Here are some Frequently Asked Questions (with answers) that we often get.
What is the Eggtimer?
The Eggtimer is a barometric-based flight computer, it records the altitude/velocity during your flight at programmable intervals, and can also be used to deploy one or two parachutes. It can also be used to trigger "airstarts", for two-stage rockets or multiple-motor models. It will save up to 32 flights, and the flight data can be downloaded to a computer in .CSV format for use with Excel and many other popular graphing programs.
The Eggtimer has some features that are unique, including the ability to trigger hobby servos as well as igniters. This makes it pretty much the only turnkey solution for non-pyrotechnic deployments. There are also three additional logic-level outputs that can be used for various purposes, and an optional driver kit is available for a nominal cost that extends their output to 8A/60V. You can also stream real-time time/altitude data out the serial port during flight, so you can get basic telemetry out of it using a 3DR or similar data radio.
How much does it cost?
An Eggtimer kit is $40, INCLUDING the data cable (which many vendors charge over $30 for alone!). You can save even more by getting the TwoFer Special: TWO Eggtimer kits with ONE data cable, for $70.
Why is it so cheap compared to the "other guys"?
Well to begin with, we sell it as a kit. You have to solder about 30 parts onto the circuit board, but they're all through-hole parts; the surface-mount parts (barometric sensor and memory) come pre-mounted on the board for you. This keeps the cost down, and we can pass those savings on to you.
We also don't include screw-terminals. Instead, there are solder pads that you use to solder pigtails, so you can use whatever connectors and/or barrier strips you want. We have found that at the price of an Eggtimer, most of our customers aren't really interested in moving it from one rocket to another, so removability isn't a big issue. You save the cost of the screw-terminals, which quite frankly are fairly expensive. Another advantage is that soldered connections aren't going to work loose with G-forces or vibration.
That being said, if you DO want a removable connection option, we sell a locking header/socket kit that you can add to the Eggtimer, so that you can create a removable wiring harness. The socket connectors lock securely into the headers, so they're not going to work loose very easily. Even with that, we still recommend soldering in the battery and switch connectors... especially if you are doing deployments.
How high can I fly with it?
The Bosch sensor that we use is factory-certified to about 30,000'. This is higher than about 99% of the flights that you're likely to have. In fact, we've really only seen one flight at that altitude... a minimum-diameter N5800. It was pretty awesome...
Is the Eggtimer "mach safe"?
Yes. We use a software filter to smooth out any sudden peaks and valleys in the velocity profile, and we inhibit parachute deployments until your rocket has slowed down under 100 ft/sec for over one second. Since that's only going to happen near apogee, you can be assured that it's not going to deploy prematurely, even if there is a mach transition "pressure-spike" that may appear to be a sudden decrease in altitude.
How can you do airstarts with only a barometric sensor? I've heard you need an accelerometer...
We use a few tried-and-true mechanisms to ensure that airstarts are only going to happen at a significant altitude and well into your flight. First, your rocket has to reach the Launch Detect Altitude, typically at least 200' (although it can be set from 50' to 1200'). Secondly, there is a Burn Timer that you set to be equal to the burn time of your motor; ignition is held off until BOTH the LDA has been reached AND the Burn Timer has expired. (You can also set an additional timer to hold off the airstart for some time after the motor burns out.) Finally, you can optionally use a hardware Breakwire to ensure that the rocket has physically left the pad.
Why do you use two batteries when everybody else only uses one?
This is part of our philosophy... safety first. By using one battery for the processor side of the Eggtimer and another battery for the deployment side, this prevents a sudden drop in voltage in the deployment battery from affecting the processor. The Eggtimer uses optoisolators between the processor and the deployment circuitry, they are totally isolated... they don't even share a common ground. It is literally impossible for a deployment voltage drop to affect the processor.
If you really want to, you CAN use a single battery... we've done it many times and it works fine IF you chose the right battery and the right deployment igniters. We like 300 mAH 2S LiPo cells, and Quest Q2G2 igniters to trigger deployment charges... they have an all-fire current of only about 200 mA, so they have virtually no effect on the battery voltage. We've also used J-Tek electric matches, they work fine too.
What kind of batteries do I need to use with the Eggtimer?
The Eggtimer's processor can be used with virtually any battery over 3v. There are two build options, one for batteries 3v-4v, and another option for batteries 4.5v-30v. The difference is that with the higher voltage option you install a voltage regulator IC; with the lower voltage option, you just leave it out. We like to use 1S LiPo batteries (nominal 3.7v) for the processor, they're small and light, and a 150 mAH cell will run an Eggtimer all day and then some.
On the deployment side, you can use any battery you want, it's rated up to 8A and 60v. 9v alkaline batteries are very popular for low-current igniters, and a 2S or 3S LiPo battery will fire just about any igniter that you're likely to come across. Each deployment channel has its own battery connection, and the channels are totally independent; you can actually have one channel using a 3S LiPo (11.1v nominal) with a high-current igniter, and the other channel using a small 1S LiPo with a hobby servo.
What kind of software is included with the Eggtimer?
None, because the Eggtimer doesn't require special software. Programming is done using a simple ANSI/VT100 serial terminal program such as Hyperterm, PuTTY, and TeraTerm. You can download them for free and they're available for virtually every computing platform on Earth. The only other requirement is that you need to have the correct USB-serial driver for your platform. The cables that we ship use the Prolific PL2303-series USB-serial chips, Prolific has drivers for Windows, MacOS, Linux, and there may also be some out there for Android as well.
To display the flight graph, you only need Excel or one of the many free Excel clones such as Kingsoft Spreadsheets. The Eggtimer's flight data is output as a standard .CSV file, so it is easy to import it into virtually any graphing or data analysis program.
How do I connect the USB-Serial cable to my Eggtimer?
The connections will be:
What's with the silly name?
Well, we originally started to build a flight recorder for TARC egg-lofting flights, and eventually we ended up expanding it to include deployments, airstarts, and whatever else we could cram into the processor's flash memory. Thus, "Egg Timer..."