Eggtimer Rocketry Eggtimer User Manual page 10

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 special-
purpose 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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