Model Rocket Continuity Check on Arduino I/O Pin

Discussion in 'The Projects Forum' started by easco, Apr 7, 2014.

  1. easco

    Thread Starter New Member

    Apr 4, 2014
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    Ladies and Gentlemen,

    Model Rocket Launch Controllers certainly do seem to be a popular topic around here and I pray you will forgive me if I ask about yet another one.

    I would like to create a Launch controller that uses an Arduino (and LCD screen) to display the results of a continuity check. (But I do NOT want to put the launch decision under control of the IC. I prefer to rely on mechanical switches for that).

    To participate in the continuity check, I thought to put one of the pins of the Arduino into "input pull-up" mode (i.e. connected to +5v through a 20k-50k internal resistor and then activated when pulled low). That pin would attach to the high side of the igniter and pull it to ground when the igniter has continuity.

    The problem I am trying to solve comes into play in the following scenario. A scout has pushed the launch button, and held it until the igniter has a chance to burn through (opening that connection like a fuse). Unless protected at that point, there is a conduction path connecting the Arduino I/O pin directly to the 12v supply. (see 10 ways to destroy an Arduino, Method #3)

    My experience with electronics has been on a 20 year hiatus. Google and I have considered a number of solutions to this problem including several circuits designed for over voltage protection in power supplies (many found here in the forums). While terribly informative (and therefore entertaining), I think I have been off in the weeds.

    The "10 ways to destroy an Arduino" link suggests a mechanism employing a Zener Diode (and a PTC fuse), but it was unclear to me how the Zener diode might affect the ground continuity test. (and the PTC fuses seem to be only available as surface mount components, I'm not sure about hand-soldering surface mount components)

    I thought to solve the problem using a simple diode as shown in the attached PDF, Continuity.pdf.

    What I would like to know is, is the diode shown in the diagram (I'm assuming something like a 1N4001) sufficient to keep the +12v supply from ruining the Arduino? Is the large current of the launch path a consideration? Will I need to take additional steps to isolate the Arduino from the +12V supply, and the large currents of the "launch path"?

    For my current requirements in the launch path, I hope to be able to launch models that use both Aerotech (First Fire) and Estes igniters. According to links on the web like this one: Electrical Current Requirements of Model Rocket Igniters, I anticipate that the current in the launch path will be around 3A with a significant peak (12.39 amps for Estes igniters in the PDF).
     
    Last edited: Apr 7, 2014
  2. wayneh

    Expert

    Sep 9, 2010
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    That will work to protect the Arduino. The input will only be pulled down to ~0.7V though, due to the voltage drop across the diode. I don't know anything about the Arduino but that should be enough to be a digital "zero".
     
  3. wayneh

    Expert

    Sep 9, 2010
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    Another approach would be a 5V zener on the input, with a resistor (say, 47Ω-470Ω) between the igniter and the input. That resistor would limit the current passing to the zener, and the zener will carry all that current to ground, ensuring the Arduino never sees more than 5V. You would choose the resistor to match up with the current rating of the zener. Aim for half of that current, for instance.
     
    easco likes this.
  4. Wendy

    Moderator

    Mar 24, 2008
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    I used to be into model rockets, still like to dable now and again.

    We are neighbors, for real. Looks like we have the same home lodge, if you use Plano Lodge 768.

    You need to send and sense a very low current, around 1 ma, and have some simple circuitry to sense the current. A comparator such as a LM393 or LM339 would work nicely.

    I don't know much about Arduino's, someday maybe. I can help you with the rest bro.

    BTW, just say wayneh last post, I like the idea. You'll note the times are the same for posting, I added this edit after the fact.
     
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    I'm sorry, but what is the Arduino for?

    You could have a simple LED turn on thru the igniter as the 20 mA making the LED glow is well below any threshold current that would cook off the rocket motor.

    Recently I've seen people doing some complicated timer things where a good reliable and safe switch is better used.

    You can even get switch covers!

    [​IMG]
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    The countdown silly! No rocket launch is complete without a count down! The OP did mention that the actual launch was under manual control.
     
  7. easco

    Thread Starter New Member

    Apr 4, 2014
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    Brother Marsden I am glad to thus meet you. We are neighbors. I am the sitting Master of Allen Lodge 1435.

    The Zener Diode solution was the one proposed in the PDF file in the original post about the "10 ways to destroy an Arduino". I was hesitant to use it because I couldn't find the PTC resettable fuse in a through-hole version. But the more I think about it, I realize that they using the PTC to limit the current against any-old-thing that might be plugged into the pin. I know what circuit is attached to the pin so so as long as I limit the current with a plain ole resistor, I think it should be fine.
     
  8. wayneh

    Expert

    Sep 9, 2010
    12,083
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    Yeah, you don't need the resistor to act like a fuse also. That's a nice extra degree of protection, but not absolutely required.
     
  9. easco

    Thread Starter New Member

    Apr 4, 2014
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    Thank you for asking. If I am honest with you and myself, the Arduino is there for the same reason that George Mallory climbed Mt. Everest. I want to display the continuity on an LCD screen to see if I can make it work. While this is an exercise in design and construction, it is also (primarily) a learning experience. Already in trying to solve this problem I have learned a lot more about Transistors (including MOSFET), SCR crowbar circuits (and other over voltage protection mechanisms) and now more about Zener Diodes.

    For that, I have found this site and its forums to be both fascinating and incredibly useful.

    Secondly, the launch controller is being constructed for use in a Boy Scout recruiting event. Having the Arduino visible, and the LCD screen prominent adds a "wow" factor that, unabashedly, is designed to both interest scouts in electronics and impress Webelos scouts who may be interested in joining our troop.

    On a more practical level, I am a Software Engineer by trade. The Arduino gives met the option of adding more interesting "control" into the controller. For example, I hope to have 3 or 4 launch pads. Through the Arduino I can "reason" about the continuity and make decisions. For example, I could provide an more interesting audible warning than a simple piezo buzzer or single frequency speaker. Imagine, for example, that with 3 pads, each pad could use an different audible cue (say... each with a different note from a C chord) AND light an LED.

    Finally, I understand the warning in your post and, like you, safety is my upmost concern. One thing I am NOT interested in doing is trusting my knowledge of Microcontrollers far enough to put the Arduino in charge of the actual launch. I've been a Software Engineer for too long, seen too many bugs, and I've been an electronics hobbiest far to briefly, to put fingers and faces at risk. You may rest assured that the launch part of the process WILL rely on just switches. Naturally, for added "wow" factor, at least one of those switches will be just like the one you sent in your picture :)


    Or a big red button... a "launch enable" button that has to be down before the launch buttons the kids have will work.

    Seems hard to go wrong with a big red button.
     
    Last edited: Apr 8, 2014
  10. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    I've seen it go wrong. A friend of mine used to do pyrotechnic flash pots for "bar bands" by winding a single strand of speaker wire around posts over some aluminum (or something ) powder. Apply the line voltage to the wire it would vaporize and set off a FLASH.

    One time the switch got left on and when he plugged the pot into his board... well, the young lady sitting next to the flash pot took it rather well.

    So use TWO buttons... an ARM switch and a LAUNCH button.

    Years back I got some huge 1" round buttons of the style used on professional arcade video games. I've yet to have a use for them but love them dearly. They would be great for a launch button. (My original intent was as a "technician saver," to start a computer controlled high voltage test I would use TWO buttons so I knew where his hands were.) (And the code wouldn't start if you just taped one down either.)

    I get you want a "wow" factor, but isn't that what the rocket ship is for? I wanna be watching the rocket not some silly screen.

    Now 12" high sever segment displays set behind the ship kinda like NASA does at their launches... THAT says "wow" to me. ;-)
     
  11. easco

    Thread Starter New Member

    Apr 4, 2014
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    You're right. The Rockets provide more wow factor at an actual launch event, however, the launch controller is also used in "static displays" at recruiting events where no rockets can be launched.

    A 12" high seven segment display for the countdown... that sound like a capital idea!
     
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