phototransistor for apogee detection

Discussion about deployment systems including altimeters, timers, air speed flaps, servo systems, and chemical reactions.
srw
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phototransistor for apogee detection

Post by srw »

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!
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rockets-in-brighton
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Re: phototransistor for apogee detection

Post by rockets-in-brighton »

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
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Spaceman Spiff
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Re: phototransistor for apogee detection

Post by Spaceman Spiff »

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"
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rockets-in-brighton
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Re: phototransistor for apogee detection

Post by rockets-in-brighton »

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
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srw
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Re: phototransistor for apogee detection

Post by srw »

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.