Hardware Wiring and Pinouts

Hardware Wiring and Pinouts

This page documents the complete hardware architecture, pinout assignments, and electrical interface circuits for reviving vintage rotary telephones with the Hosyond ESP32-S3-WROOM-1 development board.

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1. Complete Pinout Table

GPIO Pin	Function	Peripheral / Connection	Electrical Characteristics
GPIO 1	ADC1_CH0	MAX4466 Electret Microphone Analog Out	0.0V – 3.3V Analog (Internal DC baseline tracking)
GPIO 4	Input Pullup	Handset Hook Switch	Grounded (LOW) when handset is lifted; Open (HIGH) on cradle
GPIO 5	Input Pullup	Rotary Dial Pulse Switch	Normally Closed (LOW); Pulses HIGH/LOW as dial spins back
GPIO 6	Input Pullup	Rotary Dial Off-Normal Shunt	Grounded (LOW) continuously while dial is off its rest position
GPIO 7	Output PWM	IRF640N MOSFET Gate	3.3V Logic PWM driving 15Hz–30Hz AC Bell Ringing
GPIO 16	Output	MAX98357A I2S BCLK	I2S Bit Clock (16kHz $\times$ 16-bit $\times$ 2 = 512kHz)
GPIO 17	Output	MAX98357A I2S LRCK	I2S Word Select / Frame Clock (16kHz)
GPIO 18	Output	MAX98357A I2S DOUT	I2S Serial Audio Data Stream Out
GPIO 48	Output	WS2812 RGB LED	Built-in status indication LED
GPIO 0	Input	Boot / Setup Button	Hold 5s on boot to trigger Captive Setup AP

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2. Handset Audio Connections

A. Earpiece Speaker Output (MAX98357A I2S DAC / Class-D Amp)

• VIN: Connect to 5V (VBUS / External 5V).
• GND: Connect to common GND.
• BCLK (Bit Clock): Connect to GPIO 16.
• LRC / WS (Word Select): Connect to GPIO 17.
• DIN (Data In): Connect to GPIO 18.
• GAIN: Tie to GND (default 9dB gain) or leave floating (12dB gain).
• Speaker Out (+ / -): Connect to handset earpiece dynamic capsule.

B. Microphone Input (MAX4466 Electret Pre-Amplifier)

• VCC: Connect to 3.3V.
• GND: Connect to common GND.
• OUT: Connect to GPIO 1 (ADC1_CH0).
• Gain Trimmer: Set the potentiometer on the back of the MAX4466 board to roughly 50%–70% for standard conversational speech.

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3. Physical Mechanical Bell Ringer Circuit

Vintage telephones require high-voltage AC resonance (~40V–90V AC at 20Hz) to swing the internal clapper between the twin brass gongs. We drive this safely using an IRF640N N-Channel Power MOSFET paired with an inductive flyback snubber:

                  +24V to +48V DC Power Supply (Ringer Rail)
                                    │
                                    ├───[ 470nF 250V AC Cap ]───┐
                                    │                           │
                               ┌────┴───────────────────────────┴────┐
                               │     Telephone Ringer Coils          │
                               │   (Red & Black / Slate leads)       │
                               └────┬────────────────────────────────┘
                                    │
                                    ├───────|<────────┐ (1N4007 Diode)
                                    │                 │
                           Drain ┌──┴──┐              │
     GPIO 7 (PWM 20Hz) ──[100Ω]──┤     │ IRF640N      │
                                 └──┬──┘ MOSFET       │
                           Source   │                 │
                                    ├─────────────────┴─ Common GND
                                   ===

• MOSFET Gate Resistor: 100$\Omega$ between GPIO 7 and Gate to suppress parasitic high-frequency ringing.
• Gate Pull-Down: 10k$\Omega$ between Gate and GND to keep the MOSFET off during microcontroller boot.
• Series Capacitor: 470nF / 250V non-polarized film capacitor in series with the bell coils to block DC current and generate pure resonant AC swings.

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4. Rotary Dial Switch Contacts

Vintage dials (such as the Western Electric No. 7 / No. 9 or AE type 24) have two contact sets:

1. Pulse Switch (GPIO 5): Opens and closes 1 to 10 times as the rotary wheel springs back to rest. Connected to GPIO 5 with internal pullup (INPUT_PULLUP).
2. Off-Normal / Shunt Switch (GPIO 6): Closes as soon as the finger wheel is pulled from its rest stop, and re-opens only after the dial returns home. Connected to GPIO 6 with internal pullup (INPUT_PULLUP).