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Forums
4hv.org :: Forums :: Electromagnetic Radiation
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<OT> Optical comms

Author Post
Conundrum
Sat Oct 24 2009, 02:14PM Print View
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: "Somewhere in Area 51,Nevada"
Posts: 3113
Hi all.

I was looking into adding a laser detector (large area fisheye photodiode) to my balloon project, so that a signal could be sent from the ground in order to orient the transmit antenna/send commands.

Any ideas?
-A
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Proud Mary
Sat Oct 24 2009, 03:03PM
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4771
I'm not an astronomer, Andre, but wasn't there once a brief hope that pulsars might be a SETI phenomenon?

What sort of modulation would you be looking for?

Wouldn't the balloon have to be super-stabilised in some way?

What you could do with your balloon - and I will help you out if you want to do it - is to measure radiation at altitude.
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Conundrum
Sat Oct 24 2009, 05:30PM
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Joined: Thu Feb 09 2006, 05:37PM
Location: "Somewhere in Area 51,Nevada"
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yes, radiation measurement is on my "to do" list,

in order to minimise weight i plan to use an underclocked CMOS camera pointing upwards, with a pyrolytic graphite window.

i also have some spare geiger tubes but am loathe to use them due to the power requirements and sensitivity to altitude/temperature.

i have some parts on order including some accelerometers and altimeters.
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Proud Mary
Sat Oct 24 2009, 05:48PM
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Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4771
Conundrum wrote ...

yes, radiation measurement is on my "to do" list,

in order to minimise weight i plan to use an underclocked CMOS camera pointing upwards, with a pyrolytic graphite window.

i also have some spare geiger tubes but am loathe to use them due to the power requirements and sensitivity to altitude/temperature.

i have some parts on order including some accelerometers and altimeters.



Andre, I can give you an encpasulated micro 500V generator for your GM tube and a resistive barometric device. I'll also see if I have a decent end-window or waterproof GM tube for you, which will run at <500V so you can use it with the HV module.

I believe there is quite a lot of radiation up there at 50,000' so it wouldn't do any harm to read:

Transistor Sizing for Radiation Hardening
Quming Zhou and Kartik Mohanram


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Conundrum
Fri Nov 27 2009, 06:30PM
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Joined: Thu Feb 09 2006, 05:37PM
Location: "Somewhere in Area 51,Nevada"
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hmm. Was planning to use a PIC but this would probably crash spectacularly.

If synchronously clocked i could use a "quorum sensing"
setup where if one micro out of three goes flaky or fails the others flag it as "bad" and cut its Vcc line.
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Bjørn
Fri Nov 27 2009, 07:41PM
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2080
A PIC will crash no matter where you place it, sooner or later it will be hit by cosmic radiation it is just a matter of time. The die is small so it does not happen often, it is quite likely that it will fail for other reasons first. This is what the watchdog timer is for, enable it and relay your crash counter so you know if it resets.

In case of the Flash going bad you can have multiple copies of the code and let it select a random copy after reset, or use a checksum to determine if a copy has gone bad.

For your laser sensor you need a very narrow optical bandpass filter so you can filter out all wavelenghts that does not carry your signal.
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Proud Mary
Fri Nov 27 2009, 08:40PM
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Joined: Tue Feb 20 2007, 04:26PM
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Posts: 4771
I don't mean to sound negative, Andre, but how on earth will you manage to align laser sender and receiver when your balloon is traveling at maybe 100 knots at an altitude of 10km? What will happen when it goes above the clouds?

Why not make life a little easier by modifying one or more cheap surplus radiosondes operating around 405 MHz, when you can track your balloon with stacked yagis, helical antenna, corner reflectors etc, all of which are easy to make at no great cost.

I would go for the helical antenna myself, as I can see the polarization of the down signal changing through 90 deg due to the pitch, yaw, and roll of the sender antenna, and atmospheric refraction etc, whereupon a circularly polarized receive antenna would give the best advantage.

Stella
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Conundrum
Sun Nov 29 2009, 06:03PM
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: "Somewhere in Area 51,Nevada"
Posts: 3113
Interesting Bjorn.

Is it possible for a cosmic ray event to break the WDT so the PIC ends up simply resetting itself continuously?

someone on one of the electronics forums suggested using a large area RAM, continuously shunting linear bit patterns through it and detecting any change as an event, using that to reset the PIC would work.

another interesting effect I noticed is that underclocking a camera (used 4MHz xtal in place of 13.5MHz) increased the sensitivity and reduced the power consumption from around 15mA to 3mA

Could the same technique be used on a micro? clock it from the camera itself then shunt the data into a buffer to do the counting and rate compensation.

As for pointing the antenna, one way around that is to use an accelerometer to measure current tilt/spin and comepnsate by turning individual antennas on and off to "aim" the signal where it is supposed to go.

-A
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