voltage dividers are grounded to the same point as my power lead ground supplying the Fusion Brains.

  10. #10
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    Just wanted to put some closure to this thread. Thanks to you all for the guidance on using analog inputs instead of digital. I did end up making a voltage divider, but i found this jewel and it did the job.

    Now i'm going to use a second channel from that device to manage my headlight switch.
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