Eggtimer Rocketry Eggtimer, Flight Computer, Eggtimer Flight Computer User Manual

Eggtimer Flight Computer

User’s Manual

Release 1.48b Board RevC

© 2013 Eggtimer Rocketry
All Rights Reserved

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Eggtimer Features

Altimeter rated to 29,500 ft ASL, resolution to approx 1 ft.

Logs data for last 32 flights, “beeps” out maximum altitude of last flight

Flight data can be downloaded to a computer using USB interface cable (included) to .CSV format, for
use with virtually any spreadsheet, graphing, or statistical analysis software package

Reports Elapsed Flight Time, Altitude @ Time, Raw Velocity @ Time, and filtered Altitude and Velocity
@ Time

Flight memory is non-volatile, so all settings and flight data are retained with the battery disconnected,
and interrupted flights can still produce a data graph

Fully configurable flight parameters including sampling rates, deployment time/altitudes, etc.

Eight pre-programmed flight profiles, so you can select commonly-used flight profiles in the field without
needing a computer

Will work with any 3v-4v power source: 3v lithium batteries, two 1.5v alkaline batteries, 3.6v NiMh
battery packs, or 3.7v LiPo batteries

Two programmable deployment channels: Main and Drogue/Airstart. Supports two-stage multiple­deployment flights using motor ejection for booster recovery and second-stage drogue deployment

Three logic-level outputs, triggered at nose-over, Main deployment altitude, and landing detect

Supports igniters with a voltage rating of up to 60V, up to 8A, 65W maximum

Deployment outputs also support standard analog PWM hobby servos for non-pyro deployment

Uses optoisolators and bipolar driver transistors for total isolation of computer and ignition power and
immunity from false ESD or RF triggering and “brownout” protection

Uses separate battery (not included) for external deployment device; each channel can have a separate
battery so servos and igniters can be mixed

Breakwire support for arming Airstarts, 3-factor triggering for safety

Supports remote Power and Reset/Hold switches, and remote Ready/Buzzer indicators

Outputs for external igniter continuity LEDs, current-limited

Programmable using standard VT100/ANSI text terminal programs, compatible with virtually all
computers (PC, Mac, Linux)

Firmware can be updated from a standard PC using the serial cable

Weight about 20 grams, size approx. 3.9” x 1”; fits in 29mm engine tube or BT-55 body tube

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Disclaimers, Legal Stuff, Etc.

The Eggtimer is meant to be used for hobby and experimental rocketry purposes. Although
hobby rocketry has an admirable safety record, largely due to the efforts of the good people at
the National Association of Rocketry (NAR) and the Tripoli Rocketry Association (TRA),
rocketry can be dangerous if proper safety precautions are not observed. This is particularly true
with some of the advanced techniques like pyrotechnic parachute deployment and igniting rocket
motors in flight (“Airstarts”). People can and have been seriously injured by not following
recognized and accepted safety practices. We cannot be responsible for your actions.

We strongly recommend that if you are not a member of either the NAR or the TRA, you join
one of them, join a local rocketry club, and pick the brains of experienced members before you
try any kind of multiple deployment or airstart flight. The safety information included in these
instructions is by no means comprehensive or complete, and is no substitute for the supervision
and advice of experienced rocketeers.

Limited Warranty

Eggtimer Rocketry warrants that all of the parts on the packing list of the Eggtimer kit have been
included, and that they are all in working condition. If you are missing something, contact us
immediately at [e-mail] and we will send you whatever it is that you are
missing. If you are missing something really egregious (like the PC board or the processor, for
example), we may ask you to return the entire kit unbuilt, we will send you a prepaid shipping
label for this purpose. We’d especially like to see the packing list so we can figure out what
went wrong so it doesn’t happen again…

If your Eggtimer does not work properly after assembly, take a deep breath, get out the
magnifying glass and a good light, and see if you have inadvertently created a solder bridge
somewhere. Chances are pretty good that you have, or that you have installed a part incorrectly.
We are a very small company and we just don’t have the resources to repair your board, but we
will be more than happy to give you advice and we might be able to help you find your error if
you send us some high resolution pictures, to [e-mail] . We cannot take
responsibility for your assembly techniques; if you do not have experience building kits of this
nature, we recommend that you enlist some help. (Another reason for joining a rocketry club,
there is usually at least one electronically-inclined member who can be bribed with a beverage or
two to give you a hand. Engineering types love a challenge, especially it it’s easy for them but
hard for you.)

Eggtimer Rocketry warrants that when properly assembled the Eggtimer will perform
substantially according to the published documentation. This means that we spent a lot of time
trying to ensure that it’s going to work the way that we say it does, and we try to fix things that
don’t quite work right in a reasonable time. Nevertheless, we can not and do not warrant that the
Eggtimer is perfect and will meet every rocketry purpose, for the simple reason that we can’t test
every possible rocket/motor/environmental combination. It is the buyer’s responsibility to
determine the suitability of the Eggtimer for their particular purpose. If you have a problem with
this, please contact us and we will be happy to send you a prepaid return label for your unbuilt
Eggtimer kit and we will refund your purchase price.

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Quick How-To Index

How to Program Your Eggtimer….16

How to Use the Pre-Programmed Flight Settings…..27

How to Download Flight Info…..34

How to Test Deployment Charges…46

How to Set Up the Eggtimer for Multiple Deployments…..47

How to Set Up the Eggtimer for Airstarts…50

How to Use Servos with the Eggtimer……..41 (#10)

Wire Options for your Eggtimer……….13

How to Pick a Battery for your Eggtimer……………………14

How to Size Pressure Ports…..42

How to Mount Your Eggtimer……39

Beep Code Reference…..28

Quick Reference……54 (We put this last because we hope you read the rest first ☺ )

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A General Introduction to Flight Computers

A flight computer is used to record the flight path of a rocket, and to optionally deploy recovery
devices. Typically, you can use it to find out the maximum altitude (apogee) of your rocket, the
maximum velocity of the rocket, the maximum acceleration (G forces) on your rocket, and other
flight characteristics. Being able to see these after a flight helps to validate your own designs,
and a flight computer is essential for high-power rockets since the motors used in those rockets
often don’t have built-in ejection charges. Typically, such rockets have two parachutes: a
smaller “drogue” parachute that is deployed at apogee, and a larger “main” parachute that is
deployed at a much lower altitude. The drogue parachute keeps the rocket from drifting too far
away… if the main parachute is deployed at 10,000’, it is possible that your rocket could drift
over a mile from the launch site. Using two-parachute deployment scheme typically keeps the
drift down to 1000’ or less.

They can also be used to “airstart” motors in flight, i.e. to ignite the second (or even third) stage
motors. Doing this is a more advanced technique than multiple recovery deployment, and it
requires accurate software flight simulation in order to select the proper time and/or altitude to
ignite the motor. Typically, the second stage is ignited either when burnout of the first stage
motor is detected or after a short delay afterwards.

Flight computers can use either a barometric pressure altimeter, an accelerometer “G-sensor”, or
both. Each one has its own advantages and disadvantages. A barometric pressure sensor can be
used to directly determine altitude, and when read at known intervals can be used to calculate
velocity between intervals with fairly reasonable accuracy. You can in theory use one to
determine acceleration, but slight errors tend to be multiplied resulting in much larger errors.
Since they read barometric pressure which depends on temperature, and may be thrown off by
aerodynamic issues such as the transition to supersonic speeds, careful consideration needs to be
taken to ensure that the altitude readings are accurate. Barometric pressure sensors tend to be
relatively slow, so the sampling rate may be limited by the speed of the sensor. Also, errors in
barometric pressure readings are inevitable during high-speed flight, because the pressure is still
changing as the sensor is being read. Derived data such as velocity may be inaccurate at angles
that vary significantly from the vertical, since the recorded altitude will be less than the distance
that the rocket actually travels during that time interval.

An accelerometer uses the forces on the rocket to determine when the motor has fired and burned
out, and can be used to determine velocity over a known interval with better accuracy than an
altimeter. Since it reacts directly to the forces on the rocket, it is very easy to detect reaction­based events such as burnout, and therefore is the best sensor to use for “air starting” a second
stage motor in flight. It can also be used to determine altitude by integrating (adding up) the
differences in calculated distance for each time interval, however since this is “distance-traveled”
reading it may not be accurate at angles much above vertical. Also, high-G accelerometers (> 50
G, which may be experienced by high-power rockets) are expensive chips, costing a lot more
than a barometer chip. In addition, they only work well when the rocket is going up, once it
slows down before apogee the acceleration is minimal and therefore readings will not be
accurate. On the plus side, they tend to be very fast, so higher sampling rates can be used
compared to barometric pressure sensors.

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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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Programming is performed using a standard VT100/ANSI serial terminal program, connected to
your computer with a USB-serial cable. Since there is no special software required, you can use
just about any computer, running Windows, Mac, Linux, or just about anything else you can
come up with. Flights can be downloaded to a standard CSV-formatted file, and can be
imported into virtually any spreadsheet or data analysis program to produce flight graphs.

The firmware on the processor chip performs real-time data analysis during flights to detect and
report significant flight events such as launch, apogee, deployment, and landing. Hardware
breakwire support is provided to provide an extra level of redundancy for airstarts, this ensures
that the rocket has indeed left the pad and that a sudden gust of wind isn’t going to trick the
Eggtimer into thinking that it’s reached the launch detect altitude.

Getting to know your Eggtimer

Although the Eggtimer seems relatively simple, you will want to familiarize yourself with it
BEFORE you install it in a rocket, and certainly before you try flying it.

Battery (3V) – Two solder pads, marked “+” and “-“, for connecting the Eggtimer battery.
Typical draw is about 8 ma when running, recommended battery voltage is 3v-4v. DO NOT
CONNECT A 9V BATTERY TO THESE PADS OR YOU WILL DAMAGE YOUR
EGGTIMER!

Push Button (ACT) – Performs various actions when pressed.

Indicator (IND) – Glows solid or blinking, depending on the status of the Eggtimer.

Ready Indicator (RDY) – Glows on when ready for launch, blinks when flight is done or the

Eggtimer is in download mode.

Beeper (BUZ) - “Beeps” out status, warnings, altitude, and other important notifications.

Channel A (Main) Deployment (CHA) - Two solder pads, marked “+” and “-“ for connecting

an electric match or other low-to-medium current device. Deployment altitude is programmable,
from 100-2,000 ft, or at nose-over. Maximum 8A current, 60 VDC, 65 W. You can also connect
the input for a standard analog hobby servo to the “+” terminal (along with a 39 ohm ¼ W
resistor in parallel) for non-pyro deployment.

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Channel A (Main) Battery (BCHA) - Two solder pads, marked “+” and “-“, for connecting an
external battery for the deployment device (if used). Typically, these batteries are 3.7v LiPo
cells for low-current igniters such as a Quest G2Q2, or 9-12v alkaline batteries for #36 nichrome
wire or other igniters. The Eggtimer is capable of handling up to 60V/8A up to 65W, so you can
ignite even an Aerotech Copperhead igniter with it, assuming you have a strong enough battery
(i.e. 14.4v LiPo battery).

Channel B Deployment (CHB) - Two solder pads, marked “+” and “-“ for connecting an
electric match or other low-to-medium current device. This channel may be used for a “drogue”
chute, airstarting second stage or booster motors, or a chute triggered at a certain time in the
flight. Maximum 8A current, 60 VDC, 65 W. You can also connect the input for a standard
analog hobby servo to the “+” terminal (along with a 39 ohm ¼ W resistor in parallel) for non­pyro drogue deployment at nose-over (one second after apogee).

Channel B Battery (BCHB) – Separate battery terminals for the Channel B deployment. This is
provided in case you want to use two different batteries for Main and Channel B; this allows you
to mix pyro outputs and servos, since servos generally use 6v or less. If you are using both
Channel A (Main) and Channel B, and they are OK on the same battery (i.e. they’re both either
servos or both igniters), then you can jumper the BCHA and BCHB terminals together: + to +,
and – to - .

AUX1 Output – The AUX1 output turns ON (+V) for two seconds, at nose-over detect. It can
be used to provide redundancy for the CHB channel in drogue (nose-over) mode, or it can be
used to trigger a drogue parachute if CHB is being used in airstart or timed modes. This is an
unbuffered (logic-level) output, a high-current driver circuit will be required to trigger a load of
over 20 mA or over +V (typically 3V-4V). See the Eggtimer web site (www.EggtimerRocketry)
for suggested high-current driver circuits.

AUX2 Output – The AUX2 output “shadows” the function of the CHA (MAIN) deployment
output. It can be used to provide redundancy for the CHA output. Note that if CHA is disabled
(set to “0”), the AUX2 channel will also be disabled. This is an unbuffered circuit, see the
description of AUX1 for details.

AUX3 Output – The AUX3 output turns ON (+V) when a landing has been detected (AGL of
less than 30 ft for at least 5 seconds after nose-over), and stays on until the Eggtimer is reset.
This is an unbuffered output, see the description of AUX1 for details. NOTE: This output is
designed to be used with an “always on” device such as a loud buzzer or telemetry device.
Because of a quirk in the processor chip, it flashes on briefly a few times when you first start the
Eggtimer. Therefore, DO NOT USE THIS OUTPUT WITH A PYROTECHIC LOAD, or it may
fire as soon as you turn on the Eggtimer!

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Data Port – 6-pin FTDI-compatible standard header connector for connecting the USB-serial
data cable for programming and downloading data, and also for connecting a telemetry device
such as an RF serial transmitter (not included).

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External Switch/Indicator Options

The Eggtimer has several connections for external switches and/or indicators.

Power Switch (SW)

An external power switch will make it much easier to use the Eggtimer, because you won’t keep
having to disconnect the battery, or removing the jumper across the SW terminals. If you are
using multiple deployment, we STRONGLY recommend using an external power switch so you
can turn off the Eggtimer while your model is undergoing the safety check by the RSO; in fact,
most NAR and Tripoli sanctioned clubs require this. (The alternative is to have an external
Reset switch/jumper; you must have at least ONE of these, but both would be preferable if you
are doing multiple deployment, and may be required by your club if you are airstarting).

Just about any switch will work with the Eggtimer because the voltage is only about 3v and the
current is < 10 ma. However, you need to use a switch that can handle the expected G forces that
you expect the rocket to experience during flight. This is one more reason to use a flight
simulation program such as RocSim or Open Rocket; you need to know what these forces are
going to be to pick a switch.

In general, we recommend that if you use a slide switch that it is mounted so that it slides
sideways, not up and down. This will prevent G forces from possibly causing the switch to
“bounce”, interrupting the power to the Eggtimer, which is not a good thing. Any slide switch
that you use must be rated to at least twice the G forces that you are likely to see… a $1 Radio
Shack special isn’t going to cut it, spend a few bucks and get a high-quality switch.

You can also use a “push-on, push-off” type switch. Many users have had good success with
them, also mounted laterally. You can put the switch just behind one of the air ports, and actuate
it by pushing a small pin/wire through the hole. Just like with slide switches, spend the money to
get a good quality switch.

A better option would be a more positive switch, such as a rotary switch or a screw-type switch
that locks down positively. Since the forces on rockets are almost entirely along the longitudinal
axis of the rocket, the contacts on a rotary switch are unlikely to be interrupted by G forces. A
good choice is the Schurter 033.4501 rotary switch, they cost about $5. This is a special­purpose rotary switch originally designed to be a 120v/220v selector switch, but it works very
well for our purposes. You can get them from a number of online rocketry suppliers, or you can
order one directly from Allied Electronics, a direct distributor for Schurter products. They’re
actually about a buck cheaper from Allied, but you’ll have to pay shipping, so chances are pretty
good that you’re gonna come out ahead if you buy it from one of the rocketry suppliers because
you’re probably buying a bunch of stuff from them anyway.

Featherweight Rocketry also makes a good cheap screw-type switch, they use a screw to
positively lock down the contacts and completely eliminate any possibility of the switch being
jarred open. You can also make your own screw switch, Google around and you can probably
find some good examples.

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Reset Switch (RST)

When shorted, the Reset terminals put the Eggtimer “on hold”, i.e. resets it so that it does
nothing. When it is in reset mode, it draws very little power from the battery, < 2 ma. All
outputs are held low, so this is the “safe” mode when multiple deployment systems are used;
they cannot be triggered while the Eggtimer is reset. This can be used to put the Eggtimer in low
power mode while it’s sitting on the pad for a long time, and can be released shortly before
launch to help save battery life.

When the reset switch is opened, the Eggtimer begins the startup sequence. You will hear a
continuous “beep-beep” sound, the programming screen is displayed for 30 seconds, and the
flight sequence begins if no keys are pressed within this time or the button is not held down to
select a pre-programmed flight profile. After approximately 20 more seconds, the Eggtimer
begins taking altitude readings and recording, and chirps until launch to let you know that it’s
ready for flight.

If you use a multiple deployment system, we STRONGLY recommend that you have some sort
of external switch or shorting plug to hold the Eggtimer in reset mode until shortly before
launch. A lot of people have had good results with 2.5mm earphone jacks, you simply wire each
lug to the RST terminals then you short the leads on the plug together. Hang a “Remove Before
Flight” ribbon on the end of the plug, and you’re ready to go; simply pull the plug out when
you’re ready to launch to start the Eggtimer flight sequence.

You can also use the same rotary switch listed above in the Power Switch section, they’re a little
pricey at $5 each but they work very well. For larger rockets where you have the room and are
spending a lot of money anyway, it’s definitely the way to go.

The Reset terminals are also used to update the firmware on the Eggtimer, see the instructions on
the Eggtimer web site ( www.eggtimerrocketry.com ) for details.

Breakwire (BW)

A “breakwire” is used to definitively indicate that the rocket has lifted off. This technique is
used on “real” rockets, and works very well for arming multiple deployment rockets as well,
particularly for airstarts. Basically, when a rocket lifts off a wire that is connected between the
rocket and the pad breaks an electrical connection, and that event triggers a liftoff sequence.

In the case of the Eggtimer, the breakwire is optionally used to arm Channel B in airstart mode.
This provides an additional level of safety to prevent accidental airstart ignition; for the igniter to
fire, the breakwire must be tripped AND the rocket must have reached the LDA altitude AND
the Burn Timer/Airstart Timer must have expired after LDA. This virtually eliminates any
possibility of the second stage igniting on the pad. As we have mentioned before, you should
shunt the igniter or remove power to the igniter until just prior to arming the Eggtimer and
launch.

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The downside of breakwires is that it’s one more thing you have to set up on the pad. See
Appendix D for airstart and breakwire implementation tips.

Indicator Pads – .1”-spaced pads with outputs for the following external indicators:

Ready LED – In parallel with the on-board Ready LED.
BUZ – In parallel with the on-board buzzer. This must be a pizo element.
ALED – Channel A continuity LED (polarized, +/-)
BLED – Channel B continuity LED (polarized, +/-)

Ready LED (RDY, +/-)

This is an external extension of the on-board RDY (yellow) LED.

The READY LED on the Eggtimer is used to indicate that either the Eggtimer is in flight mode
(solid ON), the flight is complete (ON with the IND LED also ON), or is ready for download
(flashing ON). It also flashes along with the buzzer to indicate peak altitude after the flight, and
flashes with the IND light after programming to let you know that the Eggtimer is halted. You
can connect an external LED to the READY terminals so that you can visually confirm that the
Eggtimer is ready for flight to begin or has completed. You will need an LED capable of
lighting with approximately 10 ma of current or less, most medium-to-high intensity LEDs will
work fine. The “long” lead of the LED should be connected to the “+” terminal, the shorter one
should be connected to the “-“ terminal. If you are using an external LED in a holder with leads,
the red wire should be connected to the “+” terminal and the black wire should be connected to
the “-“ terminal.

We recommend using an “ultra bright” LED, they are easier to see outdoors but they will draw
more current. The reality is that the amount of time that the Ready LED is going to be on is very
small, and any decent battery is going to run a long time without worrying about an extra 10 ma
of current draw for a few minutes.

BUZ – Buzzer

This is an extension of the on-board pizo buzzer. The on-board buzzer isn’t meant to be very
loud, it’s designed to be used as an indicator when you’re right next to the Eggtimer to tell you
that it’s ready, or to beep out the altitude after the flight. If you want to add a louder one, you
can use these terminals. The buzzer may be a pizo electric element, or it can be a pizo buzzer
with a built-in driver. It must draw less than 20 mA.

If you want to add a VERY LOUD buzzer that helps you find your rocket, and you have a
deployment channel available, you can use the unused deployment channel in continuous mode
with a 9v-12v battery and a Mallory Sonalert® buzzer. They’re about $10. If you’re not using
CHB, just put it in drogue mode (option 1) and change the On-Time to be zero; if you’re not
using CHA, put it in Main deployment mode (option 1), choose a low deployment altitude,
maybe 200’ or so, and set the On-Time to zero. You can also use the AUX3 channel with a

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small relay to turn on the buzzer; you may need a driver circuit, see the Eggtimer web site
(www.EggtimerRocketry.com) for details. Similarly you can also use the nice little sounders sold
by Adept Rocketry, they make a very distinct sound that’s hard to miss even with other
background noise present.

ALED/BLED - Deployment Channel LEDs

ALED/BLED are designed to provide a visual indicator of deployment channel power and
igniter continuity. This lets you know immediately after connecting the igniter and powering up
the channel if something isn’t right. They use relatively little current, so leaving them on isn’t
likely to be a significant drain on your deployment battery.

The ALED/BLED outputs are current-limited with 820 ohm resistors, a 9v battery on the outputs
will source about 9ma of current to the LEDs. This is more than enough to brightly light up a
high-brightness T-1 sized LED. Note that these outputs are polarized, they are marked +/-, so
you need to make sure that you have these connected properly; the long lead of the LEDs goes to
the + output. If you’re using an LED that’s pre-wired in a holder, connect the red wire to the “+”
pad and the black wire to the “-“ pad.

A Note on Wiring

The Eggtimer is designed to have the wires directly soldered to the board. This allows you to
choose whatever method of terminating the connections you want: barrier strips, solder directly
to the switches, etc. We’ve found that simply wire-wrapping the igniters to a “pigtail” wire
soldered to the board works very well for smaller rockets. By soldering the pigtails to the board
rather than having screw terminal blocks, you eliminate the possibility that the wire may work
loose from the terminal in flight.

We recommend using #22-#26 gauge wire for wiring to the Eggtimer board, we like to use the
#24 gauge stranded wire that’s found in Cat-5 network cables. It’s cheap, easy to find, and just
the right size. It’s also twisted together in nice solid-striped pairs, so it’s easy to tell the “+”
from the “-“ wire. You can also use solid wire, but solid wire is harder to work with and has a
tendency to break after being bent a few times. These breaks can be a pain to find, because they
are typically inside the insulator jacket where you can’t see them.

If you use stranded wire, you MUST TIN THE WIRES BEFORE SOLDERING TO THE
BOARD. This is to prevent stray “whiskers” of wire strands from coming loose and bridging
pads, or breaking off and landing on the board in some random place. We’ve seen the results of
this happening, it’s not pretty, and they can be very hard to find if the lodge underneath the
processor chip or in some other hidden spot on the board.

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Eggtimer Battery Options

The Eggtimer requires a battery with a nominal voltage of 3.0v-4.0v. The typical current draw is
about 8-10 ma when operating, and 2 ma when is Reset/Hold mode. This means that you have
several battery options:

· 3v Lithium batteries, such as a CR123 or CR2

· 3v Lithium “coin cells”, such as a CR2032, if they are used for a very short time

· 3v or 3.6v rechargeable Lithium batteries

· Two 1.5v alkaline batteries wired in series for 3v, typically size AAA or AA

· 3.6v rechargeable Nickel Metal Hydride (NiMh) battery packs

· 3.7v rechargeable Lithium Ion (LiPo) cells

IMPORTANT: DO NOT CONNECT A 9V BATTERY DIRECTLY TO THE
EGGTIMER’S “3V” PADS! YOU WILL DAMAGE IT IRREPARABLY!

Because of the wide variety of rocket sizes and the different types of installations that are
possible, the Eggtimer does not come with a battery. There are a wide variety of battery options
available, which one you choose will depend on many factors:

1) The size of the rocket – Small rockets may require a smaller battery than a large rocket. If
you are trying to put the Eggtimer into the smallest possible space, a 3v Lithium “coin cell”
battery may work, but see the notes below.

2) How often you wish to replace the battery – Smaller lower-capacity batteries will have to be
replaced (or recharged) more often than larger batteries, which is the trade-off for the smaller
size

3) How much room you have for a battery – If you have a 3”, 4 pound I-powered rocket you
probably have plenty of room for a 250 maH 3.6v NiMH battery, and don’t care so much about a
few extra grams. Not so true if you’re flying it in a BT-55 “D”-powered rocket…

4) How much the battery costs – Rechargeable LiPo batteries are nice, but you may need a
special charger for them. NiMH or plain old alkaline batteries are heavier, but they cost a whole
lot less to use. Non-rechargeable Lithium cells (i.e. CR2, CR123) work very well, provide a lot
of current, are cheap and readily available, and are relatively lightweight compared to alkaline
batteries and NiMH packs.

Lithium coin cell batteries (i.e. CR2032) will work with the Eggtimer, however they have a
relatively small capacity compared to most of the other battery options. Specifically, a CR2032
battery is rated at 225 maH, but this rating is for an extremely low draw, 80 MICRO-amps,
which is what you might get from a very low power device like a digital watch or calculator.
The Eggtimer draws significantly more current that this, so you may only get 100 maH of
capacity out of the battery, and that will drop even less if the battery gets warm (as in sitting out
on the pad in the desert in 110° weather). If you want to use a coin cell battery, use the largest

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one you can fit in the rocket (CR2450’s are the best since they have the largest capacity), and test
them with a Digital Voltmeter while the Eggtimer is powered on BEFORE EVERY FLIGHT.

General Battery Procedures

1) TEST YOUR BATTERY WITH A DVM BEFORE EVERY FLIGHT!!!

If you do not have one, buy an inexpensive digital voltmeter and check the battery voltage while
the Eggtimer is powered ON, before EVERY flight. The voltage across the two Battery
terminals (B+ and B-) should be between 2.8v and 4.0v. (It may be a little higher if you have
just recharged the battery, i.e. LiPo batteries may show up to 4.2v when fully charged; that’s OK,
the Eggtimer will be fine with it). If your battery reads under 2.8v, recharge it or dispose of it
and use another battery. NEVER fly with a battery under 3.0v if you are using electronic
deployment. The Eggtimer draws very little current, but you don’t want to risk having the
battery run low at 10,000’.

Similarly, test the deployment batteries as well if you are using electronic deployment. Check
your igniter vendor’s guidelines for recommended voltage, and replace the battery if it’s close to
the minimum value, or if the voltage is less than 80% of its rated value, whichever is highest.
For example, a Quest Q2G2 igniter will fire with as little as 3.7v (a fully-charged LiPo cell), but
if you’re using as 9v battery you should replace it if it’s under 7.2v (80% of its rated value)
because its voltage drops rapidly as it begins to run down.

The Eggtimer’s built-in battery tester is designed to be used as a quick check to see if your
battery MAY be getting low. It is not a substitute for checking the battery with a DVM, it does
not have the accuracy that you’re going to get with a dedicated instrument. You NEED a digital
voltmeter in your tool box anyway; if you don’t have one, run out to Radio Shack and buy one,
they’re under $20.

2) Balance your model with the batteries in place.

AAA and NiMH batteries are fairly heavy, as are some of the larger Lithium batteries such as a
CR123. Alkaline 9V batteries are VERY heavy, almost 50 grams (nearly 2 oz!) You should
always weigh your rocket and determine the CG with the intended flight battery in place. We
strongly recommend using a program such as RocSim or Open Rocket to determine the flight
path of your rocket with the intended payload configuration (batteries, sled, etc.) and motor.

If the rocket was designed for egg lofting or multiple deployment, chances are pretty good that
just about any configuration with an Eggtimer isn’t going to cause any stability problems, since
those payload configurations are generally at least as heavy as a fully-loaded Eggtimer sled using
a 3v battery and LiPo battery for deployment. However, if you are building a rocket from
scratch or adding a payload bay onto a kit, particularly with a BT-60 or smaller body tube
diameter, you may end up with an over-stable rocket that wants to weathercock into the wind or
dance around on its tail after clearing the launch rod, due to excessive nose weight. Using a
larger motor or one with a higher average thrust (i.e. E20 instead of E9) to get more speed off the
rod will help that situation somewhat, as will using a longer launch rod. If you do the latter, you

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might want to increase its diameter, to keep it from “whipping” as the rocket travels along its
path, or better yet use a rail launcher. If you can, you can also shorten the length of the booster
to bring the CG towards the rear; you should check this carefully with a flight simulator program
first before you start cutting, however!

Programming Your Eggtimer

Before you attempt to mount the Eggtimer in a rocket or fly it, you will need to know how the
programming screens work. The Eggtimer can be programmed from just about any USB­compatible computer using the included USB-serial data cable. This cable has the interface
circuitry imbedded in the USB connector, so it doesn’t use up any board space or power on the
Eggtimer. It uses a Prolific PL-2303 serial interface chip, which is recognized as a native serial
port on Windows XP SP3, Windows Vista, and Windows 7, Windows 8, as well as by Mac OS
X and many Linux distributions. The interface is a simple text terminal, VT100/ANSI
compatible. No fancy GUI graphics, but it’s fast and easy to use, and free terminal emulator
software is readily available for every OS known to man. We like TeraTerm and PuTTY, but
any emulator will do; however, to download flight data you need to be able to “capture” the data
stream to a file, so make sure your terminal program supports this. If it doesn’t, you can get
around it by doing a Copy of the text that gets output to your screen, and doing a Paste into a text
editor such as Notepad.

If you do not already have a suitable terminal program, or you need the USB-Serial driver for the
cable, go to www.eggtimerrocketry.com, click on the Links tab, and you will be directed to a
suitable download location. Note that we try to support non-Windows operating systems since
the Eggtimer is OS-agnostic, however we’re Windows geeks so we can’t guarantee that we’ve
had a chance to try every possible configuration (especially with Linux!)

Before you try flying your Eggtimer, we recommend that you play around with the programming
screens on your workbench, until you are comfortable with it. You can’t hurt anything, and if
you totally mess up the memory you can always do a Master Reset to clear out all of your
experimental values.

To get to the Eggtimer menu screen, be sure that the power on the Eggtimer is OFF. Connect the
GND, RX, and TX connectors of the USB-serial cable to your Eggtimer, being careful to match
them up to the terminals so marked, then plug the USB end into a USB port on your computer.
Launch your terminal emulator program, configure the serial port for 19,200 baud, and connect
to the serial port. Power up your Eggtimer, and you will see this screen. You will also hear a
beep-beep sound to let you know that you’re in programming mode. When you do, hit the Enter
key a few times, this will let the Eggtimer know that you have a terminal connected and you’re
not ready to start a flight. Note: Be sure to hit Enter within 30 seconds of the beep-beeps, or it
will go into flight mode and you’ll have to reset it again to get to the programming screen.

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Eggtimer Build_1.48b Flight Settings
T:23.50
A 100 LDA <50-500 by 50> P:97804.00
B 10 Ascent Samp/sec: <4,5,8,10,15,20,25,33> A:975.39
100 Interval (ms) Main:0
C 2 Descent Samples/sec <1,2,4,5,10> CHB:0
500 Interval (ms) Batt:3.12
D 0 Main Mode <0-Off, 1-Igniter, 2-Servo CCW, 3-Servo CW>
E 300 Main Altitude <100-2000 by 50, 0 @ Apogee>
F 1 Main ON-Time <1-9,0-Cont.>
G 0 CHB Mode <0-Off, 1-Igniter, 2-Servo CCW, 3-Servo CW,
4-Airstart, 4-Airstart w/o Breakwire,6-Delay>
H 1 CHB ON-Time <1-9,0-Cont.>
I 2000 Burn Timer <100-20000 ms by 100>
J 0 Airstart/Delay <0-9900 ms by 100>
K 0 Airstart Min Velocity <0-700 by 50>
L 0 Telemetry

O Comments:

Options: Esc, A-O, ?

Options are selected by hitting the letter in front of the value (A-O), then using the + and – keys to
toggle the values up and down by the increment value (for example, 50 for the LDA setting, 1 for
most other settings). If the numeric value can be above 10, you can step up by 10 times the
increment value by hitting the “>” key. Similarly, you can step down by 10 times the increment
value by hitting the “<” key. The Esc key (usually in the upper left corner of your keyboard) exits
the screen and saves the flight settings, you will hear a long beep that confirms the save and then the
Eggtimer will halt (both lights will blink continuously). The ? key allows you to download saved
flight data (more on that later). Finally, there are a few “hidden” keys for special functions, such as
erasing all the flight memory and setting the flight settings to defaults, these are covered in the
Appendix.

Note that if you do not hit ANY keys or press the button down to select a pre-programmed flight
profile, the Eggtimer assumes that you don’t have a PC connected and you are ready to start a flight.
The beep-beep that tells you that you are in programming mode lasts for 30 seconds, then the
Eggtimer begins the flight sequence. For this reason, if you ARE programming it you should hit
Enter a few times to let it know that you have a terminal connection; once you do, it will terminate
the flight sequence and save your settings and halt after you are done programming.

Note: When you first get your Eggtimer, the flight memory is NOT initialized.
Please see Appendix A for instructions on how to clear the flight memory by
performing a Master Reset. You will need to do this before you can program or fly
the Eggtimer, or unpredictable results may follow!

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