timing device

[Sorak]

i need to put together a cheap and dirty timing device that will release a load after around 30 seconds - a minute

timing would idealy be adjustable
needs to be idiot proof - i have little electronics understanding.
must weigh under 20 grams. maybe 25.


i was thinking having something like one of those rotating egg timers with a hook attached, and when it reached a specific angle the string on the hook would slip off.

anyone got something better? or even other ideas

[2xCPU]

What other limitations do you have? eg. space
What do you have available? 5V, 240V, a decent head of water?
Is it a one off release or does it have to repeat/ be reusable?
Autonomous or remote control?
A little more info would be good.

How about one of the long BBQ matches with the load tied with a thread.

If this is home work, I want credit!

2.

[wingnut]

wind will put out your match, i prefer a sparkler for more reliability and extra sparkly brightness

[CQGLHyperion]

555 timer. Easy as to build.

[Sorak]

its going to be attached to a helium balloon. pretty much i need it to drop a 200 dollar gps without it reaching 20 km up and ~100km in what ever wind direction is blowing

so it needs to be light weight and cheap. resuable is interchangable with cheapness

[mtma]

Some sort of latex balloon connected to a pair of reversed scissors?

* I don't fancy having scissors falling on me from 20km up in the air though. But then again I'm not a fan of having a GPS fall on me either

But yeah, some contraption that unlatches your GPS based on altitude could be done with that concept technique.

[SLATYE]

30 seconds to 1 minute, to prevent it being more than 20km up and 100km away? That suggests that the balloon could be rising at over 100km/h and moving sideways at half the speed of sound...

Anyway, I'd use an AVR/Arduino for control and an RC servo to release the GPS. A pressure sensor to give you an altitude estimate is optional; otherwise you can just time it.

Alternatively you could just use a standard RC system and release it from the ground, but getting 100km range would require major modifications.

[2xCPU]

There are so many ways to solve this it's hard to know where to start.
The easiest/lightest solution will depend on what else is in the payload that you can make use of.

As SLATYE points out, the maths suggests this project is .... not fully thought through yet.

As for triggering the drop - again, I'd try to make use of something already part of the payload, like .. um ... how about the GPS!

Power everything up for launch and using a small micro, read the NMEA sentences to trigger release based on: Distance travelled, Altitude, Time elapsed etc. (which ever comes first).

its going to be attached to a helium balloon.

small details like this are probably worth mentioning in your first post.

2.

[Bastard Child]

555 one-shot timer that powers a hobby rocket motor igniter to burst the balloon?

http://www.robotroom.com/Model-Rocke...ontroller.html
http://www3.omnimodels.com/cgi-bin/w...P=M&I=ESTT2301

[Sorak]

Originally Posted by 2xCPU

As SLATYE points out, the maths suggests this project is .... not fully thought through yet.

heh, i think theres a missunderstanding. i DONT want the gps to reach full altitude. a rocket powered helium balloon capable to supersonic travel sounds awesome but not the goal.

i want it to fall after 30 seconds so i wont have to go far to fetch it. its pretty much a proof of concept for the retrieval system and low altitude ascent tracking.

i have some pretty specific weight restrictions, anything over them requires CASA permission and id rather not go through that mess.

id rather something thats mechanical, maybe wind up or low voltage, ie watch battery type.

[SLATYE]

Right. I assumed that when you said "without it reaching 20 km up and ~100km in what ever wind direction is blowing" you meant that anything less than that was fine. Apparently the intended meaning was more like "without it going more than 100m away".

For that, why not just use a standard RC TX/RX system and drop it manually?

HobbyKing Ultra Micro linear servo (1.7g)
Turnigy 80mAh 1S LiPo (2g)
FrSky VD5M 5-channel RX (2g) (this RX includes a telemetry downlink)

Total is 5.7g, plus a bit for wires and mounting hardware that you'd need on any system.

Alternatively, SuperMicro does an RX/ESC/servo all-in-one board that weighs 2.7g; adding the LiPo to that would be 4.7g, but you don't get telemetry.

[2xCPU]

Originally Posted by Sorak

i want it to fall after 30 seconds so i wont have to go far to fetch it.

Tie a string to it.

2.

Edit: Weather balloons are released and tracked everyday.
Sonde Monitor is one program that can do it very well.

[Carcin0Genic]

Im not quite sure of your goal here, but why not launch it with a hobby rocket and let it parachute down? Would look cool if nothing else
And hobby rockets are nice and easy to use, and predicting (or aiming, whatever floats ya boat) where they go is basic math.