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basic: selecting a resistor

Discussion in 'Electronic Basics' started by Costas Vlachos, Feb 8, 2004.

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  1. From your description it looks like this is some sort of active keyboard
    which requires a power supply and directly provides row/column signals which
    you can read using the uC. If that's the case, then it looks like it needs
    pull-down resistors to work. 4.7k or 10k should be fine. Don't use megaohm
    resistors as they may not be able to pull the pins to ground hard enough.

    Also, since you're posting to s.e.b, I have to ask: Are you sure the
    keyboard works in the way you describe? Normally in matrix keyboards you
    *send* signals to the columns (or rows) and *read* the rows (or columns) to
    determine the key pressed. You don't directly read all 7 pins.

    cheers,
    Costas
     
  2. Gareth

    Gareth Guest

    This is to be expected.
    You are correct that a lower value resistor will use more current, but
    only when a key is pressed; leakage from the pin of the microcontroller
    will be very small even if you soldered the pin directly to ground. To
    calculate the current use Ohm's law:

    V = I*R

    V = Voltage in Volts, I = current in Amps, R = resistance in Ohms

    When a key is pressed you have 5V across the resistor, so:

    I = 5/4700

    I ~= 1mA

    This current will flow in each of the resistors which have 5V across
    them, which is two (one for the row, one for the column), so while a key
    is pressed you will be using 2mA. You could reduce this by increasing
    the resistor value, but you make the resistor to high your device will
    more susceptible to noise or interference. I think a couple of Mohms
    would be too high.

    It is difficult to say exactly what is too high, but think you should
    consider the following:

    1) Current only flows in the resistor when a key is pressed, so consider
    how long and how often a key is likely to be pressed. For example, if a
    key is only likely to be pressed four times for 0.25 seconds in every
    hour, the average current in the resistors will be 4*0.25/(60*60) =
    1/3600 of the current calculated above. This may be insignificant.

    2) How is this device powered? If power consumption is not a problem
    then don't worry about 1mA through the resistors.

    Gareth.

    --
     
  3. goose

    goose Guest

    Hello all

    I have a microntroller connected to a keypad. It only
    reliably works when I use a resistor to ground on each
    pin on the uC that connects to the pins on the keypad.

    the keypad works as follows:
    1. 3 pins are connected to each column.
    2. 4 pins are connected to each row.
    3. 1 pin seems to be bridged to everything.

    [1] & [2] above is expected for a matrix keypad.
    [3] supplys power (+5v).

    I supply +5v to the power pin of the keypad, and
    read the other 7 pins. when any key is pressed,
    a pin for the column and a pin for the row goes
    high.

    each pin is wired as follows:

    uC pin ---------+----------- keypad pin
    |
    |
    R1
    |
    |
    gnd

    my question is what resistor should I choose for R1?

    As I understand it, the smaller the resistance, the
    greater the leakage current, no? I initially intended
    to use 4,7k resistors (which is what I tested with),
    but I now suspect that I should choose a larger
    resistor (a coupe of mega ohms?).

    tia

    goose
     
  4. vic

    vic Guest

    What you need to know to calculate the maximum resistor value is the
    leakage current of your uC pin. Let's say the maximum specified leakage
    current is 1uA, and the logic low level limit is 1V :
    Rmax = 1V/1uA = 1Mohm
    By safety you should always multiply the limit values by 2, so I would
    say 250kohm would be sufficient for this example.

    Adapt these values to those specified in the datasheet of your
    microcontroller.

    As I understand your case, the current through the resistor gets high
    only when a key is pressed, so there wouldn't be much losses on average
    anyways.
    vic
     
  5. goose

    goose Guest


    Thanks to all the people who replied
    (Costas, Gareth and Vic). Your replies
    are immensely(sp?) usefull. To answer a
    few questions:
    1. the keypad *is* self-powered.
    2. the current through the resistor
    does indeed only flow when a key
    is pressed (two pins go high).

    once again, thanks for your help :)

    bye
    goose
     
  6. Guest

    Aren't most microcontrollers better at sinking. Did you set up the
    port flags correctly. Did you code a de-bounce algorithm. If you set
    the port flags up wrong you will have high impedance which might cause
    this problem. Just if you could post the microcontroller you are
    using and keypad brand and model number. Anyways just a thought, just
    don't wish to shoot ideas into your head that will send you down it
    the wrong direction.
     
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