Launcher relay question

Ok, I'm just about to start the build on a 12v relay launcher and I'm running into trouble. In particular the relay unit. I've checked damn
everywhere locally and have not been able to locate a suitable 12v SPDT relay. Found plenty of 12v SPST relay units but no SPDT units. I've checked several auto stores and surplus stores to no avail. I ended up buying a 12v 30 amp SPST relay at rat shack and I have no doubt it'll work just fine but I'd *really* like the feature of a continuity check.
Question: Is there a way where I could use the rat shack 12v 30 amp SPST relay and somehow add a continuity check circuit for the piece of mind factor? My goal here is to have a general purpose launcher that could do the whole deal, solar igniters all the way to crapperheads. Clusters included.
Ted Novak TRA#5512 IEAS#75
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the notorious t-e-d wrote:

IMHO, there's no reason why not -- you simply supply a very low current through your indicator device, which will show you continuity, and when the relay activates (closes) it is a lower resistance than your indicator device, so all the current goes through your igniter (instead of the indicator).
Maybe I don't quite understand your question, because I don't understand why you CAN'T use an SPST relay...
Do you have some more specifics about your circuit???
David Erbas-White
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David Erbas-White wrote:

Oh I see no reason that a SPST relay wouldn't work great for a launcher. I just want the added security of having a continuity check in the mix. Thinking along the lines of Bob's el'simple relay design.
http://www.ibiblio.org/pub/archives/rec.models.rockets/PLANS/chad_pad.txt
I should also mention I'm no stranger to electronic design and theory *but* it has been 20 years or so since I've designed/built such a circuit. Rusty, yes, but I can follow directions to a tee though.
Ted Novak TRA#5512 IEAS#75
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Connect the buzz to "com" instead of "nc". Check the current. If above 50mA, add a resistor between buzz and "com".

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What he said. Essentially the buzzer is across the two switched contacts of the relay, from NO to COM.
Check the specs of your piezo buzzer before you buy: the ones I've seen are in the 10-15ma range, so no resistor should be necessary. But to quote Ronnie, "trust, but verify".
--
Bob Kaplow NAR # 18L >>> To reply, there's no internet on Mars (yet)! <<<
Kaplow Klips & Baffle: http://nira-rocketry.org/Document/MayJun00.pdf
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Bob Kaplow wrote:

Simple to do. Wire in the buzzer, put an igniter in and test it. If it goes off with a test, don't use it.:) AKS
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No, it's not that simple, unless you know that the ignitor you picked is the most sensitive ignitor you will ever encounter. Current that won't fire a copperhead will fire a solar. Current that won't fire a solar will fire a flashbulb. Current that won't fire a flashbulb will fire some ematches.
The stock estes launch systems with the light bulb will fire flashbulbs and ematches just from the continuity check! I'm not aware of any ignitor that will fire at currents any where near 10-15ma, thus this value is considered safe.
--
Bob Kaplow NAR # 18L >>> To reply, there's no internet on Mars (yet)! <<<
Kaplow Klips & Baffle: http://nira-rocketry.org/Document/MayJun00.pdf
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THe direct link to my relay launcher plan is http://www.encompasserve.org/~kaplow_r/relay_launcher.txt
No reason it wouldn't work with an SPST relay. I happened to have a SPDT relay from an old car stereo installation when I built this one.
I was thinking that the benefit of the SPDT is that you can do a safey test by touching the clips before connecting and listening for the buzzer, but if you just have SPST then if it's OK you'll still get the buzzer, and if it's shorted then you'll still get the sparks and heat.
I still don't use the optional button in the circuit for continuity check. Now that I wear bifocals, I find that even with them, it's hard for me to see what I'm doing with the clips. Wrong head position for the bifocals, glare, etc. So I connect the clips by ear rather than by eye.
--
Bob Kaplow NAR # 18L >>> To reply, there's no internet on Mars (yet)! <<<
Kaplow Klips & Baffle: http://nira-rocketry.org/Document/MayJun00.pdf
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Ted,
If you cannot find the SPDT relay, you can always spring for a DPDT. If nothing else you have a redundant set of contacts or you can double up for a bit more current carrying capacity
A good source is Digi Key
Here is the link for some good choices. The LY2F-DC12 by OMRON would not be a bad choice at about seven and a half bucks.
http://rocky.digikey.com/scripts/ProductInfo.dll?Site=US&V9&M=LY2F-DC12 LY2F-DC12

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Have you tried Fair Radio Sales? don't know if they have one but they have had all kinds of odd goodies in the past.

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the notorious t-e-d wrote:

You should be able to find 30 and 40 amp SPDT relays at Napa or Pep boys.
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Try http://www.hosfelt.com/
On Sun, 15 Jan 2006 23:57:30 -0600, the notorious t-e-d

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the notorious t-e-d wrote:

Just wire a low current piezo in a series with the contacts with a momentary switch. I check the continuity at the relay box. Really quite simple and works. It is what I am using currently in a homemade relay box with three 7.5amp-hr lead-pb batteries in parallel.
Kurt Savegnago
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I just took apart my old (late 80s?) relay box. It has such a relay. It's labeled Auto Spec TW-3415. But a web search doesn't find any references.
I did a search on "RELAY 12V 30A SPDT" and got a LOT of hits, including http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber30-070 . I'd start looking locally at car stereo shops, which is where I think mine came from.
While my relay design uses a SPDT to drop the continuity check when I launch, I don't see why it wouldn't work with a SPST relay that just bypasses the piezo buzzer the same way the Questes launchers bypass the light bulb when you launch.
--
Bob Kaplow NAR # 18L >>> To reply, there's no internet on Mars (yet)! <<<
Kaplow Klips & Baffle: http://nira-rocketry.org/Document/MayJun00.pdf
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Bob Kaplow wrote:

Well what do I know....I found the EXACT same relay at a local electronics surplus store that a local flyer clued me in on. Too bad I didn't pick that relay when I was there earlier today :(
Regardless, a beeg fat thanx to all who posted on this thread!
Wait a minute.... there's a rocketry related thread on rmr???
:)
Ted Novak TRA#5512 IEAS#75
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so, you get it built?
our club has a couple of relay launchers, and they work well when the batterys work.
not sure why you couldnt find a SPDT relay. i couldnt find a SPST relay anywhere (but did not look hard)
writes:

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Gots the parts but haven't started the build yet. I'm starting a new job tomorrow(yay, no more contract 1099 crap) and been preping for that.
Hopefully this weekend I'll get it built.
Ted Novak TRA#5512 IEAS#75
Tater Schuld wrote:

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the notorious t-e-d wrote:

No way!
I'm pretty sure it was a relay thread! :)
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