highaltitude.log.20100524

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[15:40] <DanielRichman> Spacebits got /. 'd
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[19:13] <jcoxon> evening all
[19:14] <brennen> howdy.
[19:14] <jcoxon> hey brennen
[19:16] <russss> evening
[19:16] <jcoxon> the guys doing the paris balloon launch (from the register) got in contact today
[19:16] <jcoxon> was wondering if could provide SSTV for their flight
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[19:33] <amee2k> evening everyone
[19:33] <jcoxon> evening
[19:36] <amee2k> i'm doing some stuff with microcontrollers and have just been wondering about using them for controlling bridgewire detonators
[19:37] <SpeedEvil> fucking big thyristor.
[19:37] <SpeedEvil> Or arc gap
[19:37] <amee2k> so i was wondering if there are any reference circuits for properly triggering them. power source would be an old AT/ATX PSU so 3.3/5/12V rails
[19:39] <amee2k> SpeedEvil: well, when they fire, the wire usually blows so i can likely use the overload capacity of a small one
[19:39] <amee2k> the datasheets give minimum and maximum current for reliable ignition and minimum pulse width
[19:40] <amee2k> i was wondering if any current limiting against exceeding the maximum current stated is required
[19:40] <SpeedEvil> Oh - you don't mean exploding bridgewire
[19:40] <amee2k> or any constraints on the rise time
[19:40] <SpeedEvil> yes
[19:40] <amee2k> SpeedEvil: no, the kind used for fireworks or model rocketry
[19:40] <SpeedEvil> they will not reliably fire if you exceed the current
[19:41] <amee2k> haven't inspected one in detail but they look like a blob of matchhead-like substance with the wire embedded, optionally glued shut in paper tube that can hold the fuse in the right place
[19:42] <amee2k> almost like the homemade ones. but they're cheap so i'd like to afford the additional reliability
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[19:42] <SpeedEvil> the filliment can blow without setting off the thing if current is too high
[19:43] <amee2k> hmmm i see
[19:43] <amee2k> what would i use as current limiter? preferably without making assumptions about the circuit resistance so i can chain some of them
[19:44] <SpeedEvil> A constant current device, or a PWM?
[19:44] <amee2k> that would need to be very fast PWM. they're supposed to set off very fast
[19:45] <SpeedEvil> yes
[19:47] <amee2k> hmm constant current diode?
[19:48] <amee2k> or is it better to use one built around a linear regulator
[19:48] <amee2k> i'm somewhat assuming you need rise time that is relatively small compared to the set off time
[19:49] <SpeedEvil> LM317 would work
[19:50] <SpeedEvil> 1.25 ohm from 'ref' to out - and connect ref to ground = 1A current sink
[19:50] <amee2k> on a 12V rail, i don't want to drop too much voltage either so i can chain them. 2V minimum drop across the reg + safety margin would limit my circuit to <9R to sustain the 1A minimum current
[19:50] <amee2k> hmmm interresting
[19:51] <SpeedEvil> Tjat wild ave arind 3v of headroom
[19:51] <amee2k> okay
[19:51] <amee2k> this also has the added advantage that i can keep the power rail that drives the detonators powered all the time
[19:51] <amee2k> it can't short out the PSU even if the wires short during the ignition event
[19:52] <amee2k> i'd still put it on a (fail-open) relay as safety device
[19:52] <amee2k> and wire up a ref CCFL tube from the switched rail to signal when the safety has been disabled
[19:58] <amee2k> sweet. thanks SpeedEvil :)
[19:59] <SpeedEvil> :)
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[20:37] <RocketBoy> jcoxon: thanks for the QSL card
[20:37] <RocketBoy> (great idea BTW)
[20:40] <jcoxon> np
[20:40] <jcoxon> hopefully it'll catch on and then i can recevie some!
[20:40] <jcoxon> bbl
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[21:28] <jshriver> Greetings :)
[21:28] <jshriver> are Yanks welcome to the discussion as well? :) this is the first forum or channel dedicated to HAB's
[21:28] <jshriver> so anxious to talk with people in the field
[21:29] <sbasuita> of course everybody is welcome here ;)
[21:30] <jshriver> nifty :)
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[21:31] <jshriver> I'm designing mine now, but my major bottleneck has been telemetry and i/o. I'm a ham so best best seems aprs but it's pretty expensive.
[21:31] <jshriver> er best bet can't type today
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[21:33] <jshriver> interesting reading on your website you can't use ham transmitters in the Uk
[21:34] <sbasuita> mmm uk callsigns can't go airbourne
[21:34] <RocketBoy> yeah - major bummer - but necessity is the mother of invention
[21:35] <sbasuita> but then we've made our own balloon aprs system on the unlicensed bands so its ok :D
[21:36] <jshriver> :)
[21:36] <jshriver> can you do aprs on 70cm? thought it was primarily on 2m
[21:36] <RocketBoy> jshriver - for APRS there is the TNC-X or TinyTrak - or a couple of other PIC based systems
[21:37] <sbasuita> jshriver: its not strictly aprs; its a custom distributed tracking system
[21:37] <jshriver> nifty and thanks rocketboy will look into those
[21:39] <amee2k> SpeedEvil: hmmm once a (regular non-GTO) thyristor fires, it cannot be turned off without interrupting the switched circuit through some other means, right?
[21:42] <Hiena> amee2k, exactly, It will not switch off until the Uac not drops under the Ufmin.
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[21:48] <amee2k> hmmm
[21:48] <amee2k> so if i use a thyristor to trigger the bridgewires, i should implement some way to turn off the main power rail briefly
[21:49] <RocketBoy> why not use a FET or transistor?
[21:49] <amee2k> to reset any stuck thyristors. e.g. if the wires supplying the detonator have been shorted accidentially
[21:49] <amee2k> hmmm actually
[21:49] <amee2k> good point :)
[21:50] <amee2k> thyristors look nice because once triggered they seem to exhibit very good turn-on times that are independent of the driver circuitry
[21:50] <amee2k> (which at least is a factor for FETs... as i found out the painful way lol)
[21:51] <amee2k> i suppose a darlington transistor should work too
[21:51] <RocketBoy> we use logic level FETs to initiate ignitors most of the time
[21:52] <RocketBoy> 3V is pleanty of drive
[21:52] <amee2k> i've got a pile of IRF540s and the like around... pulled lots of them out of some old CRTs a while ago
[21:53] <RocketBoy> yeah - they need more drive volts
[21:53] <amee2k> i'll have a 12V rail anyway to fire the detonators
[21:53] <amee2k> and i've used one to design a low-voltage lighting system around, but got stuck when i switched from my hacked up ATX PSU to a real lighting transformer
[21:54] <amee2k> i have some driver ICs around for them too
[21:54] <RocketBoy> you mean ignitor - a detonator is used to initiate high explosives
[21:54] <RocketBoy> dont you?
[21:54] <amee2k> oh, okay. in this case i stand corrected :)
[21:54] <jshriver> perhaps a silly question, has there been any attempts for long distance HAB's vs just height
[21:54] <amee2k> i've been using the two terms mostly interchangably
[21:55] <RocketBoy> major difference - the d word probably has the security service much more interested
[21:56] <amee2k> lol
[21:56] <amee2k> for a multi-channel setup you're probably right with LL FETs. i'll have to order transistors for the final prototype anyway, so i might as well pick some that can save me the driver IC
[21:56] <amee2k> i'm targeting an 8 channel per card setup with a backplane that runs power and an I2C bus to accept multiple cards
[21:57] <amee2k> then go from the backplane via 3x RS485 over cat3+ FTP cable to the control station
[21:57] <RocketBoy> With 12V drive a IRF540 is prbably fine - you only have to get an amp or so through the ignitor
[21:57] <jshriver> anyone here do PIC programming? or recommend a good intro site
[21:58] <RocketBoy> I use PIC - but its a long time since I started - so cant recommend an intro
[21:58] <amee2k> hmmm yeah, that would work. but i'd either need a solid-state multiplexer, or one driver per transistor
[21:59] <jshriver> 'k :) picked up a Pickit2 and had some 18k (?) samples sent..
[21:59] <amee2k> i suppose the rise time on the attiny's output pins is good enough to drive a logic level FET
[22:00] <amee2k> jshriver: never used PICs (yet) but i know a good avr site that got me started, if you're interrested
[22:00] <amee2k> oh, okay. in this case i gather not ;)
[22:01] <RocketBoy> I wouldn't worry about rise time - at worst it just means a soft start to the current
[22:01] <amee2k> okay
[22:02] <amee2k> i'm worried that a too soft start could hold me below the minimum current for too long to be reliable
[22:02] <amee2k> but then i guess the parasitics of the wire are the worse problem
[22:03] <amee2k> i've measured the rise time on the lighting controller in the order of a few hundred nanoseconds
[22:03] <jshriver> amee2k: I'd be interested in that as well if you want to paste it
[22:03] <amee2k> the lighting controller?
[22:03] <jshriver> I'll be back on in about 30min or so thanks for the info
[22:04] <amee2k> oh, wrong discussion. hang ona sec
[22:04] <amee2k> jshriver: http://tom-itx.dyndns.org:81/~webpage/how_to/atmega168/mega168_howto_main_index.php << thats what got me started
[22:05] <RocketBoy> amee2k: seen this http://ukhas.org.uk/ideas:flight_support ?
[22:05] <amee2k> good thing is, the dude who wrote it is a regular in #avr and ##electronics :)
[22:06] <amee2k> thats interresting, thanks RocketBoy :)
[22:07] <amee2k> it looks vaguely what i have on my scratchpad here
[22:07] <amee2k> i want to use the ADC channels to sense the circuit resistance too, to detect which channels look properly wired up and which don't
[22:08] <amee2k> and to confirm proper ignition... they're supposed to go open-loop when they blow, right?
[22:10] <RocketBoy> yeah they mostly go open loop - but there may be some carbon left which might give a high resistance
[22:10] <amee2k> RocketBoy: hmmm won't the 4700uF cap cause a massive surge current when the fet becomes condiuctive?
[22:11] <RocketBoy> when they fire the plasma causes a close to short circuit (much less than an ohm) for a breif period
[22:11] <amee2k> high resistance after firing won't hurt... i'd see to it that circuits >10R or so wouldn't be detected as good
[22:11] <RocketBoy> Massive - no - just the same current as a 3V battery - about 2 amps
[22:12] <amee2k> hmmm okay
[22:12] <amee2k> how about chaining ignitors with this circuit?
[22:13] <amee2k> on a 12V source i could easily put 3 or 4 in series and have a circuit resistance low enough to (theortically) sustain sufficient current
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[22:14] <amee2k> SpeedEvil mentioned further up that over-current would affect reliability so i should better use a proper (even though imprecise) current limiter
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[22:16] <RocketBoy> yeah - I'd agree I have occasionally seen failures of the usual type of ignitors diretcly on 12V
[22:17] <RocketBoy> I'd say that 6V is about the maximum you want to put directly on the usual rocketry ignitors
[22:18] <amee2k> power supply will be another old AT PSU so the 12V rail has lots of current capacity
[22:18] <amee2k> if i put in a limiter around 1A, do i still need the large cap?
[22:19] <RocketBoy> I'd say put a 3ohm resistor in series on a 12V supply
[22:19] <RocketBoy> or use the 5V line
[22:20] <amee2k> i'd like to reserve the 5V rail for the logic circuitry and run the high current stuff off the 12V rail
[22:21] <amee2k> and i should be able to put ignitors in series without reconfiguring the circuit
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[22:21] <amee2k> after all, this is for a board design. not one specific event
[22:22] <amee2k> so i'd tend to perfer the LM317, if that works
[22:24] <RocketBoy> yeah - I realized about 3 lines back this cant be for HAB
[22:25] <amee2k> actually, its intended for controlling fireworks ;)
[22:25] <RocketBoy> a current limiting circuit is probably best for chains of ignitors
[22:26] <amee2k> i don't need more than 3 or 4 effects at the same time
[22:26] <amee2k> and in the rare cases i do, i should be able to arrange it such that i can use mutiple channels
[22:27] <amee2k> but at least that much should work without setting the chain length for each channel manually
[22:29] <amee2k> right, i think that gave me some input to ponder on and come up with a draft for the schematic
[22:29] <amee2k> thanks a lot RocketBoy :)
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[22:34] <edmoore> Randomskk / jonsowman : hugh hunt is on the latest Richard Hammond thing
[22:35] <edmoore> not sure if you spotted
[22:35] <Randomskk> :o
[22:35] <Randomskk> is it on iplayer?
[22:36] <jonsowman> edmoore: nice
[22:36] <edmoore> yes
[22:36] <edmoore> he's talking about boomerangs and gyroscopes
[22:36] <edmoore> to make a change
[22:36] <Randomskk> this is series 2 episode 3?
[22:36] <Randomskk> hehe
[22:36] <edmoore> yep
[22:36] <Randomskk> awesome
[22:36] Action: Randomskk queues for download
[22:37] <edmoore> he launches into an explanation of boomerangs
[22:37] <edmoore> the correct explanation
[22:37] <edmoore> and after 2 seconds it gets dubbed over with hammond saying something completely incorrect about how they work
[22:37] <edmoore> it obviously hit the editors technicalometer
[22:37] <Randomskk> hmm get_iplayer isn't working
[22:38] <Randomskk> :(
[22:38] <edmoore> right, am going to get back to thesis. see you chaps
[22:38] <Randomskk> have fun
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[22:51] <junderwood> amee2k, did I read you correctly that you want to put initiators in series?
[22:51] <junderwood> I wouldn't bet the farm on them all taking the same time to light.
[22:52] <junderwood> Odds-on one would blow and the rest would survive
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[23:12] <amee2k> hmmm interresting point
[23:12] <amee2k> looking around on the internet though, this configuration seems to work for lots of people
[23:13] <amee2k> can anyone confirm this please?
[23:13] <RocketBoy> yeah - its reasonably standard for fireworks
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[23:13] <RocketBoy> It wouldn't be my choice for anything safty critical like a rocket
[23:13] <rjharrison> Hey RocketBoy is there a balloon order going throught
[23:14] <amee2k> and it does work reliably for an appropriately strong driver?
[23:14] <RocketBoy> rjharrison: yes
[23:15] <amee2k> yet, whats the worst that could happen? bridgewire pops without setting off the charge?
[23:15] <RocketBoy> amee2k: this is worth a read http://www.gwiz-partners.com/igniters.pdf
[23:16] <RocketBoy> its unlikly - if a ignitor fires earlier than the rest then the plasma short is likly to fire the rest
[23:18] <rjharrison> RocketBoy cool
[23:18] <amee2k> this pdf is interresting
[23:18] <amee2k> a lot in fact
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[23:19] <rjharrison> Hows it going anyhow. I may be on songs of praise soon :(
[23:20] <amee2k> they seem to work reliably even if the current is an order of magnitude larger than the spec
[23:20] <RocketBoy> HABing - closer to god?
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[23:20] <SpeedEvil> rjharrison: They want you to sing about HAB?
[23:21] <RocketBoy> amee2k:- yaeh they arn't that critical
[23:21] <amee2k> that makes it sound like both the current limiter and using a different power rail for the high current circuits is a good idea
[23:22] <amee2k> otherwise i'd require lots of bypassing to prevent the control circuitry from cutting out
[23:22] <amee2k> i've had erratic behaviour due to noise on the power rails from a switched high-current citcuit before... its highly annoying :X
[23:24] <rjharrison> Yep I'm not sure I'm happy about SoP. The only thing in common is that the balloon is possibly closer to GOD than I am
[23:25] <RocketBoy> ;-)
[23:25] <rjharrison> Right bed beckons
[23:25] <rjharrison> GPS tracking is neraly in dl-fldigi
[23:26] <rjharrison> One more night and it will be there for mac and linux chase cars
[23:26] <rjharrison> nights all
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[23:26] <amee2k> right. getting a bit on the late side here too
[23:26] <RocketBoy> nights 2
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[23:26] <amee2k> thanks RocketBoy :)
[23:27] <amee2k> oh, i was too late :(
[00:00] --- Tue May 25 2010