Phonev6

PCB Design

This directory contains the PCB design files created with KiCad.

Design History

• phonev2 (removed): Dual LM358 op-amps, raw power connector, pin headers for audio.
• phonev4 (earlier fabrication): Single LM386 power amplifier for ringer only, XL6009 boost converter, RJ9 and 3.5mm jacks. Had two design flaws: speaker output coupling (#42) and unamplified handset earpiece (#44).
• phonev5 (previous schematic): TDA2822M dual-channel amplifier replacing the LM386, fixing both audio design flaws. Adds reverse polarity protection, ESD protection, power LED, fuse, test points, and improved I2C pull-ups.
• phonev6 (current — fabricated and installed): through-hole → SMD migration of the phonev5 design, plus a make verify guardrail suite (netlist sync, footprint parity, pinout and footprint checks, ERC, DRC) that must be all-green before ordering. This is the revision currently installed in the phone.

Audio Circuit (TDA2822M)

The TDA2822M is a dual low-voltage audio power amplifier in a DIP-8 package (same footprint as the LM386 it replaces). It provides two independent amplified channels, solving both the speaker coupling and quiet handset issues.

Signal Paths

USB audio card (stereo 3.5mm)
  ├─ Left channel → J7 tip → C_inA → TDA2822 IN_A (pin 3)
  │                                    → OUT_A (pin 1) → C_outA → speaker_front+ → ringer
  │
  └─ Right channel → J7 ring → C_inB → TDA2822 IN_B (pin 4)
                                        → OUT_B (pin 5) → C_outB → speaker_receiver+ → J4 RJ9 → handset

Each channel has its own input coupling capacitor (100nF) and output coupling capacitor (100µF). The channels are electrically isolated inside the TDA2822M, eliminating the crosstalk present in the v4 LM386 design.

TDA2822M Pin Connections

Component Values

Bill of Materials

Decoupling Capacitors (C-*)

Protection Circuits

Reverse Polarity Protection (Q1)

A P-channel MOSFET (AO3401A, SOT-23 SMD; JLCPCB basic C15127) on the incoming 5V rail protects against accidental reverse-polarity connections. Q1 is placed on the power input before distribution and before feeding U1 IN+. The MOSFET's body diode conducts during normal operation with near-zero voltage drop, and blocks current when polarity is reversed.

ESD / TVS Protection (D1, D2, D3)

Three P6KE6.8CA TVS diodes (600W bidirectional, THT DO-15) protect signal lines from ESD and transients. Each clamps between the protected signal and GND—one pin to signal, one to GND. D1 protects J4 (handset); D2 protects speaker_front+ (ringer output from TDA2822); D3 protects the third signal line. These protect against back-EMF and ESD on external connectors.

Overcurrent Protection (F1)

A 1A resettable PTC fuse (F1) is placed on the incoming 5V rail, in series with Q1, before power distribution and before U1. It protects the entire board (including the boost converter input) from downstream shorts. The fuse self-resets when the fault is cleared.

Power Indicator (D4, R3)

A red LED on the 5V_MAIN rail provides visual confirmation that the board is powered, useful when debugging inside the payphone enclosure.

Connector Pinouts

J1 — Coin Validator (2x5)

The coin validator is powered from the 12V_COIN rail (U1 output). The PCB routes SoftwareSerial (600 baud) from the display Arduino pins 14 (RX) and 23 (TX), plus a reset line from pin 15. See the coin validator documentation for the serial protocol.

J2 — Keypad (2x10)

Routes to the keypad Arduino's row pins (6–9) and column pins (10–13, 18–20), plus the hook switch pins (4, 5, 21). See Arduino/PINOUT.md for the complete mapping.

J3 — VFD Display (2x13)

8-bit parallel data bus (D0–D7) plus control signals (WR, AD, RD, CS, TEST, RESET) routed from the display Arduino. See Arduino/PINOUT.md for the pin- to-VFD-pin mapping with wire colors.

J4 — Handset (RJ9)

J6 — Card Reader (2x7)

MagStripe reader connections routed to the keypad Arduino: CLS (pin 22), RDT/data (pin 0), RCL/clock (pin 1).

J7 — Speaker/Audio (3.5mm stereo)

I2C Bus

The two Arduinos communicate over I2C (SDA/SCL). The PCB provides 4.7kΩ pull-up resistors (R1, R2) to 5V on both lines — the standard recommended value for 100kHz I2C.

Power Distribution

The board is powered from an external 5V supply. Power flows as follows:

1. Incoming 5V enters the board and passes through:

   • Q1 (reverse polarity protection)
   • F1 (PTC fuse)

2. 5V_MAIN rail (after Q1 and F1) distributes to:

   • Both Arduino Micro boards (5V pin)
   • Raspberry Pi Zero WH (5V via GPIO header)
   • VFD display (5V logic supply; VFD filament uses its own internal supply)
   • TDA2822M amplifier (pin 6)
   • MagStripe reader (5V via Arduino)
   • U1 IN+ (boost converter input)

3. 12V_COIN rail: U1 (XL6009) boosts 5V → 12V. XL6009 pins: IN+ = 5V_MAIN, IN- = GND, OUT- = GND, OUT+ = 12V_COIN. The 12V_COIN rail powers the coin validator only (J1). Nothing else on the board uses 12V.

Test Points

Note on USB connectivity: Arduino ↔ Pi communication is via USB through an external hub. There are no discrete USB serial TX/RX nets on the PCB; TP6 and TP7 in older docs referred to USB serial, but that path does not exist as PCB nets. Only the test points above (5V_MAIN, GND, 12V_COIN, I2C) are actual PCB nets.

Schematic Audit

Run python3 audit_schematic.py to check component/BOM alignment, net labels, and documentation consistency. See AUDIT.md for findings and action items.

Files

Manufacturing

See JLCPCB_WORKFLOW.md for fabrication steps.

Quick summary:

1. Run ./pcb/scripts/export_jlcpcb.sh to generate jlcpcb/production_files/GERBER-phonev6.zip
2. Use KiCad's JLCPCB Fabrication Toolkit plugin for BOM/CPL
3. Upload all three from jlcpcb/production_files/ to JLCPCB

Schematic Changes Required in KiCad

The schematic file has been updated with the TDA2822M symbol and updated component values. The following wiring changes need to be completed in KiCad's schematic editor:

Audio path rewiring

1. J7 pin 1 (tip): Change net from speaker_receiver+ to speaker_receiver_pre+. This is the pre-amplified handset signal that now feeds into the TDA2822 channel B input.

2. TDA2822 IN_A (pin 3): Connect to speaker_front_pre+ via input coupling cap C_inA.

3. TDA2822 IN_B (pin 4): Connect to speaker_receiver_pre+ via input coupling cap C_inB.

4. TDA2822 OUT_A (pin 1): Connect via output coupling cap C_outA to speaker_front+ (ringer speaker).

5. TDA2822 OUT_B (pin 5): Connect via output coupling cap C_outB to speaker_receiver+ (handset earpiece via J4).

6. TDA2822 GND (pin 2): Connect to gnd.

7. TDA2822 V+ (pin 6): Connect to 5V_MAIN via C_vcc and C_dec bypass caps.

8. TDA2822 RIPPLE (pin 7): Connect via C_ripple (4.7µF) to gnd.

Protection circuit wiring

1. Q1 (reverse polarity): Place on the incoming 5V rail before distribution and before U1. Source to incoming 5V, drain to 5V_MAIN bus, gate to source (or appropriate pull-down for PMOS). Protects the entire board including U1 input.

2. F1 (fuse): Insert in series on the incoming 5V rail, before Q1 or between Q1 drain and 5V_MAIN distribution. Protects the 5V supply and boost converter input from overcurrent.

3. D1, D2, D3 (TVS): P6KE6.8CA bidirectional TVS. Each: pin 1 → GND, pin 2 → protected signal (e.g. speaker_receiver+, speaker_front+, or other external signal). Protects signal lines, not power.

4. D4 + R3: Connect from 5V_MAIN through R3 (1kΩ) through D4 to GND.

Test points

Add test point symbols connected to: 5V_MAIN, 3.3V, GND, SDA, SCL, 12V_COIN. Do not add TX/RX test points for USB serial; Arduino ↔ Pi communication uses USB through an external hub, so there are no discrete USB serial nets on the PCB.

Known PCB Errata

1. Missing trace: Reset 2 (GPIO27 → Arduino Beta RST) — The PCB is missing a trace connecting the Pi's GPIO27/GEN2 (pin 13) to the display Arduino's RST pin. Worked around with a bodge wire on the assembled board. Must be added as a proper trace in the next KiCad layout revision.

Known Issues (resolved by this revision)

1. Speaker output coupling (#42) — Fixed: TDA2822M provides fully isolated dual channels. No more crosstalk between handset and ringer.

2. Handset earpiece too quiet (#44) — Fixed: Both channels are now amplified by the TDA2822M. The handset earpiece is no longer driven at unamplified line level.

Remaining Considerations

The phonev6 schematic, layout, and generated BOM are the fabricated as-built revision. Its migration is intentionally mixed technology: protection and support parts moved to SMD while U2 remains the verified DIP-8 TDA2822M. The layout is not pending; only the GPIO27 reset bodge in Known PCB Errata remains for a future revision.

1. ALSA config update: The ALSA softvol max_dB setting in host/asoundrc.example may need adjustment since the TDA2822M provides hardware amplification. Software gain can likely be reduced.

2. Gain tuning: The TDA2822M's default gain may need adjustment for comfortable volume. If too loud, input attenuation resistors can be added. If too quiet, the coupling cap values can be increased.

3. Mounting holes: Consider adding M3 mounting holes at board corners for mechanical support inside the payphone enclosure.

Circuit Improvement Ideas

Ideas for future revisions, ordered by likely impact.

High priority

1. Bulk capacitor on 5V input — Add a 470–1000 µF electrolytic (16–25V) on the 5V rail (after Q1/F1, before distribution). Smooths supply during Pi boot current spikes and audio peaks.

2. Bulk capacitor on 12V_COIN — Add 100–220 µF (25V) near J1 (coin validator). Coin validators draw current spikes when the motor runs; the bulk cap reduces rail droop.

3. Series resistor on coin validator reset — If the Arduino drives the reset line directly to J1, add a 100–220 Ω series resistor. Limits fault current and reduces coupling.

Medium priority

4. I2C series resistors — Add 22–33 Ω series resistors on SDA/SCL at each Arduino I2C port. Dampens ringing if cable lengths increase or in noisy environments.

5. XL6009 thermal — The 5V→12V boost at ~50–100 mA can get warm in a confined enclosure. Use adequate copper around the module; consider a small heatsink if airflow is limited.

6. Undervoltage handling — The bulk input cap (item 1) helps. If the 5V supply is marginal during boot, consider explicit UVLO or reset sequencing for critical loads.

Lower priority

7. ESD on audio jacks — J5 (mic) and J7 (speaker) are externally accessible. Consider PRTR5V0U2X or similar TVS on their signal pins for robustness.

8. Power LED brightness — If D4 is too bright in a dark payphone, increase R3 (e.g., 2–4.7 kΩ) for dimmer indication.

9. TDA2822M gain flexibility — Add DNP footprints for optional input attenuation resistors so gain can be tuned without a respin.

Documentation / mechanical

10. Connector keying — Verify J1 (coin validator) is keyed to prevent reverse insertion. Document pin-1 orientation clearly.

11. Pi power path — Document whether the Pi receives 5V only from the GPIO header or also via USB; affects power sequencing and fault behavior.