I've been doing a lot of SOTA activations with Raspberry PI like SBCs, and there where always issues, like RF interference, and power supply shorts. I started to add things like galvanically decoupled audio interface, a step down power supply. It was working, but it looked like a mess.
This piece of hardware integrates all needed equipment in one nice hat. It is designed to put the RPI in a metallic box to further isolate from RF signals.
It was designed using KiCad.
- HF/VHF digital modes (PSK, FT8, etc)
- HF/VHF AX.25 based modes (Packet, APRS, etc)
- CW keying
- Clock synchronization
- Position tracking
- 3A power supply with wide input voltage range: 8 to 35V
- Ublox MAX-8/10 GPS receiver for position and time synchronization
- Inductive audio couplers to minimize GND loops. Up to 4kHz.
- PWM controlled cooling fan
- GPIO controlled PTT from the PI
- GPIO controlled CW key from the PI
- RTC
- Shutdown button
- Run LED
GPS antenna is not integrated in the device, so an external antenna connector is mounted on the PCB.
Ufortunately, the sound card of the PI can not record audio, so an external sound card must be used. I use this particular device with good results. I usually break the plastic housing, and solder a shielded cable directly to the audio interface (J109).
The GPS is needed for APRS and time synchronization. There is an onbard RTC to have some redundancy.
There are a few components that mounted or unmounted depending on particular application.
When this device is used with a handheld radio, that has a PTT and mic input (Like the FT-4X), mount C108 and R116. Please note that the value of R116 might vary based on the actual transceiver used.
If used with a radio that has separate input for audio and PTT, mount a 0 Ohm resistor to C108. You might also leave C108 as a capacitor (22uF), but it is not really needed.
C108 and R116 is placed on the layout that it can be mounted or replaced after everything else is mounted too. All you need is the dismount tha cooling fan.
If you want to have direct access to the audio interface of your radio, you can leave T101 and T102 unpopulated and short JP102 and JP103, JP104. Please note that you will loose GND separation as well.
It is designed to fit on the Raspberry PI 4. Be aware, that if the heatsink on the CPU is too high, it might conflict with the screws of the fan. If that happens, mount the heatsink 45 degrees.
A box is designed in FreeCAD using sheets of PCB. The idea is to use bare, one sided PCBs, and solder the plates from the inside. The sheets can be easily milled by a CNC machine.
There are several cutouts on the box.
In the front, you have the power inlet, and a second cutout for powering the attached LCD screen. On the right side of the front sheet, there is a cutout for the power chord to the GPS antenna, if needed.
On the right, there are anouther cutout for the audio cable.
The assembly does not include some (all) the models, hence they might violate copyright.
The hat comes with a DB9 connector to connect up your rig. I wanted to have a screw lock connector, and this was the best choice. The pin out looks like this.
| Signal name | Pin of the DB9 | Comment |
|---|---|---|
| TX audio | 1 | Audio input of your rig |
| KEY | 2 | CW key of the rig |
| PTT | 3 | PTT input of your rig |
| NC | 4 | Not connected |
| RX audio | 5 | Audio output of your rig |
| GND | 6 | Audio ground |
| GND | 7 | Audio ground |
| GND | 8 | Audio ground |
| GND | 9 | Audio ground |
| Signal name | GPIO | Commnet |
|---|---|---|
| PTT | 12 | |
| KEY | 21 | |
| DCD | 16 |
J105 provides connection to the soundcard.
| Signal name | Pin of J105 | Comment |
|---|---|---|
| RX audio | 1 | Mic input of your soundcard |
| TX audio | 4 | Speaker output of your soundcard |
| GND | 2,3 | Audio ground |
J101 provides power connection. The maximum input voltage is 35Volts.
Use u-blox MAX-8C/Q or MAX-M10S GNSS module. When the MAX-10 is used, don't populate R113.
Connect your passive or active GPS antenna to J104. Use 3.3V active antenna. PPS signal is provided on GPIO18.
When passive antenna is used, don't populate L102.
Here is an excellent article how to set up GPIO-PPS for ntpd.
BQ32000 based precision RTC is provided for backup if the GPS can't provide accurate timing.
sudo raspi-configNavigate to:
Interfacing Options → I2C → Enable
Reboot the Raspberry Pi:
sudo rebootsudo apt install i2c-tools
i2cdetect -y 1You should see the RTC at 0x68.
sudo modprobe rtc-bq32k
echo bq32000 0x68 | sudo tee /sys/class/i2c-dev/i2c-1/device/new_device
# This next one line only needed to set initial time of the RTC
# Otherwise it'll fail to read
sudo hwclock -w
# Test to read the RTC
sudo hwclock -rEdit /boot/firmware/config.txt:
Add the following line:
dtoverlay=i2c-rtc,bq32000
Save and reboot:
sudo rebootsudo systemctl disable fake-hwclock
sudo systemctl stop fake-hwclockEdit /lib/udev/hwclock-set and comment out:
#if [ -e /run/systemd/system ] ; then
# exit 0
#fiSet RTC from system time:
sudo hwclock -wSet system time from RTC:
sudo hwclock -sCheck that the RTC retains the time after reboot:
sudo hwclock -rConnect a 3V battery to BT101 for offline RTC operation.
PWM shall be enabled on GPIO13.
If you don't want PWM control, short JP101, and the fan will spin at full power all times.
There's a non-PWM control of the fan that works out of the box using the Raspberry PI OS.
Open raspi-config, under Performance Options, you shall select the desired temperature, and the GPIO pin which is 13.
To make it work, you have to add these lines to /boot/firmware/config,txt
dtoverlay=gpio-shutdown,gpio_pin=26
Ther's also a LED (D107 )that shines when the system is running. When it Doesn't, it is safe to remove power from the RPI.
To make this work, add this too:
dtoverlay=gpio-poweroff,gpiopin=7,active_low=1
These items needed for the hat for full functionality.
All sourced from Farnell, unless othervise noted. (Sorry, I'm Hungary based.)
| Item | Source designator | Comment |
|---|---|---|
| RTC Battery | 1892670 | The idea is to use heatshrink tube and some wires. |
| RTC Battery holder | 3583203 | External battery holder for CR2032 |
| Heatsink | 1211701 or https://www.alza.hu/raspberry-pi-rb-hutoborda-d4147799.htm?o=21 | |
| Standoff | 1466844 | It must be 12mm in length |
| Mounting screws M2.5x6 | ||
| GPS antenna | This or this | Use 3V active antenna |
| RPI4 | 3369503 (8GB) or 3051887 (4GB) | |
| USB audio interface | Alza | Any other do the job. Check Linux driver availability. |
| Monitor | Ebay | |
| Keyboard | https://www.pcland.hu/genius-luxemate-100-usb-fekete-hun-billentyuzet-31300725113 | Any USB Keyboard. I use the one without keypad, for its compact size. |
| Mice | Any USB, or BLE mice | |
| SD card with Raspbian image. | ||
| uHDMI to HDMI cable | Alza | |
| DB9 Male connector | To connect to your radio | |
| Data connector for your radio | ||
| Shielded audio cable | To connect the soundcard to the hat, and for the DB9 connector |