Nuts and Volts - June 2017

condensation or other contacts with conductive surfaces.

After using the board several times, we began to notice that the microcontroller frequently reset. We eventually traced the problem to radio frequency (RF) interference caused by an onboard radio tracking transmitter. We found that the RF was affecting the potential of the ground plane and thereby the reset pin.

We solved the problem by rerouting several cables, and “hardening” the reset pin by soldering a 1K pullup resistor from the reset pin to 3.3V, and a 0.1 µF ceramic capacitor from the reset pin to ground on the solder side as shown in Figure 3. We have had no trouble since.

The Sensor Board

The sensor board shown was custom designed by the authors and other club members using the freeware ExpressSCH and Express PCB. They are very easy to use — students became proficient after a few hours of use with very little guidance. We designed Version 1 as a two-layer board, but have since changed to a four-layer, making the routing of traces a bit easier as well as providing a filled power and ground plane.

The dimensions of the sensor board are the same as the PPTH, so they stack neatly. The base board and sensor board both use standard 0.1” pin spacing, so you can easily plug in your own custom circuits using standard protoboard and point-to-point wiring.

The finished boards arrived in the mail within a week. Students thoroughly tested the bare boards for pad-to-pad continuity. In addition to giving students practice using a multimeter, it also detected some gaps in traces that escaped detection prior to submitting the files for manufacturing.

The original sensor board mounted on top of the microcontroller board is proudly displayed by Maria in Figure 4. Since then, many changes have been made, but the design concept remains essentially the same. A downloadable schematic and PCB (printed circuit board) layout of the latest version (Version 3) are available at the article link for your use in your own near space ballooning projects.

Now, let’s go on a tour of Version 3 of the sensor board (Figures 5 and 6).

Status LEDs

At the lower right are four light emitting diodes (LEDs), used to indicate the status of the system. From top down: • The green heartbeat LED (LED10) shows the “pulse” of the microcontroller. It toggles on-off-on each time through the program loop, telling us that the program is running. This has been very helpful in detecting radio frequency interference and battery connection problems.

• The red SD card LED (LED11) goes on if the SD card fails to mount successfully at startup.

• The red GPS fix quality LED (LED12) is on if the number of “locked-on” satellites in less than a value defined in the code. We require at least four satellites to assure good values for longitude, latitude, and altitude.

• The red mission box LED (LED13) is on when the current location is outside of an area defined in the code by north, south, east, and west, and latitude and longitude values.

Of course, we are never in the stratosphere to watch the indicator LEDs, but they are very useful during testing and just before launch.

Visibility in direct sunlight is acceptable if the LEDs are shaded. You might prefer clear ultra-bright LEDs for better visibility. To avoid cross-illumination, be sure to paint the sides of the LEDs before they are soldered in place. Current through these LEDs is limited by four 0805 SMD resistors. SMDs (surface-mount devices) give students some challenging soldering practice.