Optical Apogee Detector
The first successful parachute deployment mechanism used on our rockets was the “Optical Apogee Detector”. Although the use of an electrical circuit and servo motor is a lot more complex than a mechanical method, I needed a mechanism which would deploy reliably each time.
Whilst browsing the web one time I stumbled across this site http://users.skynet.be/willaert/WR/optogee.htm
I was intrigued with the idea of using the difference between the light levels facing up and facing down to trigger a release mechanism. However I didn’t fancy the construction of the high voltage electro magnets, so I decided to include a servo motor control in the circuit instead.
The key components of the detector are the lm393 comparator IC and the LDR (light dependent resistors). The comparator output will be high whilst the upward facing LDR has more light exposure than the downward LDR. And low when the condition is reversed. The use of two LDR’s instead of just one means that the circuit is auto calibrating. If just one LDR was used the circuit would have to be adjusted each time to take into account the ambient light levels. With two LDR’s, as long as the ground is darker than the sky, the circuit should in theory always deploy.
The servo circuit is made up of a single 555 timer. The variable resistor adjusts how far the servo pulls when activated and the push button shorts the variable resistor to push the pin back out for the next launch.
The whole circuit requires an input voltage of 4.5-6v, we run the circuit from 3 AAA batteries, which are a good compromise between weight, size and cost.
Below is a picture of the completed circuit.
A Picture of the circuit mounted in a rocket:-