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)

• 3A power supply with wide input voltage range: 8 to 35V
• Ublox MAX-8 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

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.

Grab the schematic for a quick look.

BoM can be found here.

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.

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.

PTT is connected to GPIO17. It is buffered, and logic one means transmit.

KEY is connected to GPIO21 of the PI.

J109 provides connection to the soundcard.

There are two potentiometers (RV101 and RV102) to adjust sound level in RX and TX direction respectively. Adjust TX and RX audio levels to a certain point where you are not over driving either input. If you are using it for HF, monitor your ALC, and make sure it is not active.

J101 provides power connection. The maximum input voltage is 35Volts.

Connect your passive or active GPS antenna to J104. Use 3.3V active antenna. PPS signal is provided on GPIO18.

Here is an excellent article how to set up GPIO-PPS for ntpd.

DS3231 based precision RTC is provided for backup if the GPS can't provide accurate timing. It has internal temperature compensated crystal oscillator.

A nice tutorial how to set up the RTC on an RPI.

Connect 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.

These items needed for the hat for full functionality.

All sourced from Farnell, unless othervise noted. (Sorry, I'm Hungary based.)