DigiPi Hotspot — Vol 3: Hardware & Interface Builds

3.1 About this volume

The DigiPi software is settled — you flash a card and boot it. The interesting engineering decision is how you connect that Pi to your radio, and that decision splits cleanly into two paths depending on one question: does your radio have a USB port that carries audio and CAT? If yes, the job is almost nothing — a cable. If no, you need an interface board to bridge the Pi’s audio and a GPIO PTT line to your radio’s mic, speaker, and PTT connections. This volume walks both paths, lays out the interface-board options with rough prices and tradeoffs, explains the signals any interface must carry, and covers the supporting bits: radio choice, optional displays, power, enclosure, and flashing the card.

Every price here is approximate and drifts — components get discontinued, restocked, and repriced constantly. Prices are flagged TBD — verify against craiger.org and the vendors, and you should treat them as ballpark, not gospel.

3.2 What an interface actually has to do

Before the shopping list, understand the problem, because it makes every board choice obvious. A digital-mode interface between a computer and a radio must convey, at minimum, three signals — and ideally a fourth:

Receive audio: radio → Pi. The radio’s received audio (from its speaker/headphone or a dedicated data jack) has to reach the Pi’s sound input so the software modem can decode it.
Transmit audio: Pi → radio. The Pi’s generated tones (FT8, APRS packet, PSK31…) have to reach the radio’s microphone/data input so they get transmitted.
PTT keying. Something has to tell the radio when to transmit — to key the transmitter for the duration of a transmission and unkey it after. On DigiPi this is done properly, by a control line (a GPIO pin or a serial handshake line driving a switch across the radio’s PTT), not by sound-activated switching.
(Optional) CAT control. Frequency and mode control — reading and setting the radio — over a serial/USB channel. Not strictly required for FM packet/APRS, but very desirable on HF.

3.2.1 Why VOX is a poor substitute

You can avoid a PTT wire by using VOX — the radio’s voice-operated-switch, which keys the transmitter whenever it hears audio. It is tempting because it needs no control line. It is also a bad idea for serious digital operation, for concrete reasons:

Timing slop. VOX has attack and hang delays. It keys slightly late (clipping the start of a packet or an FT8 transmission) and unkeys slightly late (holding the transmitter up after the data ends). For time-critical modes like FT8 and packet, that slop causes missed decodes and wasted airtime.
False triggering and no clean control. Any stray audio can key the transmitter, and the software has no positive confirmation the radio is actually transmitting.
It cannot do CAT. VOX solves only the keying half of one signal; it does nothing for frequency/mode control.

A real PTT line — GPIO through a small switch, or CAT-keyed over USB — is deterministic: the software decides exactly when the transmitter is on. That is why every path below provides genuine PTT, and why VOX is the fallback of last resort.

3.3 Type A: USB-CAT rigs — a Pi and a cable

The easy path. Modern transceivers with a built-in USB interface — Icom IC-7300, Icom IC-705, Yaesu FT-991/FT-991A, and Xiegu rigs such as the X6100 — carry everything over a single USB cable: receive audio, transmit audio, CAT frequency/mode control, and PTT keying. The radio presents itself to the Pi as a USB sound card plus a USB serial port, and DigiPi’s rigctld/Hamlib layer drives the CAT side.

For these rigs the “interface build” is: plug the Pi’s USB into the radio. No hat, no sound card, no PTT circuit, no soldering. This is the configuration to aim for if you are buying a radio with DigiPi in mind and HF data (FT8, JS8Call, ARDOP-Winlink) is your goal — the whole station reduces to a Pi, a USB cable, and the rig.

TBD — verify. Confirm the exact USB behavior per radio model (which enumerate as a single composite USB audio+serial device vs needing a separate CAT cable), and any per-rig quirks or settings, against craiger.org and each radio’s manual. Described generally; not bench-confirmed.

3.4 Type B: non-USB rigs — add an interface board

The other path covers everything without USB CAT: FM handhelds (Baofeng and the like, e.g. the F8HP), Yaesu FTM-400/FTM-300/FTM-200 and other dual-band mobiles, Retevis radios, and older HF rigs with only a mic/accessory jack. These need a board that provides a sound-card front-end and a PTT switch wired to the radio’s connectors. Below are the mainstream options DigiPi supports, roughly from most turnkey to most DIY.

3.4.1 Masters Communications DRA-Pi-Zero (~$75) — the clean solution

The Masters Communications DRA-Pi-Zero is purpose-built for this project: a board designed for the DigiPi image that mounts directly on the Raspberry Pi Zero’s 40-pin header. It integrates the sound-card audio (in and out) and the PTT keying into one tidy assembly that stacks on the Pi, so the whole DigiPi-plus-interface becomes a single small sandwich with no loose modules or flying leads. If you want the least-fuss, best-integrated Type-B build and do not mind paying for it, this is the one to reach for.

TBD — verify. Confirm the DRA-Pi-Zero price (~$75), current availability, exact model/variant compatible with the DigiPi image, and which radio-side cables/connectors it needs, against craiger.org and Masters Communications. Price/model drift likely.

3.4.2 DigiPi Hat (~$40)

The DigiPi Hat is a lower-cost purpose-built board for the project — the same idea (integrated audio + PTT on a hat) at a lower price point than the DRA-Pi-Zero. It is a reasonable middle ground for a clean build on a budget.

TBD — verify. Confirm the DigiPi Hat’s price (~$40), where it is sold, its exact feature set (does it include CAT, what PTT method), and how it differs from the DRA-Pi-Zero, against craiger.org. Described from the fact sheet; not bench-confirmed.

3.4.3 Fe-Pi Audio Z v2 sound card (~$24–34) + a PTT solution

The most modular — and often cheapest — approach separates the two jobs. The Fe-Pi Audio Z v2 is a small, well-regarded I2S sound-card hat (~$24–34) that gives the Pi clean audio in and out. But a sound card alone does not key the radio — you still need PTT, and the Fe-Pi is paired with one of two options:

N7EBB RIB (~$25). A Radio Interface Box — a ready-made PTT interface that provides isolated keying (typically an opto-isolated switch) between the Pi’s control line and the radio’s PTT. Buy-it-and-wire-it, no soldering of active parts.
A DIY PTT circuit. For a few dollars in parts you can build the PTT switch yourself: a 2N7000 N-channel MOSFET and a 100 kΩ resistor. The Pi’s GPIO pin drives the FET’s gate (through the resistor), and the FET switches the radio’s PTT line to ground when commanded. When the GPIO goes high, the MOSFET conducts and pulls PTT low, keying the transmitter; when it goes low, the FET turns off and the radio unkeys. It is about as simple as an interface gets — one transistor, one resistor — and it is a genuinely good little learning build. (Add the radio-side connector and, for HF, watch grounding/isolation.)

This Fe-Pi + PTT route is the sweet spot for the hands-on builder: cheap, understood down to the component, and flexible about which radio it drives.

TBD — verify. Confirm the Fe-Pi Audio Z v2 price (~~$24–34) and DigiPi compatibility, the N7EBB RIB price (~~$25) and where to buy it, and the exact DIY PTT wiring (2N7000 pinout, GPIO pin used, resistor value/placement, whether a flyback/isolation is recommended) against craiger.org’s build notes. Circuit is described in principle; confirm the reference schematic before soldering.

3.4.4 Digirig Mobile + cable

The Digirig Mobile is a popular, compact, commercial radio interface (sound + CAT/PTT in a tiny puck) that pairs with a radio-specific cable. It is not a Pi hat — it connects over USB — but it is a proven, well-supported interface many operators already own, and it works with DigiPi as the audio/PTT front-end for a non-USB radio. The cost is one more small box and a matched cable rather than an integrated hat.

TBD — verify. Confirm Digirig Mobile compatibility with the current DigiPi image, which radio cable you need for your rig, and pricing, against craiger.org and digirig.net. Described from general use; not bench-confirmed.

3.4.5 AIOC “All-In-One-Cable” (~$25) — for Baofeng/Kenwood HTs

The AIOC (All-In-One-Cable) is an open-hardware USB gadget that plugs into a Baofeng/Kenwood-style (K1) two-pin HT jack and presents a USB sound card plus a serial PTT — the entire interface for a cheap HT in a single small inline module, around $25. For the “I have a $25 Baofeng and want to try VHF packet/APRS/Winlink” operator, the AIOC is the minimal-cost, minimal-clutter answer: HT, AIOC, Pi. It is USB-connected (like the Digirig), so it is not a hat, but it is the least you can spend to get a proper (non-VOX) interface onto a handheld.

TBD — verify. Confirm the AIOC price (~$25), that its connector matches your specific HT, its DigiPi compatibility, and any firmware/config needed, against craiger.org and the AIOC project. Described from the fact sheet.

3.5 Choosing the radio

The interface is half the decision; the radio is the other half, and it follows from which modes you care about:

A cheap FM HT or mobile — for VHF/UHF data. For APRS, packet, and VHF Winlink, an inexpensive FM radio is all you need. A Baofeng F8HP with an AIOC, or a Yaesu FTM-series mobile with an interface board, covers the local data modes cheaply. This is the low-cost on-ramp to DigiPi.
An HF SSB rig — for the weak-signal and HF-Winlink modes. For FT8, JS8Call, and ARDOP-Winlink, you need an SSB transceiver on HF. If it is a USB-CAT rig (IC-7300, IC-705, Xiegu X6100), you are back on the easy Type-A path — a single cable — and you get CAT control for free. This is the configuration most HF-focused DigiPi users aim for.

Many operators end up with both over time: a handheld for local APRS/packet and a USB HF rig for the weak-signal bands, with DigiPi happy to drive either.

3.6 Optional GPIO displays

DigiPi runs headless, but you can add a small GPIO-attached display if you want at-a-glance status without opening a browser:

Adafruit 1.3” TFT (~$16) — a small, cheap color TFT that mounts on the GPIO header.
2.8” ILI9341 (~$45) — a larger display for more on-screen information.

These are conveniences, not requirements — the browser remains the real interface. Note the Vol 1 caveat that the experimental HDMI display feature is a separate thing and is not compatible with the Pi Zero 2 W; these small GPIO/SPI panels are the display path that suits the Zero 2 W.

TBD — verify. Confirm the display prices (Adafruit 1.3” TFT ~$16, 2.8” ILI9341 ~$45), that both are supported by the current DigiPi GPIO code, and exactly what each shows, against craiger.org. Described from the fact sheet.

3.7 Power and enclosure

DigiPi’s power needs are modest — it is a Raspberry Pi, powered from USB (5 V). The Pi Zero 2 W in particular draws little, which is why it suits battery-backed go-kits: a USB power bank or a small 12 V-to-5 V (USB) converter runs it comfortably for extended periods. Budget headroom on your supply for the radio’s own transmit current, which dwarfs the Pi’s draw.

For an enclosure, anything that houses a Pi (plus a stacked hat, if you use one) works; the DRA-Pi-Zero/DigiPi-Hat sandwich is compact enough for a small printed or off-the-shelf case. The design intent is a tidy, semi-permanent little box that lives with the radio rather than a laptop you set up each session.

TBD — verify. Confirm any specific recommended power supply, current draw figures, and enclosure suggestions against craiger.org. Described from general Raspberry Pi practice.

3.8 Flashing the SD image

Finally, getting the software onto a card. The process is standard Raspberry Pi imaging:

Get the image — download the ready-to-flash DigiPi SD image. As covered in Vol 1, the packaged image is Patreon-gated from $1 (the underlying software is open source on GitHub).
Unzip it — the download is compressed; extract the image file first.
Flash a card — write it to a ~8 GB (or larger) microSD card using Raspberry Pi Imager or balenaEtcher. Use the “use custom image” option and point it at the unzipped file; do not apply the imager’s OS-customization settings, since DigiPi’s own first-boot flow handles configuration.
Boot and configure — insert the card, power the Pi, and follow the Wi-Fi-join and Initialize flow from Vol 1 (and detailed in Vol 4): join SSID DigiPi, hand it your home Wi-Fi, then set callsign and grid on the Initialize page.

TBD — verify. Confirm the current minimum SD-card size (fact sheet says ~8 GB), the exact download/unzip steps, and whether Raspberry Pi Imager’s customization should be left untouched, against craiger.org. Described from general practice plus the fact sheet.

3.9 Choosing among the interface options

To distill the Type-B decision into plain guidance:

Want the tidiest, no-fuss build and don’t mind the cost? The DRA-Pi-Zero (~$75) stacks on the Pi Zero, is built for the image, and leaves nothing dangling. It is the “buy it and be done” choice.
Want purpose-built but cheaper? The DigiPi Hat (~$40) is the same integrated idea at a lower price.
A cheap Baofeng-class HT and minimal spend? The AIOC (~$25) is the whole interface in one cable — the lowest-clutter way onto an HT.
Already own a Digirig, or want a proven USB puck? The Digirig Mobile + cable works as the front end for a non-USB radio.
A hands-on builder who wants to understand every part? The Fe-Pi Audio Z v2 (~$24–34) plus either the N7EBB RIB (~$25) or the 2N7000/100 kΩ DIY PTT is the most educational and often the cheapest, at the cost of a little wiring.

There is no single right answer — it is a tradeoff of money, integration, and how much you enjoy building. What every option shares is that it provides real audio in both directions and a real PTT line, which is the whole point.

3.10 Grounding, isolation, and RF-in-the-shack

One hardware caution that applies to any Type-B build, especially on HF: RF getting back into your interface is a real and common failure mode. When you transmit, stray RF can couple into the audio and control lines, causing erratic PTT, garbled transmit audio, or a Pi that locks up mid-transmission. The purpose-built boards (DRA-Pi-Zero, DigiPi Hat) and the RIB generally include some isolation and filtering to mitigate this; a bare DIY sound-card-plus-MOSFET build is the most exposed. If you chase intermittent gremlins that only appear on transmit — and only on certain bands or power levels — suspect RF ingress first: add ferrite chokes on the audio and USB leads, keep leads short, and ensure a solid station ground. This is ordinary digital-mode station hygiene, not specific to DigiPi, but it bites new builders often enough to name.

TBD — verify. Confirm which of the interface options include galvanic isolation (transformer/opto) on audio and PTT, and any RF-mitigation guidance, against craiger.org and each vendor. Described from general digital-mode practice.

With the card flashed and the radio wired, you have a complete DigiPi station. Vol 4 collects the reference material — modes, interfaces, default access, glossary, sources, and a single consolidated checklist of every TBD item flagged across these volumes.