By Per Magnusson, SA5BYZ

In addition to the RF and processor boards described in previous articles, there are three simpler boards that make up the electronics of the receiver.

The following files are available for download:

• Schematics PDF
• Layouts PDFs
• Layouts as gerbers
• BOMs (Bill of materials)

Link to a zip-file with he files:

LCD Board

The LCD board is very simple. It essentially just contains the LCD and the connector towards the processor board. The LCD is a 16×4 alphanumeric one called EA DOGS164W-A without backlight. It can also be used as 3×16 where one of the rows is twice as high and this mode is used during a race to enlarge either the gain setting and the RSSI indicator, or, more rarely, the frequency setting.

This is all there is to the schematic:

C1 seems to be required by the internal circuitry of the LCD, but is not really mentioned in the datasheet, except that it is present in an example schematic. The decoupling capacitor C2 could probably be skipped as the power consumption is extremely low. The backlight pins are connected, but a module without backlight is used here. The display is configured to use I2C as the interface by grounding the IM1 pin (SPI is another option). Applying a reset pulse after power up is recommended.

The datasheet does not mention what the limits are for the supply voltage. Nominal is 3.3 V, but it seems to run fine on 3.1 V also.

The PCB

The PCB has two layers and is 0.6 mm thick. It is 52 x 47 mm in size. This board is easy to solder manually. The assembly drawings for top and bottom are shown below.

The layout consists mostly of ground planes and this board serves double duty by also being part of the top of a shield box around the processor.

The assembled board looks like this:

The large frame of exposed metal on the bottom side connects to the rest of the shield (formed by a 3D-printed part with copper tape) around the processor.

USB/Headphone Board

This board has the USB and headphone connectors as well as provides the connection for the power switch. It is quite simple without any active components. The schematic is shown below.

J1 is the FPC connector towards the processor board. J2 is the headphone connector wired such that it should work with all (or at least most) 3.5 mm headphones, regardless if they have a microphone or not. J3 is the USB-C connector for charging and firmware updates. Both the headphone jack and the USB connector are waterproof types with gaskets towards the panels, but the waterproofing is unlikely to be perfect because of the uneven surface of 3D-printed walls.

The 5.1 kohm pull-downs on CC1 and CC2 of the USB-C connector, signals to a connected USB host/charger that it should enable 5 V on VBUS. See the article “All About USB-C: Resistors and Emarkers” on Hackaday for more details. If we were to draw more than 500 (or is it 900?) mA from VBUS, we should really check the voltage on CC1/CC2 to see that the source is able to provide that much current. But here it is not strictly necessary. If the receiver is connected via a USB-A to USB-C cable, then these CC pins are not connected and do not matter.

The power switch connects via wires to the through-hole pads TP1 and TP6. Grounding POWER_SW# enables power to the receiver. The pull-up R3 to HP_DETECT# is not populated as there is already another pull-up on the processor board side. SIGGEN/P1 is not used in this 2 m receiver (although it is used for an unshielded inductor that can act as the antenna for a very weak test transmitter for antenna tuning in an 80 m receiver that uses the same board).

VUSB is the voltage from the USB-C connector used for charging the LiPo battery.

The USB_P/N signals carry the USB data.

RIGHT/LEFT_HP are the headphone audio signals. PROG# connects to an optional push-button. This is the processor programming signal that can take it out of some software lock-ups, but is usually not needed. Another PROG# push-button exists on the main board.

HP_DETECT# gets pulled low when plugging in headphones. This is currently not used by the software, but a way to use it as the power-up signal for the receiver is described in the text about the processor board.

J1 is located on the bottom side of the board, while the rest of the components are on the top side. I soldered all these components myself, although both the USB connector and the FPC connector are quite fine pitch. Using a solder paste stencil may help dispensing a suitable amount of solder paste, but someone reasonably experienced with SMD soldering should be able to solder all these connectors with the help of a stereo microscope, a good soldering iron with a thin tip, thin solder wire and perhaps a little flux and solder wick.

Here are the assembly drawings:

And 3D views of the board:

The oval hole in the middle of the board allows a 3D-printed eccentric piece to push the board towards the wall of the box to compress the gaskets of the connectors. This may make the gaskets somewhat more effective, although one probably needs to use e.g. silicone to make the seals truly watertight.

The two copper layers look like this:

Some attempt was made to keep the differential impedance of the USB data lines close to 90 ohms. This board is 0.8 mm thick.

Front Panel Board

The front panel has four buttons, a rotary encoder and three LEDs. These are all supported by a PCB that allows all these parts to be connected via a single FFC cable to the processor board. The front panel board is glued (preferably by epoxy) to the inside of the front panel of the 3D-printed case.

This board is also a very simple 2-layer design. The schematic is shown below.

J1 is the FPC connector towards the processor board. It has 0.5 mm pitch, so requires some skill when soldering. P1 is a pin header to which the membrane keyboard connects. I used the same keyboard from Adafruit in a previous receiver, but apparently it was changed so that the common pin ended up on the other end of the row of pins. I therefore had to patch my boards to accommodate this, but the issue has been corrected in the design shown here. The numbering of the button signals may still be incorrect both here and on the processor board though… Since I was lazy, I did not create symbols and footprints for the LEDs and encoder. Instead, I used test points and manually placed them suitably near holes in the board so that the components could stick out through these and further out through the front. Here are 3D views of the board:

To help reduce water ingress around the encoder, a gasket can be printed using flex/TPU material and put on the encoder before soldering it to the PCB. Adding some silicone to both sides of the gasket is probably a good idea.

The photo below is from an earlier revision of the board where the keyboard connector had to be patched (by offsetting it one notch and adding a new ground connection). The published version does not have this bug.

I made the power LED yellow, the charging LED red and the charging-done LED green. The legs of the LEDs need to be bent in a funny way to reach down to the surface of the PCB. Surface mounting through-hole LEDs like this is perhaps not quite the norm.

This board is 0.6 mm thick to keep weight down.

The legs of the angled pin header need to be cut to be flush with the side of the board that gets glued to the front panel.