Model Rocket Launch Controller

Discussion in 'The Projects Forum' started by Sirius.Black, Nov 10, 2009.

  1. Sirius.Black

    Thread Starter New Member

    Nov 10, 2009
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    Hi All -

    I am new to this forum as of today :) My 11-year old son and I have set out to design and build our own model rocket launch controller that puts out sufficient amps for 3-4 engine clusters.
    As for me I am pretty good with reading electrical schematics and then building the circuits...etc. However, we are having difficulty with the design. I have looked at a few designs that I found on the internet, which for me are either overboard because they've the capacity to power multiple launch pads, or they're too simple - like the basic Estes or Quest launch controller. I'm looking for something in between and am content running the system off of a 12v hobby or motorcyle battery.

    What I have in mind for the safety interlock is a series of 3 lighted toggle switches (w/ protective covers) that must be turned on in sequence "red-yellow-green" in order to "arm" the controller. Once armed, I have both a Radio Shack pizzo-siren as well as an LED array that will strobe as audible and visible warning indicators. Then I have a large momentary push button switch to use for the launch button.

    So, my humble problem (and forgive me for being so novice), is designing the circuit so that it functions the way we want it to and there are a few main problems that I've not been able to overcome:
    1. I understand the concept where there is circuit continuity once the igniter is hooked up, but I cannot seem to overcome how to keep the power low enough so that it is just a continuity 'check' and doesn't inadvertantly burn the igniter?
    2. I haven't been able to figure out how to wire the switches so that they are activated in sequence? If I wire them in parallel then flipping one will be as good as all; if I wire them in series then nothing will light up, the siren won't go off, nor the LED array until all switches are flipped on.
    We built a simple controller this past weekend that kinda-sorta worked, however, it's powered w/ 8 AA batteries and when I hook it up to the igniter, my switches work like I want pretty much, but the voltage to the igniter is about 6v and when I press the launch button the voltage increases to about 9v but the igniter doesn't burn - which (unless I'm wrong) tells me that I don't have the needed amps. I was basing this off of I diagram I found in one of the Evil Genius electronics books. In the book there is an 'advanced' circuit that uses a 12v relay and I know nothing about relays and more so this circuit shows using a second 12v battery wired in to the relay and this makes no sense to me as I've never seen a launch controller with 2 12v batteries??????

    I would really REALLY appreciate some guidance!

    Thank you:(
     
  2. Wendy

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    Mar 24, 2008
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    Excellent! Wish I could have gotten my kids interested, I loved it when I was a kid. A simple light bulb will limit current, but if that is too low tech, an LED and resistor will do it. Qty 8 AA batteries should be enough (12VDC), unless you're using a really old Estes igniter (which was a straight simple length of nichrome wire). Are you measuring the voltage at the igniter? What gage wire are you using? If it is too small that could be a problem, I used 18 gage lamp cord, which is common and cheap from the hardware store. For the LED @ 12V you need around 1KΩ for the resistor, this will give you a good continuity check.

    I'm just the first who saw this post, there are a lot of folks who were into the hobby around here. Expect lots of replies. Another thought, do a search, I think you'll see a lot of old posts.

    If you draw your schematic we will be happy to help you tweak it.

    I never had much luck clustering myself. Battery power is part of the issue, you might think about going to a 12V sealed lead acid battery, they have lots of omph. I've never liked the parallel igniters, which is the standard way its shown, because if one engine lites off and starts the whole shebang moving the other igniters and be pulled out of the other engines. Series is a non-starter, period.

    The one reliable cluster ignition system I found was from Centuri (I don't think they exist anymore), they used a fuse type mechanism, basically a rubber feeling pyrotechnic compound around a nichrome wire. What I did was stuff the fuse into the engine, put a magnesium flash bulb into it and taped it all down. Guaranteed ignition every time, but I don't think the flash bulbs are available anymore either. Part of the reason it was so reliable is the ignition source traveled with the engine until the engine blew it off.
     
    Last edited: Nov 10, 2009
  3. SgtWookie

    Expert

    Jul 17, 2007
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    I, too, grew up with model rockets. Had a blast with them, too. :)

    The idea of requiring three switches to be flipped in a certain sequence is intriguing, but unless you're using a microcontroller, will make the circuit very significantly more complex and higher in cost.

    The more complex a circuit, the greater the likelihood that something in it will break, or otherwise fail in an unexpected manner.

    Military-specification equipment reduces such breakage/failures by using high-reliability components manufactured and tested under strict guidelines/rules. These types of parts can be purchased from various suppliers/distributors, but they are very expensive; costing 10 to 100 times (or even more) than the average commercial specification component.

    To keep things simple, reliable, affordable and most importantly SAFE, I suggest more on the order of:

    1) A removable ARM key. The person who is connecting the wiring harness to the rocket motor(s) removes the key from the launch controller and takes it with them to the pad. This could be as simple as a 1/4" stereo phono plug that is wired as a short from tip to ring; ground circuit left open.

    2) If desired, a 2nd ARM switch can be added; or it could function separately as a TEST switch.

    3) Dual LAUNCH switches, spaced far enough apart that one needs two hands to press them, and configured in such a manner that both switches could not be accidentally activated by dropping the launch control panel. The switches should be momentary contact type, normally open.

    Relays should be avoided. Mechanical failure is not a good thing.

    I suggest that MOSFETs should be avoided in igniter control circuits, as they frequently short when they fail. This would cause an instant launch when the ARM key was inserted.

    Standard bipolar transistors are readily available, and in proper configuration can handle quite a bit of current. They are also much more tolerant to static electricity than MOSFETs are.
     
  4. Sirius.Black

    Thread Starter New Member

    Nov 10, 2009
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    Thanks Bill -

    I will work on sketching my schematic this evening and get it attached.
    In response to your other questions....
    I've never had any problem with clustering igniters. I have an Estes Maxi-Force that flys on 3-D engines...very well too I might add...and so far I've not had a problem; the beasties all touch off perfectly using a 12v car battery.

    So, all of the toggle switches I have along with the siren and LED array are 12v DC rated (I'll have to double check the current rating). Funny, I hadn't given much thought to the wire gage, but I was thinking of likely using 18-20ga. wire for the internal circuitry; speaker wire to run out to the launch pad; and 12ga. to the 12v battery - which by the way will be of the lead-acid automobile/motorcycle variety.

    As far as measuring the voltage, yes, I'm measuring it at the igniter.

    Estes makes an advanced launch controller for powering the larger sized rockets and I used to own one. If I remember it too was powered with 8 AA batteries and I never had any luck with it.

    Regarding the controller we built up this weekend, it too uses 12v switches, but it's plumbed with 14ga. wire mostly all the way to the igniter.

    I'll get my schematic sketched up and hopefully you all could give me some good advice from there!
     
  5. SgtWookie

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    Jul 17, 2007
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    Here is an on-line wire resistance calculator: http://www.stealth316.com/2-wire-resistance.htm

    Plug in your length in feet, click on the Amps box, plug in your Amperes, and then click Calculate.
    Don't forget that you have two wires going from the control panel to the pad. If your cable will be 30 feet long, then you will have 60 feet of wire.

    It is OK to use smaller gauge wire on the panel itself, as the wire lengths there will be quite short.

    Don't forget; the panel itself needs a fuse just in case a direct short develops somewhere; otherwise you will have a mass of melted wiring! :eek:
     
  6. Sirius.Black

    Thread Starter New Member

    Nov 10, 2009
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    Okay, here is the schematic draft that I'm working on. Again, I really need help with the switching sequence and the resistance so that my switches have to activated in sequence in order to 'arm' the controller as well as ensure that once it is armed that my circuit is only a 'continuity' verification of the igniter connections. (Please see attached)
     
  7. SgtWookie

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    OK, there are a couple of problems with your circuit.

    1) The ARM switches are in parallel, not in series. This means that only one of them has to be closed in order to supply current to the LAUNCH button.

    2) There is no way to check continuity through the igniter unless the arm switch is closed. This can mean running back and fourth several times to the launch pad.

    Here is a launch controller I drew up a couple of years ago for someone:

    [​IMG]

    The ARM switch is a phono jack that you plug a phono plug into.

    The continuity light will illuminate when there is a current path from the launch pad.

    In case the alligator clips were accidentally shorted together, the fuse will blow when the launch buttons are pressed, rather than the wires melting.
     
  8. Wendy

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    Mar 24, 2008
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    Something that occurs is it might be nice to have a local continouity test for each ignitor for the cluster, I've never seen it done, but it should be easy. What is the maximum number of engines for each rocket?
     
  9. Sirius.Black

    Thread Starter New Member

    Nov 10, 2009
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    Hi Sgt. Wookie -

    1st of all thank you very very much for your help! As I stated in my original posting - while I am readily able to readi and interpret schematics (and build from them); I'm certainly quite novice at circuit design.:D

    I like your approach of using the stereo jack - do I need to solder the internal jack connector points together somehow to short it out? I will incorporate this into my build! The buzzer and LED seem to be similar to what I will be using and I see the addition of the fuse and 3 220-ohm resistors (which I will need to use the 680-ohm for the 12v per your text note. The use of 2 buttons for launch is a neat idea as well, but I'd need to space them far enough apart on my panel so that both cannot be pressed by one hand.

    What are the voltage ratings for your buzzer and LEDs? My LED array is 12v as well as my buzzer. Also, what gage wire do you recommend for running out to the launch pad?:rolleyes:

    So my only remaining question then is how (or am I able to even) do I sequence my 3 toggle switches? As you saw, in my schematic draft, they are shown in parallel. The switches I intended to use are lighted toggle switches (Red, Yellow, and Green). So I was hoping that as I swtiched each on "on" that it would light up accordingly. Once I flipped the last switch then that would 'arm' the circuit and my siren as well as my LED array would be activated.
    However, as you point out - with them in parallel, only 1 switch need be turned on for the circuit to close; however, if I wire them in series then none will light up until all are closed.:confused:

    Thanks again!
     
  10. Wendy

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    Mar 24, 2008
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    The arming key is an old concept. A lot of them on the older launchers use a hole in the plastic with two metal strips screwed into the plastic with a gap less then the diameter of the metal pin that fits into the hole, a stereo jack is a simple off the shelf equivalent to this.

    You use a stereo jack as a key, that is internally shorted to complete the circuit.
     
  11. SgtWookie

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    Well, this would require at least a separate return line for each igniter - or else some sort of multiplexing arrangement down at the launch pad, which could quickly complicate things very significantly, as well as making it more difficult to build/troubleshoot/repair.
     
  12. Wendy

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    Yeah, but still remember that Saturn V model flipping lazily through the air, not one of my better lauches, and while the rocket flew again it was never as pretty.
     
  13. SgtWookie

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    That's OK, most (if not all of us) started out that way too! ;)

    Not the jack, the stereo plug. You solder a jumper between the tip and ring connection. You leave the ground disconnected.

    As Bill_Marsden asserted, this is an easy-to-obtain substitute for the launch controller safety pin that was in the 60's version of the Estes launch controller; I had one of those. Back then, the launch controllers were a kit that you assembled. The contacts for the arming switch were a couple of pieces of brass bent at nearly a 90° angle and were screwed to the inside of the black plastic housing.

    My kit had battery terminal clamps for use with a 12v auto battery, but I frequently just used a 6v lantern battery.

    The resistors on the LEDs are calculated by:
    Rlimit >= (VoltageSource - LED_ForwardVoltage) / Desired_LED_Current
    The buzzer resistor - it depends on what voltage it's rated for, and how much current it draws.

    That's the whole idea.

    You have an array of LEDs?
    If they're rated for 12v you could probably use them as-is.
    See the wire voltage drop calculater.
    16 gauge would probably work fine. You could use speaker wire, or lamp cord wire. Big Orange and Big Blue hardware stores both sell those by the foot or by the roll.

    I haven't looked at that part yet.
    It might turn out to be rather complicated, and more complexity is something that you really don't need.

    I don't know anything about your switches, other than they are colored red, yellow and green and are illuminated by some means; whether LEDs or incandescent.

    Usually, switches have separate circuits for the internal lights. But unless you supply more information, we're kind of dead in the water.
     
    Last edited: Nov 11, 2009
  14. SgtWookie

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    Ouch! :( I have an old Gemini sitting a couple feet from me that I built in the 60's. It has some "battle scars" as well. ;)
     
  15. whatsthatsmell

    Active Member

    Oct 9, 2009
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    All the 12v lighted switches I have seen have a separate ground provision for the light itself. This would allow the light to light while in series provided the switches are turned on in the same sequence that they are wired.

    Hope I was able to explain that so it makes sense. ;)
     
  16. Sirius.Black

    Thread Starter New Member

    Nov 10, 2009
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    You guys have been soooo much help!!

    Attached are the info sheets for the lighted switches I have as well as the launch switch.

    As "Whatsthatsmell" replied about having a separate ground for the light - on these toggles I see 3 terminals. So, now if I wire the switches in series where would I be tapping the ground to? (I must sound stupid:eek:)

    To "Sgt. Wookie's" comment - yes, my LEDs are in a bar - I think there are 6 in the row. They can flash, or scan...etc. and they are 12v rated.
     
  17. SgtWookie

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    ok, looking at your 1st attachment, there are a number of switches listed. Out of all of those, I'll take a wild guess and assume that you obtained these:
    35-090 - red
    35-092 - green
    35-094 - amber
    55mA 12v lamp, contacts 16A, 12v.
    3 terminals
    It does not indicate which terminals are connected to what.

    One terminal will be common to both the lamp and the switch contact.
    One terminal will be the other end of the lamp.
    One terminal will be the other side of the switch contact.

    It will need to be wired up so that the single switch contact gets the +12v (or daisy-chained from the prior switch), the common terminal will be the output, and the terminal connected to the other end of the lamp will go to ground.

    With the 12v lamp being 55mA, it is probably an incandescent lamp, meaning polarity shouldn't be a problem.
     
  18. whatsthatsmell

    Active Member

    Oct 9, 2009
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    If you refer to Sgt. Wookie's launch controller circuit, you would wire the three switches in series between the 5A fuse and the R1 resistor for the buzzer. The ground lead can then be hooked up to the negative side of the circuit after the J1 cutoff switch.

    Doing this will still requires the J1 cutoff to be in place to fire the rockets, even though your switches are lighted, but still allow the "igniter connected" led to function.
     
  19. SgtWookie

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    Looking at your 2nd attachment, I just see a variety of push-button switches that indicate they are illuminated by 12v (except for the one on the bottom right, which is not illuminated) but no wiring diagram for them is shown.

    If they are wired in series, each bulb would receive about 6v. They would be dim.

    It appears that they have both NC and NO contacts. You would use only the common terminal and the NO terminal.
     
  20. SgtWookie

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    Jul 17, 2007
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    There are two ways to connect the switches from your description.
    One way, the ARM switches will not truly be arming switches; they will simply be interrupting the buzzer circuit to eliminate the warning.

    The other way, the igniter connected portion will not function until all of the ARM switches are closed.
     
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