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Circuit Questions

SRLMSRLM Posts: 5,045
edited 2012-10-03 19:52 in General Discussion
I have three circuit questions to ask:

1. In the following circuit, should I have series resistors in line with each of the diodes? The specs are:
--- Batt is 0 to 4.5 volts
--- P10 is 0 or 3.3 volts (Propeller I/O)
--- EN123 is a digtal input, logic low of 0 to 0.4v, logic high of 1.2v to 6v, and leakage current of +-1uA
I want to have EN123 be high if either P10 or SW is asserted. So, this is similar to the logical "OR" condition.
attachment.php?attachmentid=95972&d=1349162369


2. In the Quickstart reference circuit both the blue and the red LEDs are powered through the same 100Ohm resistor. From what I've learned, each LED must have it's own resistor. Does the Quickstart schematic have a flaw? For comparison, the Eddie control board (newer?) has a 270Ohm resistor for each LED.


3. How can I have two sources connected to the BOEn of the Propeller chip, and allow either (or both) to reset the Propeller?
--- Source 1: FTDI chip, standard circuit (see the circuits above).
--- Source 2: Logic 0 or 3.3v (pulled up, similar to I2C).
Enable_OR.png
668 x 560 - 8K
Eddie_control_board_LED.png
918 x 631 - 95K
quickstart_FT232_LED.png
895 x 761 - 99K

Comments

  • RDL2004RDL2004 Posts: 2,554
    edited 2012-10-02 04:36
    It's not a problem to use one current limit resistor for multiple LEDs. However, the brightness of the LEDs will drop lower for every additional LED that turns on. This is probably not a major issue with just two indicator LEDs, but can be very noticeable and annoying with something like a seven segment digit displays.


    edit: as Kwinn pointed out in #6 but I failed to mention, the LEDs must have a similar voltage drop for one resistor to work.
  • Mark_TMark_T Posts: 1,981
    edited 2012-10-02 08:21
    1) What voltage is "+++"?
  • SRLMSRLM Posts: 5,045
    edited 2012-10-02 09:07
    Mark_T wrote: »
    1) What voltage is "+++"?

    That was my attempt to indicate "anything". For practical purposes, let's say 6volts (more than battery or Propeller IO).
  • tonyp12tonyp12 Posts: 1,951
    edited 2012-10-02 09:27
    In your circuit are the diodes LED-diodes?

    Does the EN123 input have built-in Pull-up resistor? (that is pretty common)


    ORering is best done with open-drain layout, the Prop pin can be turned to an input to emulate that.

    sharing a 100ohm, maybe the blue and red LED is never on at the same time? then is is perfectly OK.
  • kwinnkwinn Posts: 8,697
    edited 2012-10-02 10:33
    1 Circuit in post 1 is fine as long as EN123 can handle the battery voltage. The left diode blocks it from reaching P10.

    2 Depends. If both diodes do not need to be on at the same time it is ok. If both are on at the same time the red one will be on due to it's lower forward voltage. So it really depends on what is needed.

    3 A signal diode on each source with the anodes connected to BOEn.
  • SRLMSRLM Posts: 5,045
    edited 2012-10-02 10:34
    tonyp12 wrote: »
    In your circuit are the diodes LED-diodes?

    Does the EN123 input have built-in Pull-up resistor? (that is pretty common)


    ORering is best done with open-drain layout, the Prop pin can be turned to an input to emulate that.

    sharing a 100ohm, maybe the blue and red LED is never on at the same time? then is is perfectly OK.

    -- I put the diodes in to prevent back current flow from one source to another. They are not LED.
    -- The datasheet for EN123 says "Logic Input Pulldown Resistance" of 400-1200kOhms
    -- Can you provide a schematic or a description of how it's connected? I'm unfamiliar with most EE concepts, and the internet isn't a help here.
    -- I don't know. Does the FTDI do that? I wonder...

    My main concern with question 1) is how to prevent the 4.5v of the battery from going back into the Prop, and vice versa.
  • Dave HeinDave Hein Posts: 6,347
    edited 2012-10-02 10:47
    The circuit looks fine to me, except that you may not need the pulldown resistor. It depends on whether the spec for EN123 means that it already has a pulldown resistor, or that you need to provide one. It sounds like it already has a pulldown resistor.
  • tonyp12tonyp12 Posts: 1,951
    edited 2012-10-02 11:08
    EN123 is that the pin name or the actual device? (can you share the device name please)

    If it does have ~800k Ohm pull-down,
    it's very weak pull-down and is there just in case the mcu pin boot up as a input-pin and the voltage will not slowly creep up to a logic high.

    Do you want the signal to pin EN123 on/high most of the time? or is low (gnd) 90% of the time?
    And device could actually have active-low enable, so until I know ic name.
    Designing a good pull-up/down layout is more of a consern when running on battery.
  • frank freedmanfrank freedman Posts: 1,983
    edited 2012-10-02 11:58
    SRLM wrote: »
    I have three circuit questions to ask:

    1. In the following circuit, should I have series resistors in line with each of the diodes? The specs are:
    --- Batt is 0 to 4.5 volts
    --- P10 is 0 or 3.3 volts (Propeller I/O)
    --- EN123 is a digtal input, logic low of 0 to 0.4v, logic high of 1.2v to 6v, and leakage current of +-1uA
    I want to have EN123 be high if either P10 or SW is asserted. So, this is similar to the logical "OR" condition.
    attachment.php?attachmentid=95972&d=1349162369


    2. In the Quickstart reference circuit both the blue and the red LEDs are powered through the same 100Ohm resistor. From what I've learned, each LED must have it's own resistor. Does the Quickstart schematic have a flaw? For comparison, the Eddie control board (newer?) has a 270Ohm resistor for each LED.


    3. How can I have two sources connected to the BOEn of the Propeller chip, and allow either (or both) to reset the Propeller?
    --- Source 1: FTDI chip, standard circuit (see the circuits above).
    --- Source 2: Logic 0 or 3.3v (pulled up, similar to I2C).
    Question 1: these used to be called steering diodes in some of the Siemens systems where this circuit was used to control a function from multiple points Many times different footswitches or other control signals typically on +24V control ckts. And yes it is an or function, done this way so that one voltage source could not feed back into a less positive source. That was why the diodes rather than tying all inputs to say a terminal strip.

    For item 3, you may need a zener ckt to limit the voltage out of the diode summing point. Then (no prop doc on hand) put that signal into the reset pin or via inverter if it should be /reset. Just quock thoughts.
  • SRLMSRLM Posts: 5,045
    edited 2012-10-03 18:45
    tonyp12 wrote: »
    EN123 is that the pin name or the actual device?

    EN123 is the name of the input pin on the device.
    tonyp12 wrote: »
    Do you want the signal to pin EN123 on/high most of the time? or is low (gnd) 90% of the time?
    And device could actually have active-low enable, so until I know ic name.
    Designing a good pull-up/down layout is more of a consern when running on battery.

    EN123 is high most of the time (active high), and yes: the system is running on battery.
  • SRLMSRLM Posts: 5,045
    edited 2012-10-03 18:46
    Thanks everyone. I've received the answers that I'm looking for, and so I'll mark this thread solved for now. I'll post my final schematics when I'm done and have tested everything.
  • tonyp12tonyp12 Posts: 1,951
    edited 2012-10-03 19:52
    Instead of diodes, you could use two 200k Ohm resistors.
    With the internal 800k pulldown, you will create a 3/4 voltage divider.

    But P10 should be output + high for on and turned to a input for off, as to not 1/2 volt divide the push down button.
    Or you could use a 20k ohm for the button and it still will "win" e.g result is higer than 1.2v

    Though a 800k is pretty weak, the good part is the extra 200k Ohm will reduse the battery drain a little more
    And you should not use the 100k pulldown you have in the picture.
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