QRP U4B Ultimate4 Balloon Tracker Instruction Manual
- June 5, 2024
- QRP
Table of Contents
QRP U4B Ultimate4 Balloon Tracker
Introduction
The U4B tracker was developed over a period of 7 years from 2015 to 2022, in collaboration with Dave VE3KCL who has launched 83 test flights from Toronto, Canada. With flight duration from 2 hours to 305 days (10 months, almost 17 laps around planet Earth), they all taught us something and were great fun. U4B is designed to be an easy to use, lightweight, low-cost module that can be configured as simply as entering your callsign, yet for more advanced owners can be flexibly extended with more sensors and as much complexity as you like. Automated tracking maps and utilities are available on the QRP Labs website, both for simple tracking purposes and for downloading your own telemetry.
The U4B PCB contains
- 33.0 x 12.7mm PCB (plus removable protrusion with micro-USB connector)
- Weight: 1.8g (with micro-USB protrusion removed)
- 32-bit ARM microcontroller running QDOS (QRP Labs Disk Operating System)
- 128K disk (implemented on EEPROM chip)
- 27mW (approximately) transmitter using Si5351A synthesizer
- TCXO referenced frequency stability
- Band coverage 2200m to 2m
- LM75 temperature sensor
- Status LED
- USB interface for configuration, programming and easy firmware update (just copy the new firmware file into the apparent USB Flash drive).
Simplest possible operation
Just connect to U4B with a PC terminal emulator, and configure it with your callsign. Register the flight name, details and channel on the QRP Labs website. Fly!
More flexible and advanced features
U4B contains a wealth of flexibility and hardware expansion options that you can use to customize your flight:
- 19 GPIO pins – of which 9 can be configured as analog inputs and 8 are easily accessible via PCB edge pads; all 19 can be used as digital input or output control pins
- I2C bus for connecting additional sensors e.g. pressure, humidity
- A BASIC programming language with a full-screen text editor, compiler and debugger
- 128K Disk storage for your programs and data; BASIC can read/write data files
- Command-line utility
- Telemetry over WSPR for relaying your additional sensor data
The U4B radio transmitter can transmit the following modes:
- QRP Labs tracking and telemetry over WSPR
- WSPR (including extended mode and slow 15-minute WSPR)
- JT9 (1, 2, 5, 10, 30 minutes)
- JT65 (modes A, B, C)
- Hellshreiber (standard, DX, and slow multi-tone FSK)
- CW (standard speed, QRSS, FSKCW and DFCW)
- Customized “Glyph” patterns can produce a unique idenfier on QRSS
SAFETY FIRST
- If you are anyone else gets seriously injured (or worse) that’s seriously going to take the fun out of the whole thing. So pay massive amounts of attention to safety, PLEASE…
- Launch in a wide flat area: First off, the best place to launch is a flat area, a WIDE flat area, with no obstacles for hundreds of meters in any direction; particularly obstacles involving power lines. A balloon can often travel along for a while almost horizontally in the slightest breeze you don’t even realize exists, so don’t assume anything is going straight up. Again, nowhere near or even in sight of power lines!
- Determine wind direction: Ideal launch conditions are zero wind. But recognizing that rarely does one have such a luxury, it is necessary before everything, to determine the wind direction; then choose the launch site such that the balloon has the longest clear space to ascend into.
- Launch from ground level: Don’t be tempted to launch from a roof, or a balcony, even if it seems like a good idea (maybe you think it will get you higher than nearby obstacles). In the heat of the moment, your attention could be so focused on getting your precious dream airborne that you’ll take unreasonable risks, balancing precariously near the edge just to dislodge it or stop that last bit of antenna catching on something… and before you know it, you’ll fall off to your doom. Better to stay on the ground, do a bit of extra thinking to find a suitable launch site.
- Mind hydrogen: remember, Helium is a non-explosive, non-flammable gas. Hydrogen has greater lift, is cheaper, a renewable resource, and leaks more slowly – but is explosive. So if you decide to use hydrogen, do some proper research about how to handle it properly, at the minimum following all safety instructions directed by whatever container of the stuff you’ve got hold of.
- Aircraft: An oft-repeated objection to high altitude ballooning is that it’s a danger to aircraft. Having spoken to numerous commercial and private pilots about this, I have yet to meet one who did other than laugh at the thought of something that tiny and delicate being any kind of danger to an aircraft. Modern aircraft simply have too many built-in redundant systems and safety features, over-engineering etc for such a small thing to be a hazard. Nevertheless, better not tempt fate, or risk any undue attention, by selecting a launch site anywhere near an airport. Stay even further away from any military facility.
- Rules, regulations: It’s your responsibility to find out what laws, rules, regulations etc apply in your country. QRP Labs is not going to tell you, QRP Labs doesn’t know. No idea at all. So make sure you undertake all necessary research and figure out what you can, and cannot do. Then make your own decisions accordingly. Test test test… OK this isn’t really part of safety. Well maybe it is, if you consider preserving your sanity an important goal. TEST! Test, re-test, test every possible scenario you can think of. Solar power in the dark, solar power as the sun comes up, goes down, spinning around, clouds, blah blah. Whatever you can think of. If you’re writing your own BASIC program try to think of all the possible routes the code can take and think of a way to check they all work properly. Test everything so thoroughly on the ground. There’s already enough to go wrong with the wind, clouds, tangled antennas, leaks, miscalculation of gas volume, blah blah… at least get the U4B set up correctly! Once it’s left your hands, you’re very very unlikely to ever see it again. So you had better be sure it WORKS before you let go! This includes not breaking off the USB tab until the last moment – because testing and fixing something is a whole lot harder without that.
- Common sense: This list is not exhaustive! Above all, use common sense! Best to delay a launch, and come back another day – there should be plenty – then risk injury or life.
Know your U4B
U4B is a 33.0 x 12.7mm PCB, using 0.6mm thick FR4 PCB for low weight. SMD components are mounted on both sides of the PCB
Parts list
- Part Value Package Description
- C1 10uF 0402 Capacitor
- C2 10uF 0402 Capacitor
- C3 0.1uF 0402 Capacitor
- C4 0.1uF 0402 Capacitor
- C5 0.1uF 0402 Capacitor
- C6 0.1uF 0402 Capacitor
- C7 0.1uF 0402 Capacitor
- C8 0.1uF 0402 Capacitor
- C9 0.1uF 0402 Capacitor
- C10 0.1uF 0402 Capacitor
- C11 0.047uF 0402 Capacitor
- C12 22uF 0402 Capacitor
- C13 22uF 0402 Capacitor
- C14 10uF 0402 Capacitor
- C15 0.1uF 0402 Capacitor
- D1 S4 SOD-323 Diodes
- D2 S4 SOD-323 Diodes
- D3 S4 SOD-323 Diodes
- D4 Red LED 0402 LED
- L1 2.2uH 2012 Inductor
- L2 2.2uH 2012 Inductor
- R1 1K 0402 Resistor
- R2 22K 0402 Resistor
- R3 1.5K 0402 Resistor
- R4 22K 0402 Resistor
- R5 100K 0402 Resistor
- R6 280K 0402 Resistor
- R7 22K 0402 Resistor
- R8 22K 0402 Resistor
- R9 22K 0402 Resistor
- R10 22K 0402 Resistor
- R11 1.5M 0402 Resistor
- R12 100K 0402 Resistor
- R14 100K 0402 Resistor
- R15 5.6K 0402 Resistor
- R19 1K 0402 Resistor
- T1 SI1967DH-T1-E3CT-ND SOT363 Dual MOSFET transistor
- TCXO 25MHz 3225 TCXO
- U$1 SIM28M 10x10mm SIM28M GPS
- U1 STM32F103CBT6 TQFP-48 STM32 chip
- U2 LM75 SOIC-8 I2C Temperature sensor IC
- U3 24M01 SOIC-8 128K I2C EEPROM IC
- U4 RT9013-33 SOT23-5 3.3V LDO voltage regulator
- U5 SI5351A MSOP-10 Si5351A Synthesizer
- USB MICRO-USB MICRO-USB MicroUSB
- XTAL 8MHz 3225 Crystal
Track layout
GPIO pins 8-18 pad locations
Parts layout, top side
Parts layout, bottom side
Schematic
Circuit explanation
The U4B tracker has a rather simple design and does not require highly detailed explanation.The brains of the device is an STM32F103CBT6 microcontroller. A 32-bit ARM Cortex M3processor having 128K of program memory. In the U4B, 12K is used for the bootloader and 116K for the application firmware. Although the maximum speed of the microcontroller is 72MHz, it is run at a much lower speed in this leisurely application, to reduce current consumption. There is an 8MHz crystal as the system oscillator; internally this is used to generate a 4MHz CPU clock during flight, or 18MHz when a USB connection to a host PC is present. The USB connector is a micro- USB type, located on a protrusion from the top edge of the nominally rectangular U4B PCB. This section of the PCB is a break-away tab, in other words a line of holes in-line with the top PCB edge can be snapped to remove this section for flight, thereby reducing the weight by several grams. The U4B may be powered from the USB host via a diode located on the breakaway tab. The other two components present on the breakaway tab are the necessary 1.5K Fullspeed USB pull-up resistor R3, and a 22K resistor R7 which the microcontroller uses to “sense” when a USB connection is present, and enable its USB peripheral accordingly. The Si5351A Synthesizer IC is used as the “radio transmitter”, configured by the microcontroller over the I2C bus. It can be used in two ways, under firmware control.
- Low power mode: Clk0 (connected to output pad RF_1 via a capacitor) is driven at the operating frequency, Clk1 (connected to output pad RF_2) is grounded. Power output into a 50-ohm load is found to be approximately 9mW.
- High power mode: Clk0 and Clk1 are driven at the operating frequency in anti-phase (180- degree phase difference). The power output into a 50-ohm load across RF_1 and RF_2 is found to be approximately 27mW
If Clk0 and Clk1 are connected to opposing legs of a dipole antenna, the power
output can be conveniently controlled by BASIC commands, set to low or high
power mode. There is no Low Pass Filter in the U4B, which has the advantage of
allowing it to operate on any band; but the disadvantage that there is no
harmonic attenuation; however given the very low power output this is not
normally considered a problem. If it troubles you, you could add an external
filter on a small PCB with appropriate SMD capacitors and inductors. The 22K
resistors R2 and R4 provide some protection against static build-up on the
antenna during flight. The synthesizer is powered via half of a dual P-channel
MOSFET T1, under control of the microcontroller via the SW_SYNTH signal.
Similarly, the SIM28M GPS is powered by the other half of the P-channel MOSFET
under microcontroller control. In use, the microcontroller ensures that the
GPS and Synthesizer are never both powered on at the same time; this limits
the maximum current consumption of the device, easing the design of the power
supply (normally, solar panels), particularly during early morning or late
afternoon operation when the sun is likely to have a low angle of incidence on
the solar panels if a horizontal panel arrangement is used. The
microcontroller receives serial data from the SIM28M GPS receiver module via a
USART (serial) connection at the default 9600-baud data rate of the GPS
module. Both transmit and receive serial data signals are implemented so that
the microcontroller can write commands to the
GPS; in particular this is necessary to be able put the GPS into “balloon
mode” with the appropriate command.
Power to the GPS receiver is filtered by the 2.2uH inductor L2 to try to keep
microcontroller digital noise out of the GPS receiver circuits. Power to the
microcontroller Analog Supply pin is similarly filtered by L1. The battery
backup connection to the GPS is always powered via diode D2, even when the GPS
main power is switched off. This is essential to retain the GPS memory so that
at next power-up a “warm start” is available, which has a very quick satellite
acquisition time. In addition to the Si5351A Synthesizer chip on the I2C bus,
two other devices share are connected
to the I2C bus: the LM75 temperature sensor IC U2, and the 24M01 128K Serial
EEPROM IC U3. These latter two devices are kept powered on at all times.
7-bit I2C addresses used by devices in the U4B are
- 0x48 U2, LM75 Temperature sensor IC
- 0x50 U3, 24M01 128K Serial EEPROM IC
- 0x60 U5, Si5351A Synthesizer IC
It is essential that if additional devices are connected to the I2C bus, you
must ensure that they do not use these three I2C addresses. These devices may
be read and written using BASIC commands.
There’s a 3.3V LDO voltage regulator IC, U4, RT9013-3.3 which has an enable
input, controlled via a potential divider network R5, R6, and R11. This is set
up to provide a hysteresis such that the
“turn-on” voltage is somewhat higher than the “turn-off” voltage; it avoids
rapid on/off oscillation of the power supply in low light conditions. The
SOLAR input is connected to the BATT output and
voltage regulator input, via a diode D1. This prevents reverse current into
the solar panel in the dark when a connected battery still has a charge.The
maximum rated input voltage of the RT9013-3.3 is 6.0V (absolute maximum
rating) and recommended maximum operating voltage 5.5V. Care must therefore be
taken not to use so many solar cells that 6.0 is exceeded. Remember that solar
panels get BETTER as they get colder. In full sunshine at altitude, -55C
temperatures can increase voltages and currents by up to 20%. The U4B can be
used with rechargeable batteries such as LiPo of various capacity; the higher
the battery capacity, the longer operation after dark will be possible, but
the higher capacity batteries are also heavier which will result in lower
flight altitudes. There are many trade-offs to consider. Alternatively for
very light-weight flights, a battery can be eliminated altogether; an ultra-
capacitor can be used or even nothing at all. However arguably some form of
storage is useful to avoid drop-outs in the early morning and late afternoon
if the panel is rotating and tilting. The microcontroller can read the
available battery voltage V+ on its PB1 port, via a potential divider
R8/R9. The U4B has 8 GPIO pins presented as pads along the lower edge of the
PCB on the bottom side; these are known as pins 0 to 7 in the BASIC
programming. They can be written and read as digital outputs or inputs (0V is
“0”, +3.3V is “1”); or they can be read as 12-bit analog inputs where a value
of 0 is Ground, and 4095 is +3.3V; these inputs can be used for reading analog
sensors. A further 11 GPIO pins are digital-only and known as pins 8 to 18 in
BASIC programs. Most applications will not need such a large number of GPIO
pins but if you do, you will need to solder very fine wires to very tiny SMD
pads on the top side of the board as identified in the diagram in section 5,
above
Basic flight connections
The HF antenna needs 1/4-wavelength on each leg (for example, each leg is 5m for 20m operation). A typical flight configuration has the top end of the top leg tied to the balloon, and the U4B payload suspended directly in the middle of the dipole. The GPS antenna can be made of simple #28 (0.3mm) enameled wire; a 1/4-wave at 1575Mhz is about 45mm. The feedline consists of 1-inch of twisted wire, about 6 turns works well. The above is just an example of a minimal flight tracking system. You can add your own sensors and complexity as you see fit! There are a lot more practical tips about flight system preparation and launch, in the FDIM 2021Conference proceedings article at http://qrp-labs.com/u4b
Resources
- Refer to the operating manual for details of the terminal applications (for configuration of U4B, text editing, writing BASIC programs, and testing etc.)
- Refer to other pdf publications on the U4B page on the QRP Labs website http://qrplabs.com/qdx
- The operating manual also explains how to update the U4B firmware (using a USB connection to a PC), and how to set up tracking on the QRP Labs website.
- For updates and tips relating to this kit please visit the QRP Labs QDX kit page http://qrp- labs.com/ u4b
- For any questions regarding the assembly and operation of this kit please join the QRP Labs group, see http://qrp-labs.com/group for details
Document Revision History
- 1.00 18-Apr-2021 First draft version 1.00