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1 A small experimental mobile phone Acoustical Apogee Detection - See more at: http://forum.maxthon.c on Fri Jul 25, 2014 9:14 am

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This method uses the fact that there is a lot of noise produced by air flowing past the rocket during ascend. It can be noticed in many on board videos e.g. on youtube. Close to apogee the noise level drops almost to zero. This change in noise can be used to trigger the deployment mechanism.
Drawback: If the rocket's trajectory has a significant horizontal component the noise level will never vanish completely. As a consequence the parachute will not deploy.
AAD seems to be unexplored. That's why we go for it.
Rather than launching a rocket only with acoustical apogee detection or implementing a backup recovery system we decided to design a small experimental payload. This will be launched with a standard rocket with deployment timer.
The first attempt is a simple detection circuit using an electret microphone and a two stage amplifier followed by a low-pass filter. The result is a DC voltage which is dependant on the noise level recorded by the microphone. The output voltage is limited to 1.4 V in order to protect our data logger to which the signal is fed. We hope to find a correlation between vertical speed and noise level.






Schematic of the detection circuit. Since the signal level is unknown the gain of the second stage is variable. The input of the datalogger is protected by D2 and D3.
The PCB with AAD experiment.
The microphone has been obtained from a discarded mobile phone headset. Microphones can be found in almost every handheld device. 
The tiny PCB without housing. There is also a small button which can be used for many water rocket purposes ;o)
The experimental payload has been completed and waits for its first launch.
Simulation of a launch: The data-logger (hidden in the white foam of the payload section) records the output voltage of the circuit. Noise is simulated with a small FM receiver.
The simulation result: A noise peak at liftoff decreasing to a very low level at apogee being followed by constant noise (assuming that the parachute deploys). The parachute deployment would be triggered when the noise is minimal.
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