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the opposite of a RELAY

donperry

Jun 12, 2011
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Hey guys my first post here

I am building an ejection system for my scratch built rocket.
This part of the rocket will be responsible for ejecting the parachute for a safe landing back to earth

My plan is to run a wire thru the delay fuel and have that wire burn just about when the fuel is done.

That wire I'm imagining is connected to 2 AA batteries that will hold another circuit open. When that wire is burnt i want the other circuit to close and set off my ematch that will ignite the parachute ejection charge.

What device can i use to accomplish this?
 

donkey

Feb 26, 2011
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you can use a spdt relay just connect it so the relay is no, when the connection to power is lost it will go nc activativng the second circuit. being a rocket mass = extra fuel so have you got the second circuit running of the same batteries or are you using extra?
http://imageshack.us/photo/my-images/13/parachutet.png/ is a very crude attempt the fuse in it is the wire you will burn. i am assuming it might be good to put a resistor in there so as not to overheat your fuse wire making an early release. i hope someone can edit this further but it seems to me like it would work in this case. also for a an issue of weight would watch batteries work? they are so much lighter and still provide 1.5v, some can provide 3v which is 2 aa batteries. anyway i am a beginner but i hope i have helped you a little
the program i use has a few issues with schematics but they label the parts so i hope its straight forward
 

donperry

Jun 12, 2011
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thank you for quick reply donkey.

My problem is that whatever power source i'm using must be strong enough to burn a steel wool wire (about an inch, maybe less) and cause an ignition (on second circuit)
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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We've had a few model rocketry related questions. Do a search on these forums for "ematch" and maybe "rocket" see what you find.
 

donperry

Jun 12, 2011
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Yes i have, so far they are using altimeters.

MY setup is a little bit different tho,
 

donkey

Feb 26, 2011
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could you use some sort of pressure switch within the fuel housing? when it is empty the switch closes and activates the secondary switch. maybe a big task. also what weight differential is there between a 9v and 2 AA is that an option? as i said i am new to this and have only the basic ideas. i will see how i go with further ideas some maybe a little hard in practice however.
 

donperry

Jun 12, 2011
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I can carry about 150G of extra weight comfortably. It's 200 newtons/second H motor
 

donkey

Feb 26, 2011
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i hope you understand what i meant by pressure switch(may be called something totally different). like a fuel gauge in your car, when you get close to empty you get the fuel indicator. instead of the fuel indicator you activate the parachute though. am researching it now to see if its viable
 

donperry

Jun 12, 2011
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Oh that? Oh not, I'm using solid fuel so no pressure to check on.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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OK, so your plan is to have a circuit formed through the "delay fuel" and have that circuit broken, thus triggering the ejection charge.

Isn't the idea of this delay fuel that it directly triggers the ejection charge? why interpose something in between?

In any case, the circuit that we've shown before to set off an ematch could be triggered by your opened wire. You need the wire to pull down the trigger. A pull up resistor would then activate the trigger when the wire burns through.
 

donperry

Jun 12, 2011
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I do not like the traditional method as I have to use clay bulkhead and drill a hole etc, etc.
also the motor can only be used once in that setup (the way i make them) So i want to separate the ejection section from the motor itself.

I cannot find the diagram you speak of. is it the same as this one?http://imageshack.us/photo/my-images/13/parachutet.png/
 

davenn

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Sep 5, 2009
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what do you want across that output ?
as it stands there is no voltage across that output to power some thing else

what does it take to fire the ematch voltage/current wise?

so maybe something along these lines ?

attachment.php



Dave
 

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donperry

Jun 12, 2011
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I will be using a 9V battery to ignite the charge. Is the transistor setup less burdensome on the battery than the relay?
 

poor mystic

Apr 8, 2011
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Hi Donperry
This sounds like fun!

AA batteries are very heavy and could be replaced by a much smaller and lighter capacitor which you could charge just prior to lift-off. I wouldn't be surprised if this were more reliable, too, since the capacitor could be permanently soldered and glued into place instead of held by spring pressure as is usually the case with batteries.
Because your electronics need not dissipate power for very long you could use a really tiny transistor in the way Steve suggests.

So many people must have put so much work into this already but some folk do enjoy the challenge of re-inventing wheels. It's not as though it's easy for an inventor to find a genuine challenge that is legal and cheap enough for a hobby.

I've included an attachment showing both my interpretation of steve's suggestion and my own idea about the capacitor.
 

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donperry

Jun 12, 2011
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thank you.

I'm kind of reinventing the wheel, and somewhat making it spin a different way.
Most people use altimeters that trigger the charge, a mercury switch that activates once the rocket tilts, and some build the charge into the engine/motor using a delay grain. I'm avoiding the cost of the altimeter and the design on integrating the charge in the motor.

Mercury switch sounds expensive, but you guys could advise me better.

If this works i wouldn't be surprised if more people use it :)
And this internet thread will be the point of reference for many to come :D

question:
would the capacitor have any chance of leaking out into the resistor?
 

poor mystic

Apr 8, 2011
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I hope to put you off playing with Mercury for life!

Wow. Please do not use mercury for every possible reason including explosion hazards and toxicity in the environment, and to yourself.

I would not want to use any metal whatever and definitely not mercury anywhere near a rocket fuel mixture since metals often cause explosions, filthy ones if mercury is involved. My burnable link would be made of graphite. I'd stay away from horribly toxic mercury entirely.

The resistor would of course consume power while current flowed through it and through the burnable link. A little bit of current would then be used to charge the gate of the MOSFET and the remaining charge would flow through the MOSFET and trigger your mechanism. You could certainly store oodles of energy in the capacitor at a high voltage; I think it's a much better solution than AA cells.
 

poor mystic

Apr 8, 2011
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I don't like it. Try the guys on the chemistry forum, I don't think they'll like it either.
 

davenn

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Sep 5, 2009
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Hi Donperry
This sounds like fun!
AA batteries are very heavy and could be replaced by a much smaller and lighter capacitor which you could charge just prior to lift-off.
SNIP
I've included an attachment showing both my interpretation of steve's suggestion and my own idea about the capacitor.

Don was referring to a 9V battery, not AA's. but even so, a charged up supercap may do just as good.
dont know if I have seen any around 9V. Most are 3.3V or 5V, but may have enough zap to activate the ematch. it would have to be experimented with

so what's the point of the zener ?


Dave
 
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