Nuts and Volts - July 2014

and exploration using BalloonSats if the flight computer is large enough.

An Extreme BalloonSat Flight Computer

As mentioned, the core of the BalloonSat Extreme is Parallax’s BASIC Stamp 2pe. The BS2pe has

16 memory banks of 2 kb each for a total of 32 kb of memory. So, there’s lots of room for programs. Since BASIC is the programming language of the BS2pe, the learning curve for new students is pretty shallow.

The power of Parallax’s version of BASIC means that in most cases, only a single 2 kb memory bank is sufficient to operate a near space mission. This leaves the remaining 30 kb of memory available for data storage. That’s enough memory for 15 separate experiments.

However, a flight computer can’t collect and store data unless it can interface with a wide variety of sensors. So the BalloonSat Extreme flight computer is designed with five external ports.

First is the flight computer’s analog port. The BS2pe doesn’t have built-in analog-to-digital conversion capability, meaning it can’t process sensor voltages directly into digital values. Therefore, it relies on a MAX186 IC to provide this service.

This ADC is pretty slick. It has eight channels and digitizes voltages with 12 bits of resolution (for a total of 1024 voltage bins). Its maximum voltage is capped to 4.096 volts, so with those 12 bits of resolution, it’s capable of measuring voltage changes as small as one millivolt.

To make interfacing analog sensors easier, each channel of the analog port provides a connection to +5V, ground, and one channel of the MAX186. All it takes to operate an analog sensor is to plug it into one channel of this port.

The second external interface is the servo port. This port has three male headers that connect servos using the standard Futaba connector.

The BalloonSat Extreme uses a separate battery pack for the servos.

The reason why is a separate battery pack prevents a stalled servo from draining the main battery and crashing the flight computer due to a low voltage brown-out.

Next is the camera port where three of the BS2pe’s I/O pins connect to relays. These relays take the place of a camera’s shutter switch, and allow the flight computer to trigger up to three cameras to take a picture. With three cameras, a BalloonSat can simultaneously record images in all directions.

The fourth port is the digital port. Five BS2pe I/O pins are assigned to this one and like the analog port, each channel in the digital port provides ground, + 5 volts, and an I/O connection to the BASIC Stamp. The digital port can operate experiments like a Geiger counter — a sensor that produces a five volt pulse every time a cosmic ray passes through.

The last port is the GPS port. This port is a male DB- 9 connector firmly bolted and soldered to the printed circuit board (PCB). It connects to one I/O pin so the BS2pe can collect data like mission time and altitude. The port uses one of the DB- 9’s pins for serial data, pin 4 for positive five volts, and pin 5

for the ground connection. Because of the connection to power, a GPS can start providing position data as soon as it’s plugged into the port.

There remains one more external connection: the commit pin. It’s the method used to start the BS2pe program operating experiments and collecting data. That way, the BalloonSat Extreme doesn’t record a significant amount of data on the ground.

Construction

There are three steps to assembling the BalloonSat Extreme: making the PCB; soldering components to the PCB; and wiring the flight computer. I’m fond of using DALPRO (now CamCon Chemical) products to make PCBs. However, you may be more familiar with other products. Whatever the method, use the bottom copper pattern available at the article link to make your PCB.