Custom Products for the Digital Radio Amateur Enthusiast 		  Print this Page

I2S Digital Radio Adapter Interface Board
For Digital Two-Way Radio Applications

Product Support Documentation
Model DRA-Pi-Zero


Top side of DRA-Pi-Zero circuit board - shown considerably enlarged.

Bottom side of DRA-Pi-Zero circuit board - shown considerably enlarged.

DRA-Pi-Zero with APRS® connection - shown considerably enlarged.


Schematic Image - Click to download a high quality PDF.

Overview of DRA-Pi-Zero
Things that may not be otherwise obvious:

  • Use with KM6LYW's DigiPi Software - providing a very high quality but small form factor (Pi Zero) sound card.
  • An optional (DIYmall type) mini TFT display is available optionally at purchase. Adafruit displays do not fit properly.
  • NO Case is provided - and no commercially produced case fits that we're aware of.
  • Two radio connectors are provided - a 1/8" (3.5mm) TRRS (tip - ring - ring - sleeve) and a Mini-DIN-6.
  • NO radio cables are supplied, but some are optionally available.
  • The TRRS connector is compatible with DigiRig Radio Cables - providing an easy connection to your radio.
  • Kits come with the 28 pin surface-mount IC preinstalled - but other SMT parts do exist and need installed (as small as 0805).
  • Kits are only recommended for VERY experienced builders.
  • RX audio feeds the MIC input of the chip, which by default feeds both the Left and Right channels.
  • No channel steering header is needed (or provided) for RX audio.
  • The larger pot (R12) adjusts the receive audio level - more information below.
  • The 1200 / 9600 "RX" header selects between these two receive audio baud rates (types of audio).
  • The smaller pot (R14) adjusts the transmit audio level - more information below.
  • The LEFT / RIGHT "TX" header selects which channel feeds the transmitter - no soldering required.
  • The TX audio signal is fed from the high-power headphone outputs to the TX header.
  • TX audio feeds pin 1 of the MD6 and Ring 1 (the ring closest to the tip) on the TRRS.
  • PTT is available at MD6 pin 3 and Ring 2 (the ring closest to the sleeve) on the TRRS.
  • The PTT is driven by a transistor with its bias coming from pin 32 (GPIO 12) of the 40 pin header.
  • The Red PTT LED is in parallel with the coil of the PTT relay.
  • The Blue Bluetooth LED is from from pin 29 (GPIO 5) of the 40 pin header through a current limiting resistor.
  • The Green Carrier Detect LED is driven from pin 36 of the 40 pin header through a current limiting resistor.

    Parts List:
    If you purchased our kit, refer to the parts list and make sure you have all of the components you need to build it.
    Click here for Parts List.

    Construction/Assembly:
    Click here for assembly instructions and construction notes - with large photo.

    Jumper Settings:
    Click here for Header / Jumper settings and other board connection assignments.

    Mini-DIN-6 Pinout:
    Click here for MD6 pinout in text.
    Click here for an image of the Mini-DIN-6 data jack.

    3.5MM TRRS Pinout:
    Click here for TRRS Pinout.

    Installation:
    This device is intended to be installed onto a Raspberry Pi Zero 2W (NOT included) - but will work on any Raspbery Pi with a compatible 2X20 pin header and acceptable power supply. If your Raspberry Pi has an optional heatsink on the prcessor - it will need to be carefully removed to provide the necessary clearance for the DRA-Pi-Zero to fully seat on the 2X20 header. A thermal pad of the proper thickness (not supplied or included) can be installed optionally to allow the Mini-DIN-6 radio connector to be used as a heatsink with some Raspberry Pi's.

    Compatibility:
    The DRA-Pi-Zero has been tested with DigiPi VER 1.9-1 software on a Raspberry Pi5, 4, 3, and Zero 2W, and works as intended.

    Configuration:
    Please refer to the instructions on the DigiPi website for configuration settings using this device.   See Craig's unboxing video.

    Refer to the schematic for the pinout of the MD6 and TRRS where all of the logic signals, power connections, and audio signals are listed.

    Custom TRRS radio cables are available from the Digirig Store. They have several pre-made cables that work with the DRA-Pi-Zero for many radios.

    Recommended powering requirements:
    Raspberry Pi's don't like to be under powered. We recommend a high-current capable power supply for the system. Something capable of 2 amps or more is best. Separate 3.3V powering is provided for the digital and analog sections. The board is supplied with 5 VDC from the 40 pin connection from the Raspberry Pi. Onboard linear regulation is provided for the higher current analog 3.3V loads (CODEC Analog) insuring low-noise and high signal to noise ratio.

    Recommended receive audio input level:
    The DRA-Pi-Zero accepts the widest range of audio compared to any other similar radio adapter interface. The input signal is attenuated by potentiometer R12, giving the broadest range of acceptable levels. If your receive audio level is adjustable or programmable (at the radio), we recommend around 2.0 volts P-P. This allows very good signal-to-noise ratio and low cabling cross talk. This level results in a 50% rotational setting of the potentiometer. Additional audio level setting is available from AlsaMixer - advanced Raspberry Pi operational understanding is required.

    Transmit audio output capability:
    The transmit audio output level is adjusted by the smaller pot (R14). The LEFT / RIGHT "TX" header selects which channel feeds the transmitter - no soldering is required to make the selection. The TX audio signal is fed from the high-power headphone outputs to the TX header. Maximum TX output level is 3.0V P-P with a 500 ohm load and 2.8V P-P with a 30 ohm load - TX pot wide open and Alsamixer set to "Max White" (no red). Most portable radios don't need additional AlsaMixer adjustment - the default setting of 37 is usually good for HT's. But - some radios require elevated transmit audio (many radios like Yaesu and Kenwood mobiles. These radios might require a master level setting in the 70's - especially when driving the modulator directly.

    PTT Keying:
    The DRA-Pi-Zero uses a relay for PTT switching. Some radios don't like a transistor, and the use of a relay eliminates any keying issues due to the junction loss of a transistor. FET's and other transistor switching is fast - but the reed relay we use is almost as fast. For the protocols this hardware supports - it's plenty fast enough. The 2N7000 is a popular PTT transistor but will lock up in the presence of RF (RFI). The last thing we want is for the radio interface to lock PTT "on" solid - and require a hard reboot to resolve it.
    More below.

    Red "PTT" LED:
    The Red LED indicates the successful assertion of Push To Talk (PTT). This LED is in parallel with the coil of the relay. PTT logic is driven from pin 32 of the 40 pin Raspberry Pi 2X20 header.
    Pin 32 is labeled "GPIO 12" on the official Raspberry Pi header description page. There is no provision to use a separate / different Raspberry Pi GPIO (pin / function) to illuminate the Red LED or actuate the PTT relay.

    Online ordering available from the DRA-Pi-Zero main page.

    Email Kevin Custer for support, ordering information, order by check, or support of this exciting product.
     
     

    Product of Masters Communications, all rights reserved.
    Specifications may change without notice.
    Images property of Kevin Custer - W3KKC
    Board layout by Kevin Custer - W3KKC.
    HTML August 4, 2024, W3KKC All Rights Reserved!