Eggtimer Rocketry Eggtimer User Manual page 6

Generally, accelerometers are more accurate for events logged when the rocket is going up, and
barometric pressure altimeters are more accurate for events that happen when the rocket is going
down. However, since most of the critical events such as ejection tend to occur in the descent
phase, an altimeter-based flight computer can do almost everything that a computer with both an
altimeter and an accelerometer can do, at much less cost. With careful software data filtering
and a judicious choice of settings it is possible to detect burnout with relatively decent accuracy
with a barometer-based flight computer, and it is possible to use a barometric based flight
computer to do Airstarts if at least two non-barometric events are used to trigger it.
Meet the Eggtimer
The Eggtimer is an altimeter-based flight computer with two deployment channels. It can record
up to 32 flights, of varying length; typical flight time for most rockets is 1-2 minutes or less, but
flights can be recorded for much longer periods if necessary. Altitude samples are taken at
programmable intervals, depending on the phase of the flight: Burn, Coast, or Descent. In
general, Burn and Coast intervals are the shortest, with the descent interval being much longer
because the rocket is descending at a relatively slow and constant rate so a faster rate won't
result in more meaningful data. There is a wide variety of settings available to accommodate
different rocket and motor combinations.
The Eggtimer is designed primarily to be used with medium power rockets, typically from "D"
to "G" power, or Level 1 and 2 HPR rockets using "H" through "K" power. The Main (Channel
A) deployment channel is programmable so that it may be triggered at apogee or at a fixed
altitude from 100-2000 feet, and you can select the number of seconds that the trigger is turned
on. The "Channel B" output can be used as either a Drogue deployment channel that fires at just
after apogee, or it can be used to airstart a second stage a programmed time after reaching
Launch Detect Altitude. Since most of the motors that the Eggtimer is designed to be used with
come with built-in ejection charges, the Main deployment channel can be used for the main
parachute, the built-in ejection charge can be used for the drogue parachute, and the Channel B
output can be used for airstarts, so the Eggtimer can support two-stage multiple deployment
rockets.
The deployment channels are capable of triggering a medium-current Estes igniter or #36
nichrome wire using an appropriate battery (such as a small 7.4v LiPo pack). You can also use a
low-current igniter such as a Quest Q2G2 with a single 3.7v LiPo battery. The Eggtimer
supports inexpensive pulse-width modulated (PWM) analog servos, which are typically used
with radio control models, so it can also be used with non-pyro deployment mechanisms, and can
even be used with large water rockets.
There are also three unbuffered logic-level outputs, they are set to turn on at nose-over, main
deployment altitude (shadowing the Main channel), and at landing detect. With the addition of
suitable buffer and driver circuitry, these channels can provide redundancy for the primary
channels, and the AUX3 (landing) channel can be used to turn on a sounding or tracking device
when the rocket lands.
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