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back to article LOHAN's cold heart beats beneath silicon breast

Our crack Low Orbit Helium Assisted Navigator (LOHAN) electronics team has made some cracking progress over the past few weeks, putting together the beating heart that will control our audacious Vulture 2 spaceplane mission. Click here for a bigger version of the LOHAN graphic A couple of months back, we decided to fire the …

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Boffin

And before anyone reminds me...

Yes, I did forget to write 'turn on the processor power' between turning all the switches off and turning switch 1 on...

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Anonymous Coward

Boobies

Balloon

Or

Orbital

Ballocket

Ignition

Electronics

System

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Anonymous Coward

But the important question is

But the important question is, "does it have a beeper?"

It simply MUST have a beeper that beeps when armed.

A count-down display is also essential.

And of course, a red and a blue wire.

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Re: But the important question is

Regrettable, in space, no-one can hear you beep.

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Re: But the important question is

You're not going in to space.

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Re: But the important question is

Shouldn't it go "Ping!"?

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Re: But the important question is

You also need to put in the most expensive machine in the office.

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Boffin

nice board

Who did you use to fab your circuit boards? They're really pretty, and I'm always looking for a good place to do it. I've done some with ExpressPCB for example, but their proprietary software isn't my favorite, though their prices and build quality are decent.

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Happy

Re: nice board

Ha, I just looked at that photo of the pcb designer again, and it looks a LOT like ExpressPCB's tool.

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Anonymous Coward

"working on the board's schematic"?

Looks like the PCB layout to me, not the schematic...

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Re: "working on the board's schematic"?

The schematic's on the laptop, just visible...

The PCB software is Eagle; not sure what Anthony used for the renders.

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Boffin

Re: "working on the board's schematic"?

That's because it's probably a computer rendered pre-view, as the article indicated.

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Mushroom

Short Circuit?

I'm not sure what kind of batteries you're using on the igniter, but is forcing a short-circuit in the event of a mission timeout such a good idea? Doesn't it risk causing a fire that could lead to a rather unwanted detonation of the rocket motor anyway?

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Re: Short Circuit?

The idea of the short circuit is that the fuse blows, so the batteries can't fire the motor.

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Go

Schematic review

R5 to R9 are 10k, and so the LED current with a 12V source will be about 1mA and you probably won't be able to see them in the sunshine. I suggest a magnitude less.

I would be concerned that the frequency of the fail-safe timer is rather temperature dependant. I would recommend a crystal oscillator (32kHz with +/-20ppm spec over -40 to +85 are tens of pence).

There seems to be a lack of local decoupling on the micro or any of the other ICs/Modules.

Fuse, or otherwise protect, the incoming supplies, otherwise a loose ground wire accidentally touching of the through hole components will take tracks off you circuit board.

IC5 will dissipate significant power. Make sure it has a decent heatsink (useful to keep the electronics warm) or use a switch mode module (about a fiver in small volume) that are nominally drop in replacements for the old 7805 devices.

Keep up the good work!

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Re: Schematic review @ Ed and those below...

1) Very conservative design. For the day job I build robots that steer drills three miles underground...

2) The LEDs are 2mA high brightness types. They'll be shaded when we start. One of them goes through the relay coil and I don't want that current to persuade the relay to trigger.

3) Failsafe timer isn't critical. We expect the electronics pack to remain close to room temperature. The oscillator is around a hundred hertz.

4) Decoupling is there but there's not a lot of it; I may add more.

5) IC5 dissipates milliwatts; it won't need a heatsink though it will need a mica insulating shim.

6) Relay because that way I can get a physical separation between the coil and the contacts, and guarantee that the immediate aftermath of powering up the processor doesn't send a random glitch to the igniter.

7) BJT rather than FET because FETs are not too happy under five volts. I'm constrained to 3v3 by the other hardware, or I need a lot of level conversion bits - extra complexity.

8) Two transistors not because I expect a transistor to die but in case an output from the processor GPIO fails.

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Mushroom

ORGASMIC

Online

Real-time

GPS-based

Altitude

Sensor

Module and

Ignition

Controller

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Hope there's no moisture in the relay can as it might be frozen in position when you try to fire the igniter.

Strange choice, using a transistor to switch a relay, _ALL_ modern pyro systems use mosfets to fire igniters. Igniters usually fire with <1 amp for a few 10's of millis. I have used SP8K4 dual mosfet in a small sop8 package. Make sure you don't knock the flight package when you launch, you don't want to bounce the relay contacts.

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Boffin

Have you checked your relay for susceptibility to vibration/shock? It's not common, but since they are mechanical devices it _is_ possible to get a momentary contact if the shock is in the wrong direction.

You're not trying to switch a particularly heavy load, so I would have thought solid-state stuff would be adequate.

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I'm glad it is not

High altitute

Ignition

Timing

Launch

Electronic

contRoller

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Hmmm...

So you have two transistors for redundancy switching a single relay?

I'm pretty sure I know which components I would have doubled up for redundancy, and it wouldn't have been the solid-state semiconductors.

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Boffin

Re: Hmmm...

>So you have two transistors for redundancy switching a single relay?

I don't see a problem with that. Years ago, I made some boards to control model railroad snap-relays from a PIC, and for each port, I used two pins of a nice darlington transistor chip (with on-chip back emf protection, tailor made for this) to source the relay. Relays are noisy.

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Boffin

Re: Hmmm...

The chip was an ULN2803A. I remembered just after sending the message. Great for this kind of thing.

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Re: Hmmm...

Nice chip, it looks familiar.

My query was actually that they'd doubled up on the semiconductors, but not on the electromagnetic mechanical part... The part which could freeze solid at those altitudes.

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Anonymous Coward

Safe

High-altitude

Ignition

Timing

System

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Pint

Is that really???

...an HP-41C being used to support the design of this critical LOHAN component????

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Re: Is that really???

It does appear to be one of HP's greatest. I still have mine, though nowadays I tend to use its virtual version on my iPhone (i41CX) and my desktop (V41)

RPN forever!

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Re: Is that really???

Well spotted. I have an HP11C for the office desk, too. Had to change the batteries only last month... first time since 1987. Nothing lasts, these days.

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Backronym

Battery-Operated On-Ballocket Jumpstart/Off-switch Board

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Stratospheric

Launch

Apparutus

Providing

Pre-ignition to

Experimental

Rocket

yeah, struggling with the PP....

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Or more simply

Stratospheric

Launch

Activation

Gadget

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1

Bulgarian

Balloon

Using

Leccy

Guaranteed

Automatic

Rocket

Igniter and

Altitude

Neutralizer

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Binary Response at Altitude

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I like this one. Nice and simple, and before launch you have to make sure LOHAN's BRA is on.

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