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  1. #61
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    12th September 2015
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    Quote Originally Posted by UhClem View Post
    I can see several single point failure modes that would result in an energized output. With no warning unless you tap the clips together.

    1) RLY1 contacts shorted. A not uncommon failure mode.
    2) Q1 fails energizing RLY1
    3) The Arduino fails turning on Q1.



    At the bare minimum there should be an audible warning if the relay is closed. Better would be to include that and remove the single point failure modes.

    The simplest being to have the relay switch the high side when the arm key switch on the controller is enabled. Then use a low Rds(on) FET to switch the low side. You might need a FET driver to fully enhance it. (The MIC5018 is my favorite.)

    In addition a pad side safe/arm switch that disconnects the igniter outputs from the electronics is a very good idea. (DARS has had good results with 20A DPDT toggle switches.) Otherwise this system is totally dependent on the software operating correctly. Proving software correctness (not only what you write but in the Arduino libraries and in the XBee) is hard.

    The keyswitch is wasted as it serves no useful purpose.
    Then to the point of driving the high side...
    I am attaching a modified schematic to try and capture your thoughts...

    Launch Pad_Control Shield Alt.pdf


  2. #62
    Join Date
    9th May 2009
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    1,531
    Quote Originally Posted by UhClem View Post
    Proving software correctness (not only what you write but in the Arduino libraries and in the XBee) is hard.
    That was one motivation for suggesting doing away with the Arduino and letting the XBee do everything (though it assumes that the people at Digi know what they're doing). That said, it's not very complex code, although it's amazing how simple code can have complicated and unexpected errors...

    I'd be a little more concerned about the power-up behavior of the XBee and the Arduino. One hopes that the XBee would never ever drive an output during power-up, but extensive searches of the datasheet failed to find an explicit statement to that effect. Likewise for the AVR in the Arduino, although that datasheet is a lot longer.


  3. #63
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    31st December 2009
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    Las Cruces, NM
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    I agree with David's assessment of what we can see of the circuit so far. I know Dave's background as an EE with decades of experience. My professional background is in my LinkedIn page (see my sig below). I don't know Dan Fox's background. But, I appreciate his willingness to share his design and get some feedback.

    Here's my non-wireless simple launch circuit that incorporates much of the safety features David mentioned. I've shared this on the web many times. At least 10,000 launches have been done based on this design over the past 20+ years. Adding a simple/secure wireless link in between could be done without introducing too many additional failure modes. XBee + Arduinos + libraries + control software does not make a simple wireless link, but it's accessible to hobbyists without professional EE background.

    Link: relayer.pdf

    Some requirements that are met with this (non-wireless) design:
    * Shorting a connector cannot activate the relay.
    * There is a physical disconnect to safe the pad.
    * There is a physical disconnect to safe the controller.
    * Autoreset thermal breaker to protect wiring if there are shorted leads.
    * If the relay is stuck closed for any reason, there is an alarm.
    * No MOSFETs to fail, or be susceptible to RF flooding.
    * Continuity at both sides (~5mA test current, could be less with an added opamp).

  4. #64
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    12th September 2015
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    I spent 38 years designing test equipment and in product systems for several automotive and automotive electronics companies.
    I am the designer of the Wilson F/x launch system for 17 years and hold an Amateur Extra license.
    I don't claim to know everything but I try to stay active in helping people learn.

  5. #65
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    3rd February 2012
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    So Cal (ROC, TRASD, SCRA)
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    2,337
    Quote Originally Posted by mikec View Post
    That was one motivation for suggesting doing away with the Arduino and letting the XBee do everything (though it assumes that the people at Digi know what they're doing). That said, it's not very complex code, although it's amazing how simple code can have complicated and unexpected errors...

    I'd be a little more concerned about the power-up behavior of the XBee and the Arduino. One hopes that the XBee would never ever drive an output during power-up, but extensive searches of the datasheet failed to find an explicit statement to that effect. Likewise for the AVR in the Arduino, although that datasheet is a lot longer.
    Arduino ATMEGA328P I/O pins are configured as inputs on boot, so they'll be at whatever state your pullup or pulldown resistors have them. The only exception that I've seen is the D13 pin... it glitches on startup, probably because it's used as part of the internal bootloader handshake.

  6. #66
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    12th September 2015
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    Quote Originally Posted by jsdemar View Post
    I agree with David's assessment of what we can see of the circuit so far. I know Dave's background as an EE with decades of experience. My professional background is in my LinkedIn page (see my sig below). I don't know Dan Fox's background. But, I appreciate his willingness to share his design and get some feedback.

    Here's my non-wireless simple launch circuit that incorporates much of the safety features David mentioned. I've shared this on the web many times. At least 10,000 launches have been done based on this design over the past 20+ years. Adding a simple/secure wireless link in between could be done without introducing too many additional failure modes. XBee + Arduinos + libraries + control software does not make a simple wireless link, but it's accessible to hobbyists without professional EE background.

    Link: relayer.pdf

    Some requirements that are met with this (non-wireless) design:
    * Shorting a connector cannot activate the relay.
    * There is a physical disconnect to safe the pad.
    * There is a physical disconnect to safe the controller.
    * Autoreset thermal breaker to protect wiring if there are shorted leads.
    * If the relay is stuck closed for any reason, there is an alarm.
    * No MOSFETs to fail, or be susceptible to RF flooding.
    * Continuity at both sides (~5mA test current, could be less with an added opamp).
    You might want to check the terminal designations on your relay it seems from the diagram they might be mis-labeled. NC should be NO and NO should be NC.

  7. #67
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    31st December 2009
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    Las Cruces, NM
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    Quote Originally Posted by kc9qzf View Post
    I spent 38 years designing test equipment and in product systems for several automotive and automotive electronics companies.
    I am the designer of the Wilson F/x launch system for 17 years and hold an Amateur Extra license.
    I don't claim to know everything but I try to stay active in helping people learn.
    Thanks, Dan. Good to know. And it's much appreciated that you are here, open to review and suggestions.

    I used to design telemetry systems for NASA, including critical squib-firing circuits for mission events. I've also design mixed-mode RF/analog/digital/software for life-critical medical products and military systems.

    Now that we are mixing together help for individual hobbyists with a commercial venture, some of us may be more reluctant to help for free.

  8. #68
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    12th September 2015
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    Macy IN
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    Quote Originally Posted by jsdemar View Post
    Thanks, Dan. Good to know. And it's much appreciated that you are here, open to review and suggestions.

    I used to design telemetry systems for NASA, including critical squib-firing circuits for mission events. I've also design mixed-mode RF/analog/digital/software for life-critical medical products and military systems.

    Now that we are mixing together help for individual hobbyists with a commercial venture, some of us may be more reluctant to help for free.
    I am not getting paid for this project, it is just something I feel can help folks who are new to electronics and are interested in launch systems get a start.
    I am putting together a "systems mech" to show how one could use the proposed Arduino and Shield to make a safe system. Hopefully sometime today I will get time to put it together and post it here.

  9. #69
    Join Date
    12th September 2015
    Location
    Macy IN
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    Quote Originally Posted by jsdemar View Post
    I agree with David's assessment of what we can see of the circuit so far. I know Dave's background as an EE with decades of experience. My professional background is in my LinkedIn page (see my sig below). I don't know Dan Fox's background. But, I appreciate his willingness to share his design and get some feedback.

    Here's my non-wireless simple launch circuit that incorporates much of the safety features David mentioned. I've shared this on the web many times. At least 10,000 launches have been done based on this design over the past 20+ years. Adding a simple/secure wireless link in between could be done without introducing too many additional failure modes. XBee + Arduinos + libraries + control software does not make a simple wireless link, but it's accessible to hobbyists without professional EE background.

    Link: relayer.pdf

    Some requirements that are met with this (non-wireless) design:
    * Shorting a connector cannot activate the relay.
    * There is a physical disconnect to safe the pad.
    * There is a physical disconnect to safe the controller.
    * Autoreset thermal breaker to protect wiring if there are shorted leads.
    * If the relay is stuck closed for any reason, there is an alarm.
    * No MOSFETs to fail, or be susceptible to RF flooding.
    * Continuity at both sides (~5mA test current, could be less with an added opamp).


    Based on your drawing, here is the same basic mech for the pad using the shield....

    DIY PAD SYS MECH.pdf

  10. #70
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    31st December 2009
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    Quote Originally Posted by kc9qzf View Post
    Based on your drawing, here is the same basic mech for the pad using the shield....

    DIY PAD SYS MECH.pdf
    I don't understand it at all.


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