Welcome to the Water Rocket Forum, sponsored by The Water Rocket Achievement World Record Association.

The largest, most sophisticated and ground breaking group supporting you, the serious water rocket flyer! Whether you are a beginner or an expert, the WRA2 has something for everyone.

A water rocket is a type of model rocket using water as its reaction mass. The pressure vessel (the engine of the rocket) is constructed from thin plastic or other non metallic materials (usually a used plastic soft drink bottle) weighing 1,500 grams or less. The water is forced out by compressed air. It is an example of Newton's third law of motion.

phototransistor for apogee detection

Discussion about deployment systems including altimeters, timers, air speed flaps, servo systems, and chemical reactions.
Post Reply
srw
New Member
Posts: 2
Joined: Wed Aug 26, 2009 8:18 am

phototransistor for apogee detection

Post by srw » Wed Aug 26, 2009 9:03 am

I'm just getting started in the hobby and have had some success with a simple rocket made from two 2L bottles, one as the pressure vessel and one as a crushable nose cone. Through trial and error I got the nose balast dialed in so that the rocket flies straight, but it also comes straight back down within 10-15m of the launcher.

It flies great, but for safety and durability I want to add parachute deployment. I came up with this idea then saw it mentioned in an older forum post. The sensing rig will be two phototransistors mounted parallel to the rocket, one facing up and one facing down. Use a comparator and debouncer circuit to detect apogee when the downwards-facing phototransistor sees higher intensity light than the nose. This would detect when the rocket has turned into its descent, since the sky should be brighter than the ground. The comparator output would be used as the enable line on a 555 timer to drive a servo release. It will add a little drag to the rocket, but I think it will work. Has any one tried it?

Thanks!



User avatar
rockets-in-brighton
Senior Member
Senior Member
Posts: 431
Joined: Fri Jun 20, 2008 12:33 pm

Re: phototransistor for apogee detection

Post by rockets-in-brighton » Wed Aug 26, 2009 3:20 pm

srw wrote:I'm just getting started in the hobby and have had some success with a simple rocket made from two 2L bottles, one as the pressure vessel and one as a crushable nose cone. Through trial and error I got the nose balast dialed in so that the rocket flies straight, but it also comes straight back down within 10-15m of the launcher.

It flies great, but for safety and durability I want to add parachute deployment. I came up with this idea then saw it mentioned in an older forum post. The sensing rig will be two phototransistors mounted parallel to the rocket, one facing up and one facing down. Use a comparator and debouncer circuit to detect apogee when the downwards-facing phototransistor sees higher intensity light than the nose. This would detect when the rocket has turned into its descent, since the sky should be brighter than the ground. The comparator output would be used as the enable line on a 555 timer to drive a servo release. It will add a little drag to the rocket, but I think it will work. Has any one tried it?

Thanks!
Yes, this has been tried, and successfully, although I don't recall if it included the refinement you describe of two photodetectors and a comparator. If you search the forum for Recovery Guide, you should find a link to a pretty comprehensive list of techniques, and you should be able to find the prior art. Sorry, the mini browser I'm using makes it hard to paste a link in here.


Cheers
Steve
Rockets-in-Brighton
WEB: http://groups.google.co.uk/group/rockets-in-brighton

User avatar
Spaceman Spiff
WRA2 Member
WRA2 Member
Posts: 681
Joined: Sat Feb 24, 2007 11:06 am

Re: phototransistor for apogee detection

Post by Spaceman Spiff » Fri Aug 28, 2009 7:41 pm

srw wrote:I'm just getting started in the hobby and have had some success with a simple rocket made from two 2L bottles, one as the pressure vessel and one as a crushable nose cone. Through trial and error I got the nose balast dialed in so that the rocket flies straight, but it also comes straight back down within 10-15m of the launcher.

It flies great, but for safety and durability I want to add parachute deployment. I came up with this idea then saw it mentioned in an older forum post. The sensing rig will be two phototransistors mounted parallel to the rocket, one facing up and one facing down. Use a comparator and debouncer circuit to detect apogee when the downwards-facing phototransistor sees higher intensity light than the nose. This would detect when the rocket has turned into its descent, since the sky should be brighter than the ground. The comparator output would be used as the enable line on a 555 timer to drive a servo release. It will add a little drag to the rocket, but I think it will work. Has any one tried it?

Thanks!
The photo detectors could in theory be buried in the body and peer out through windows or through the bottles themselves so they would not add any drag!


Spaceman Spiff
"What goes up, must come down"

User avatar
rockets-in-brighton
Senior Member
Senior Member
Posts: 431
Joined: Fri Jun 20, 2008 12:33 pm

Re: phototransistor for apogee detection

Post by rockets-in-brighton » Thu Sep 03, 2009 7:33 am

Finally remembered to look up the prior art, http://home.people.net.au/~aircommand/r ... de.htm#_70 and http://home.people.net.au/~aircommand/r ... de.htm#_71. These refer to Bernard Willaert's design that triggers a solenoid (http://users.skynet.be/willaert/WR/optogee.htm) and Nick Fisk's variation on it that substitutes a servo (http://www.fisk.me.uk/blog/water-rocket ... -detector/). Both used a comparator circuit as you described.


Cheers
Steve
Rockets-in-Brighton
WEB: http://groups.google.co.uk/group/rockets-in-brighton

srw
New Member
Posts: 2
Joined: Wed Aug 26, 2009 8:18 am

Re: phototransistor for apogee detection

Post by srw » Thu Sep 03, 2009 7:43 am

rockets-in-brighton wrote:Finally remembered to look up the prior art, http://home.people.net.au/~aircommand/r ... de.htm#_70 and http://home.people.net.au/~aircommand/r ... de.htm#_71. These refer to Bernard Willaert's design that triggers a solenoid (http://users.skynet.be/willaert/WR/optogee.htm) and Nick Fisk's variation on it that substitutes a servo (http://www.fisk.me.uk/blog/water-rocket ... -detector/). Both used a comparator circuit as you described.
Thanks for posting the links. I will end up with something similar, except it will be packaged in a 1L bottle. I've been cutting the bottom off of a bottle and sliding it over the pressure vessel as a crushable nose cone. The deployment system will have a cap mounted on the bottom so I can screw it into onto nose when I want to use it. I've been protoyping the ejection mechanism and will post the results in a few weeks when its finished.



Post Reply