Saturday, July 15, 2023

Gamma Dog hardware - PCB v4.0

I finished the design of PCB v4.0 for the Gamma Dog and OSH Park service came through again with excellent manufacturing quality boards.

The improvements in the schematics and layout are not huge but this version incorporates the progress in the development, which continued after PCB v3.0.

Gamma Dog Main PCB v4.0

Bottom layer with a Marie Curie quote.

Changes from PCB version 3.0:

  • Changed the placement of the DAC board and EEPROM board connectors. The new version of the 12-bit DAC daughterboard is slightly larger due to additional QT connectors and it wouldn't fit in the old footprint. 
  • Added HV voltmeter circuitry for measuring and adjusting the HV PMT Bias in the field - the measurement range is 0 - 1255V (max). The High-Voltage reading is displayed when entering the menu system's diagnostic screen and will allow for field adjustments when changing different detectors. The circuit provides also HV PS status data for the Self-Diagnostics / Health-Check during startup. The accuracy is better than the on-screen resolution of 1V.
  • Added the I2C digital potentiometer for the Digital Audio Volume Control to the main board (MSOP-10 package) - with the previous version it was an in-line add-on board as this feature was developed after fabrication of PCB v3.0
  • Added support for the LED in the GREEN/DOWN button - this LED now serves as "Charging" (flashing) and "Charge complete!" (solid) indicator during the internal battery charging.
  • Added an SMD jumper allowing the user to configure the power source for the Audio Power Amplifier module - options now are either the regulated 3.3V from the MCU board or the direct battery power (4.1V max) (affording the highest possible audio volume).
  • Re-organized (separated) and moved the DOWN Button and Audio Amplifier connectors - the GREEN/DOWN button now has a dedicated 3-pin connector (previously, it was shared with the Audio Power Amplifier), and the connector is placed right next to the BLUE/UP button connector, closer to the button's location.
  • Removed all of the unnecessary and unused component footprints, related to the old analog circuit for setting Minimum Pulse Height Threshold with a trimmer-pot.
  • Improved the matching between the motherboard's pads and the corresponding HV PS module Output and GND pads for easier installation of the piggybacked PS module.
  • Various small changes in component placement, trace routing and overall layout optimizations.

The Motherboard PCB with SMDs, sockets, headers and connectors installed. Visible, on the top-right is the HV voltmeter's VD string of precision resistors. 
The input impedance of the voltmeter is >1 GOhm in order to reduce any voltage drop caused by the measuring circuit.

Second Level of boards.
DAC, EEPROM and HV Power Supply + Pulse Amplifier.

Third level. 
The nRF52840 MCU module along with various sensors and RFI Shielding Can installed around the HV circuit.

Fourth Level completes the board stack.
Latching Relay module and RFI shielding cover installed.

Calibrated and tested V4 boards ready for the housings.

The official Firmware version is now v4.5, supporting the new hardware features and changes the way how unit-specific data is handled. 
The unique "Identity data" of each Gamma Dog unit, such as calibration offsets for various voltage dividers, pulse amplifier DC bias value, exact DAC reference voltage, serial number, etc is now stored in a dedicated "Read-Only" area of the EEPROM and not hard-coded in the firmware as it was before. 
Each unit is flashed with its "identity" during manufacturing, removing the need for unit-specific firmware code and simplifying development and future firmware updates.

New feature in the V4 hardware - High-Voltage Digital Meter - 0V to 1255V.
The PMT Bias reading is shown when entering the menu system. 
The value is constantly measured and displayed at a rate of 4Hz until a button is pressed. 
Adjusting the detector voltage in the field only requires a small screwdriver.

"Behind the scenes" - the machining work of the front panel. 
A 1/4" thick Plexiglas bezel for the display is already glued with black RTV sealant inside the display opening. The edge of the bezel is painted black to reduce internal reflections of the edge.
The display board is attached with self-tapping screws in blind holes. The blind holes for the audio boards are threaded and the board is secured with 5mm nylon stand-offs and 2.5mm nylon screws.

This picture shows all of the UI components and interconnects installed on the Control Panel. 
The wiring is done with JST connectors for a quick, easy, and clean assembly / disassembly.

The main board is mounted on brass stand-offs and connected to the Control Panel's UI components.
 There are only two connectors beyond this point - the white LIPO battery pack connector seen in the lower-right corner and the female BNC connector for the scintillating detector, seen just to the left of the battery connector cable.

The Complete electronics package of Gamma Dog V4. 
As a finishing touch, a 3D-printed bezel for the speaker opening retains a piece of open-cell foam to protect the speaker from dust and debris. Under the foam there is a second, fine metal speaker grill for mechanical protection.
Another cosmetic change in the V4 enclosure is a change in the way, the display and speaker board are mounted on the control panel. I used blind screw-holes from the back side to avoid exposed screw-heads on the front and the look now is nice and clean. The only screw-heads visible are the ones holding the main board. 
A self-diagnostics routine is executed on startup - the very first stage (pictured) is the check of all UI components.

Menu System in firmware version 4.5. In this video the Bluetooth connectivity is disabled, and the menu list skips the two BT-related items. Total of 20 Menu Items.

 At this point, I consider the firmware to be "fully matured" and bug-free.

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