Phase 2 — Full Schematic

Component selection and detailed schematic design for the PillStim development board, with all ICs, passives, and test infrastructure specified and sourced.

Design Approach

Pill-First Component Selection

All active components and circuit topologies were selected with the pill form factor as the primary constraint. ICs were chosen for their availability in chip-scale or ultra-small packages (SC70, SOT-23, MSOP, WLCSP) that will fit within the 10 mm × 20 mm capsule. Passives target 0402/0201 in the final pill; the development board uses larger 0603/0805 equivalents for ease of hand assembly and probing.

Development Board Strategy

The dev board replicates the pill circuit with larger package variants, plus additional test infrastructure: rotary switches for scope probe routing, an Analog Discovery 3 header for digital IO and waveform injection, a UART interface, selectable electrode loads (tissue model, LED indicators, capacitive), and a power source selector (battery, USB/UART, or ST-Link). This allows full characterisation of the stimulation circuit before committing to the pill-scale layout.

Schematic
PillStim full schematic
Full development board schematic — click to zoom. Includes MCU, Dickson charge pump, H-bridge current stimulator, feedback sensing, and test infrastructure.
Circuit Blocks
MCU STM32L052K8T6 — Cortex-M0+, 12-bit DAC, 32KB flash, LQFP-32
Charge pump 6-stage Dickson — 74LVC2G04 clock driver, RB521S-30 Schottky diodes, 100nF flying caps
Storage Capacitor bank — separate sub-sheet, MLCC array
Current stimulator LTC2063 + BSS138 sink — zero-drift op-amp, dual-range sense (330Ω / 3.3kΩ)
H-bridge BSS84 / BSS138 — complementary MOSFET, level-shifted PMOS gates
Feedback TMUX1104 mux + LTC2063 buffer — VHV, CSV, current sense, routed to MCU ADC
Power 2× SR927 series — 3.1V, with external battery JST connector
Dev-only blocks: Scope selector (2× SS-10-16NP-LE rotary), electrode load selector (tissue model / LED / cap via SS-10-23NP-LE), Analog Discovery 3 header (2×15 pin, digital IO + waveform injection), UART (JST-SH 4-pin), power selector (battery / UART / ST-Link via SS-10-23NP-LE), and test signal injector with diode protection. These will not be present in the pill.
Bill of Materials
Development board BOM. All components sourced from DigiKey and Mouser.
Ref Value MPN Package Price
Integrated Circuits
U2 STM32L052K8Tx STM32L052K8T6 LQFP-32 $3.93
U1, U3 LTC2063 LTC2063ISC6#TRMPBF SC70-6 $3.74
U4 TMUX1104 TMUX1104DGSR MSOP-10 $3.09
U5 74LVC2G04 SN74LVC2G04DBVR SOT-23-6 $0.70
Transistors
Q1–Q6, Q12, Q13, Q15, Q16, Q22, Q23 BSS138 BSS138 SOT-23 $0.28
Q10, Q11, Q14, Q21, Q24 BSS84 BSS84-7-F SOT-23 $0.21
Diodes
D1–D10, D13 RB521S-30 RB521S30T1G SOD-523 $0.15
D11, D12 Green LED 150060GS75000 0603 $0.15
Capacitors
C1–C5, C9, C10, C12, C14–C16, C20, C21 100nF CL10B104KB8NNNC 0603 $0.10
C11, C17–C19 10µF CL21A106KAYNNNE 0805 $0.25
C13 1µF CL10A105KA8NNNC 0603 $0.10
C8 1nF CC0603JRNPO9BN102 0603 $0.10
Resistors
R18–R20, R22, R23, R25–R33, R35, R37, R42, R43 1MΩ RC0603FR-071ML 0603 $0.10
R36, R44 100kΩ RC0603FR-07100KL 0603 $0.10
R17, R21, R34, R45–R47 10kΩ RC0603FR-0710KL 0603 $0.10
R16 3.3kΩ RC0603FR-073K3L 0603 $0.10
R38–R41 1kΩ RC0603FR-071KL 0603 $0.10
R14 330Ω RC0603FR-07330RL 0603 $0.10
R15 0Ω RC0603JR-070RL 0603 $0.10
Battery
BT1, BT2 SR927 SR927W 9.5mm coin $2.36
Connectors
J3 VBAT_EXT SM02B-SRSS-TB(LF)(SN) JST-SH 2-pin $0.44
J8 Analog Discovery 3 FTSH-115-01-L-DV-K 2×15, 1.27mm $6.96
J10 UART SM04B-SRSS-TB(LF)(SN) JST-SH 4-pin $0.61
Switches
SW1 Reset RS-282G05A3-SM RT SMD tact $0.85
SW4, SW5 Scope selector SS-10-16NP-LE SP6T rotary $3.49
SW6–SW8 Load / power / test SS-10-23NP-LE DP3T rotary $2.91
Total $56.85
Pill variants: All active ICs (STM32L052, LTC2063, TMUX1104, 74LVC2G04) are available in chip-scale packages suitable for the pill. Passives will move from 0603/0805 to 0402/0201. Dev-only components (switches, connectors, LEDs, test headers) will be omitted from the pill BOM.
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