Battery-less Audio Node Design


I’ll turn this concept into a grounded project specification first, then verify whether the named off-the-shelf parts can actually support a 2 mm³ battery-less audio/RF node before attempting a schematic. I’ll rename the blank project, capture the requirements, and do a feasibility/power-component check against available part data.
I renamed the project and captured the concept in Project Specification. I also completed a datasheet-grounded feasibility check in Feasibility and Power Analysis.
Conclusion: the exact 2 mm cube off-the-shelf design should not proceed to schematic as stated—the microphone and EFR32-class SoC packages alone exceed the volume, and the 01005 buffer capacitor is orders of magnitude too small for RF/audio burst energy. Recommended next step is to choose a revised target: larger proof-of-concept PCB, true custom bare-die/SiP research package, or reduced-function NFC-style node.
Design a larger proof-of-concept PCB around these components.
I’ll design a practical proof-of-concept schematic around the same RF SoC, PDM mic, Schottky rectifier, and GRM02 buffer concept. I’ll size the power path first, add the components and support circuitry, wire the schematic, then run review and document the firmware/power notes.
Designed the larger proof-of-concept schematic around the EFR32MG22, SPK0641HT4H-1 mic, SMS7621 rectifier bridge, GRM02 benchmark capacitor, larger reservoir cap, AP2112K 3.3 V rail, SMA RF test port, coil input, SWD header, reset, decoupling, and RF match.
I documented the implementation in Proof-of-Concept Design Notes and added firmware bring-up guidance in Firmware Starter. Schematic connectivity is complete; remaining review issues are expected layout airwires and generic-part MPNs to resolve before fabrication. Next step is PCB layout setup and placement—ready when you want me to proceed.
Set board stackup
Auto-place components
Run ERC/DRC
Resolve BOM MPNs
Add test points

U2
R3 P2 - R4 P1
D1 2 - D2 2
U1 PAVDD - C3 P1
U1 PAVDD - C3 P1
J4 P2 - D2 1
U1 IOVDD - U1 RFVDD
L1 P2 - C12 P1
U1 RESETn - R1 P2
D1 2 - D2 2
U1 PAVDD - C3 P1
U1 PC00 - MK1 CLOCK
D1 2 - D2 2
L1 P2 - C12 P1
U1 PAVDD - C3 P1
U1 RF2G4_IO - L1 P1
D1 2 - D2 2
R3 P2 - R4 P1
MK1 SELECT - R2 P1
U1 PA01 - J2 Pin_4
U2 EN - C1 P1
L1 P2 - C12 P1
D1 2 - D2 2
U1 RF2G4_IO - L1 P1
U1 PA02 - J2 Pin_2
U1 RESETn - R1 P2
U2 EN - C1 P1
U1 PC00 - MK1 CLOCK
J1 P1 - U2 VIN
MK1 SELECT - R2 P1
U1 PAVDD - C3 P1
L1 P2 - C12 P1
U2 VOUT - C2 P1
U1 PAVDD - C3 P1
J4 P1 - D1 1
U1 HFXTAL_I - Y1 OSC1
J4 P2 - D2 1
U1 RESETn - R1 P2
U1 DECOUPLE - C7 P1
J4 P2 - D2 1
U1 PA02 - J2 Pin_2
U1 PC01 - MK1 DATA
U2 VOUT - C2 P1
U1 RESETn - R1 P2
J4 P1 - D1 1
U1 RESETn - R1 P2
U1 PAVDD - C3 P1
U1 HFXTAL_O - Y1 OSC2
U1 PAVDD - C3 P1
U1 HFXTAL_O - Y1 OSC2
U1 HFXTAL_I - Y1 OSC1
U1 PC01 - MK1 DATA
U2 VOUT - C2 P1
J4 P1 - D1 1
U1 PAVDD - C3 P1
U1 PA01 - J2 Pin_4
U1 RF2G4_IO - L1 P1
U1 DECOUPLE - C7 P1
R3 P2 - R4 P1
U1 IOVDD - U1 RFVDD
J1 P1 - U2 VIN
D3 1 - D4 1
C12
Capacitance
1.3 pF
C7
Capacitance
1 uF
GND
C2
Capacitance
10 uF
MK1 GROUND - C8 P2
MK1 GROUND - C8 P2
J1
D3 1 - D4 1
MK1 GROUND - C8 P2
GND
D3 1 - D4 1
GND
D3 1 - D4 1
GND
C1
Capacitance
10 uF
S1 3 - S1 4
GND
GND
GND
S1 3 - S1 4
GND
C8
Capacitance
0.1 uF
C9
Capacitance
47 uF
S1 3 - S1 4
C5
Capacitance
0.1 uF
C3
Capacitance
0.1 uF
C4
Capacitance
0.1 uF
D3 1 - D4 1
GND
D3 1 - D4 1
MK1 GROUND - C8 P2
GND
GND
D3 1 - D4 1
D3 1 - D4 1
D3 1 - D4 1
J4
C6
Capacitance
0.1 uF
C13
Capacitance
18 pF
GND
GND
MK1 GROUND - C8 P2
C11
Capacitance
1.2 pF
GND
J2
J3
C10
Capacitance
0.1µF
R3
Resistance
1 MΩ
R1
Resistance
10 kΩ
R2
Resistance
0 Ω
L1
Inductance
2.6 nH
U1
D4
MK1
D2
D3
R4
Resistance
330 kΩ
Y1
S1
D1
Proof-of-Concept Design Notes
Architecture Implemented
This proof-of-concept PCB uses a bench-powered 3.3 V rail for reliable bring-up while preserving a separate RF/inductive harvesting experiment block.
Functional Blocks
  1. Bench power input: J1 accepts external 5 V input.
  2. 3.3 V regulation: U2 AP2112K-3.3TRG1 generates the main 3V3 rail.
  3. MCU/radio: U1 EFR32MG22C224F512IM32-CR handles PDM audio capture, harvested-voltage sensing, and 2.4 GHz radio packet burst experiments.
  4. Audio input: MK1 Knowles SPK0641HT4H-1 PDM microphone.
  5. RF interface: J3 SMA edge connector with a preliminary 2.4 GHz matching network based on EFR32MG22 datasheet typical values.
  6. Harvesting test front end: J4 coil/rectenna input, D1-D4 SMS7621-060 low-barrier Schottky bridge, C10 GRM02 01005 benchmark capacitor, and C9 larger 47 uF reservoir capacitor.
  7. Debug/programming: J2 10-pin header, RESET button S1, SWDIO/SWCLK/RESET/3V3/GND signals.
Power Budget
The POC design is intentionally bench-powered because the original battery-less concept has insufficient harvested energy for continuous audio/radio operation.
Estimated peak 3V3 budget:
  • EFR32MG22 6 dBm TX: about 8.4 mA typical.
  • SPK0641HT4H-1 performance mode: up to 0.710 mA at 1.8 V equivalent; actual 3.3 V operation is within its 1.6 V to 3.6 V range.
  • MCU active overhead estimate: about 2.1 mA for high-speed active code.
  • Miscellaneous/debug margin: about 1 mA.
  • 1.5x design margin result: about 18.3 mA minimum budget.
The AP2112K-3.3TRG1 is rated for 600 mA output, so it has ample margin for the POC board and additional debug loads.
Key Schematic Nets

Table


NetPurpose
5V_INExternal bench input to LDO VIN and EN
3V3Main regulated rail for U1, MK1, and debug header
GNDCommon return path
PDM_CLKU1 PC00 to MK1 CLOCK
PDM_DATAMK1 DATA to U1 PC01
PDM_SELECTMK1 SELECT tied low through R2 for low-channel PDM timing
RESET_NU1 RESETn, pull-up R1, reset button S1, and debug header
SWCLKU1 PA01 to debug header
SWDIOU1 PA02 to debug header
HFXTAL_I / HFXTAL_O38.4 MHz crystal candidate Y1
RF_MATCH_ICU1 RF2G4_IO side of RF match
RF_ANTSMA antenna/test connector side of RF match
COIL_A / COIL_BExperimental coil/rectenna AC input
VHARVRectified harvested DC reservoir
HARV_SENSEDivider output to U1 PA03 for firmware measurement
Datasheet-Grounded Design Choices
  • EFR32MG22 QFN32 power pins are tied to 3V3 in a non-DCDC/bypass-style POC configuration. VREGSW is intentionally unconnected.
  • U1 DECOUPLE has a 1 uF capacitor; EFR32 datasheet notes minimum effective capacitance should remain at least 0.6 uF.
  • EFR32 debug pins are assigned from the datasheet: PA01 = SWCLK and PA02 = SWDIO.
  • EFR32 HFXO requires a 38.4 MHz reference for BLE/Zigbee operation. Y1 is a library candidate; its load capacitance and ESR still need datasheet confirmation before production.
  • EFR32 typical 2.4 GHz match values from the datasheet were captured as 1.2 pF, 1.3 pF, 2.6 nH, and 18 pF. This network must be RF-tuned in layout with the chosen board stackup and antenna/SMA geometry.
  • SPK0641HT4H-1 requires all ground pins connected and a bypass capacitor next to VDD. The microphone datasheet warns against Class 2 dielectric capacitors near the microphone due to piezoelectric effects; C8 should use a non-piezoelectric dielectric such as C0G/NP0 if available at the selected value.
Current Review Status
  • Electrical schematic connectivity is in place with no unresolved ERC category returned in the latest electrical review query.
  • DRC airwires are expected because the PCB has not been placed/routed yet.
  • Manufacturing review reports missing MPNs on generic passives and generic headers. This is acceptable for proof-of-concept schematic capture, but must be resolved before fabrication by selecting exact orderable MPNs for every generic passive/header.
Known Follow-Up Before Layout
  1. Confirm Y1 crystal datasheet: 38.4 MHz, load capacitance compatible with EFR32 HFXO, ESR within EFR32 limits, and frequency tolerance suitable for the target protocol.
  2. Replace generic passives with selected manufacturer MPNs or assign exact vendor part numbers.
  3. Decide board size; a 30 mm x 30 mm to 50 mm x 50 mm two-layer POC board is reasonable for hand probing and SMA/coil experiments.
  4. Place C3-C7 near U1 supply/DECOUPLE pins, C8 near MK1, and RF match components tightly between U1 RF2G4_IO and J3.
  5. Route RF_ANT as a 50 ohm controlled-impedance trace; use a solid ground reference under RF routing.
  6. Do not treat the SMS7621 bridge as production power management; it is an experiment front end only.
  • Architecture Implemented

  • Functional Blocks

  • Power Budget

  • Key Schematic Nets

  • Datasheet-Grounded Design Choices

  • Current Review Status

  • Known Follow-Up Before Layout