The 14-pin PIC16F688 seems almost custom
designed for an application like this. It’s physically
small (especially in its SOIC surface-mount
package) and can draw less than 1 mA from a
three volt supply, but still has 4K of program
memory, several 10-bit analog-to-digital converters
(ADC), an internal clock oscillator, and internal
pull-up resistors on port A and MCLR which help
minimize the parts count.
The schematic in Figure 3 shows how the PIC
interfaces with the rest of the components. Port A
is used for ICSP, RS-232 data output, the I2C
interface to the EEPROM and any external I2C
devices, mode selection (flight mode or download
mode), and switched power to the accelerometer
and telemetry transmitter. Port C handles analog
data collection from the accelerometer and
external sensors, as well as digital I/O. Figure 3
also includes a simple external interface circuit (J4,
J5, U4, and associated components) and external
five volt power source for use when downloading
collected data to a PC via RS-232, if your PC’s
serial port (or USB to RS-232 converter) won’t
work reliably with the three volt signal output from
the computer when using its onboard battery.
The telemetry transmitter is based on a design
by Harry Lythall SM0VPO. Inductor L1 is 6. 5 turns
of bare solid 22 gauge wire wound around a 3/16”
form with an overall length of about 1/2”. You can
stretch or compress the inductor a bit to tune the
transmitter to the design frequency of about
144 MHz, so that crystal X1 can lock in that
frequency in third overtone mode; 144 MHz is at
the bottom end of the amateur radio two-meter
band, so you can use a two-meter portable
transceiver to listen to the telemetry. Be sure to
follow Federal Communications Commission (FCC)
rules for licensing and station identification.
Alternatively, check out http://transmitters.
tripod.com/begin.htm for component values that will
yield a transmit frequency in the FM broadcast band and
some possible variations on the design.
If desired, solder an antenna wire to the inductor one
turn from the end connected to the positive supply
voltage, being careful to route the antenna away from
metal or other objects that may destabilize the transmitter.
I’ve gotten a range of several hundred feet outdoors with
a good quality receiver using no antenna wire at all.
The code uses pin RC4 to generate modulation
signals for the telemetry radio, but as shown in Figure 3
there is no direct modulation connection to the
transmitter. Close proximity of the PIC I/O line to the
transmitter provides a modest level of modulation; if
desired, you can connect RC4 via a simple resistor-capacitor interface to the base of Q1 or to an external
transmitter for more precise modulation control.
Multifunction interface J1 serves several different
purposes depending on whether you’re programming the
PIC, collecting data during flight, or downloading data to
your PC. During flight, pins 1 and 2 are jumpered to
provide power to the computer, and pins 4 and 5 are
jumpered to set the computer to flight mode.
After a flight, the jumper is removed from pins 4 and
5 to set the computer to download mode, and pins 3 and
4 provide the RS-232 data (9600 baud, 8N1) to your PC
via the external power/interface circuit described earlier.
Pins 2 through 6 of J1 provide ICSP control of the PIC,
but note that these pins are not in the standard ICSP order
(a compromise to accommodate the other functions J1
supports), so you will have to make a simple adapter cable
to your favorite ICSP programmer. The physical
arrangement of the pin functions in J1 ensures that the
computer can only be configured for one mode at a time.
January 2015 41
■ FIGURE 1. ExpressPCB MiniBoard containing two BT-5s
and one each of the BT- 20, BT- 50, and BT- 60
flight computer boards.
■ FIGURE 2. MiniBoard cut apart into individual
flight computer boards.