The “Micro-DTMF Touch Tone Receiver/Decoder”
board by Byonics (see Resources sidebar) converts the
DTMF signal to four-bit binary coded decimal (BCD),
which can be read by the microcontroller through header
J20. The microcontroller can thus respond to any of up to
16 remote commands from the ground.
For example, the payload could be cut free from the
balloon at any time, or a sampling device turned on, or a
sensor activated. We are looking forward to testing this
Also near the center of the board is a piezoelectric
buzzer. The buzzer provides a “beep” when the
microcontroller is powered up. It also provides a
continuous warning tone several seconds before the line
cutter relay is activated.
Along with the red warning LED, the buzzer has
captured our attention moments before launch,
prompting us to quickly power-down before we were left
holding the payload with no balloon. The microcontroller
can also be programmed to sense a landing, and make
the buzzer chirp periodically to assist with finding the
payload in tall grass or dense woods.
An SD card socket is located along the upper right
edge of the sensor board. This SMD gave students an
extra soldering challenge. We use a 2 GB SD card,
providing plenty of capacity for data files. Data is written
to the card in a .csv format that is easily read by your
favorite spreadsheet program.
High Current Driver
The board includes a high current driver consisting of
a TIP120 transistor (a Darlington pair) driven by an
optoisolator. This circuit is intended to easily provide up
to two amperes of current. Two spring-loaded terminal
blocks are provided along the right edge of the board:
one for an independent power source and one for the
This can be used to power a motor, or Federal
Aviation Administration (FAA) required lights during a
night flight. Pads are provided at D4 if you need a flyback
diode on the load side, or just leave it out.
Pins P28 and P29 on the Propeller chip support I2C,
and are dedicated to communicating with the external
EEPROM on the microcontroller board. They can also be
used to communicate with additional I2C devices.
However, we decided to use pins P0 (SDA) and P1 (SCL)
for additional devices.
Access to these pins is provided through header J14.
Both pins are pulled up by 4. 7 KΩ resistors R3 and R17,
so all you have to do is plug in your I2C device. Keep your
Many I2C modules — including the Parallax altimeter
(#29124) — include their own pullup resistors, so resistors
R3 and R17 are not needed. Be sure to check your
device datasheets for specific requirements.
Stuffing the Board
Stuffing the sensor board is easy. Simply follow the
silkscreen labels and Parts List. Most components are
through-hole. Start with the low profile components
including the surface-mount resistors, then work up. An
inexperienced student will probably need about six hours
to complete the build. As with the microcontroller board,
clean the completed sensor board and apply conformal
All of the pin headers described are the polarized
locking type (we used Molex KK™brand) to provide
reliability and insure correct polarity.
Until Next Time
That wraps up the overview of the hardware. Every
time I look at the schematic and the PCB layout, I think of
things I want to add, delete, or revise. I am sure you can
The open-ended nature of these projects is one of the
things that makes them so interesting. Next month, we will
introduce the program that puts the hardware to work.
34 June 2017
Many sensors including
thermistors and the TMP36
GPS5HAOEM GPS module
Micro-DTMF Touch Tone
PCB design tools and