this is a parachute release mechanism for a model rocket, i built it for a water rocket but other types would work. It uses an Arduino pro mini (http://www.sparkfun.com/products/9218) which is really good for this as it weighs less than two grams and is tiny. The mechanism fits in a 2 litre drinks bottle which can be attached to the top of your rocket, you could easily scale this project up or down for different sized rockets.
the device works with a tilt sensor, when the rockets power tails of then it will tip forward as the front is heavier, this will (hopefully) trigger the tilt sensor which will turn the servo releasing the door .
What you'll need:
Arduino Pro mini (or a full sized one would work fine)
tilt sensor http://www.oomlout.co.uk/tilt-switch-p-243.html?zenid=b02b94c613289937d7d01f66b4264578
pliers and wire clippers
FTDI breakout board( to program the arduino)
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Step 1: Making the Frame
the frame that holds the parachute is made out of cardboard, i drew around the bottom of the bottle and cut it ouut then used it as a template for the other side. i also cut a long piece of card slightly thinner than the diameter of the circle and cut a rectangular hole in the middle of it for the servo. Push the servo through this hole and screw the screws into the card. I used superglue to attach the two cardboard circles to the strip (like in the picture).
i cut out four cardboard triangles to support the circles and glued them in place. i made a flap that goes over the servo to protect it from the parachute and made two boxes of card for it to rest on and glued a bath sponge cut in half to the flap, this is so that when the door on the bottle opens then the parachute is pushed out.
Step 2: The Plastic Bottle
the plastic bottle is a two litre bottle that i cut the base off. i also cut a flap out, i cut it off completly then taped it back on with electrical tape, this made a good hinge tat flapped open easily. i also cut a small hole for the elastic to go through as it hooks onto the servo, it wraps around the bottle and loops around a lego technic washer ( you could use anything that shape) that is glued through a hole in the back. the door is pulled open by a elastic band which is superglued to the door and the rest of the bottle
there is a straw taped along one edge of the door, this stops it from closing completely and also from getting jammed, there are also
toothpicks taped to the inside of the door to made it more sturdy and stop it from bending
Step 3: The Electronics
the electronics are really simple so i haven't drawn a circuit diagram,the red battery lead is soldered into the RAW pin through the switch,the black lead to the ground pin. the white wire of the servo goes to a digital pin ( i used pin 9), the red lead goes to the vcc pin and the back lead to ground, i soldered all three wires to a male header that connects to the servo. the tilt sensor is soldered into pin 2 on the arduino and the ground pin right next to it, be careful when soldering not to hold the soldering iron on too long as you can damage the board
Step 4: Programming the Device
the device uses an edited version of the debounce example sketch. debouncing works by reading the tilt sensor twice in a very short space of time (about 200 milliseconds works well) and comparing them, it is necessary as with a rocket sometimes the vibrations can set of the sensor without it actually tilting, the debounce sketch makes that much less likely
Step 5: Possible Improvements
for the parachute i used a plastic bin liner, i cut allong the middle to open it out, covered the edges in masking tape and atached fishing wire to the corners for the strings.
to use the parachute you need to slide the cardboard with the electronics stuck to the top it into the bottle, turn the Arduino on before this, then you attach it to the top of your rocket, stick the end of the parachute strings onto the body of the rocket and the main part under the door, holding it down against the elastic band. Take the elastic and tie a loop in one end and hook it onto the servo arm, tie the other end of the elastic to the lego washer on the back,
you should probably add an led so that you don't leave the device on by mistake, i originally planned to have the switch positioned so i could open the lid to access it but it would have made it hard to pull the cardboard out unless i used really long wires.