Ring Generator

This page documents the ring generator implementation for the Ma Bell Gateway, which produces the 90V AC 20Hz signal required to ring vintage electromechanical telephone bells.

Overview

Requirements:

Voltage: 90V RMS (approximately 127V peak, 254V peak-to-peak)
Frequency: 20 Hz (North American standard)
Cadence: 2 seconds ON, 4 seconds OFF (controlled by ESP32)
Current: 20-40 mA RMS per ringer
Power: 2-4W when active

Design Choice:

The Ma Bell Gateway uses a commercial telecom ring generator module rather than a custom oscillator/transformer design. This matches the overall design philosophy of using dedicated ICs for each function (like the HC-5504B SLIC), providing reliable operation without custom analog circuit design.

For alternative approaches (555 timer, ESP32 PWM, etc.), see the design documentation at Ringer Signaling.

How Ringing Works

The ringing system involves coordination between the ESP32, SLIC, ring generator module, and relay:

┌─────────────────────────────────────────────────────────────────────────────┐
│                           Ring Signal Flow                                   │
└─────────────────────────────────────────────────────────────────────────────┘

                                                Normal State (On-Hook)
                                                ┌─────────────────────┐
                                                │  -48V Battery Feed  │
                                                │  (from SLIC)        │
                                                └──────────┬──────────┘
                                                           │
┌──────────┐    GPIO 13     ┌──────────┐    RD Pin    ┌────┴────┐
│  ESP32   ├───────────────►│  SLIC    ├─────────────►│  Ring   │
│          │    (RC Pin)    │ HC-5504B │    (Relay    │  Relay  │
│          │                │          │    Driver)   │  DPDT   │
└──────────┘                └──────────┘              └────┬────┘
                                                           │
                                                Ring State │
                                                ┌──────────┴──────────┐
┌──────────┐                                    │                     │
│   48V    ├───────────────►┌──────────┐       │    ┌───────────┐    │
│  Supply  │                │  Ring    ├───────┴───►│ Telephone │    │
│          │                │  Gen     │  90V AC    │  Ringer   │    │
└──────────┘                │  Module  │  20Hz      └───────────┘    │
                            └──────────┘                             │
                                                                     │
                                          Tip/Ring (green/red) ◄─────┘

Sequence:

Incoming call detected - ESP32 receives ring notification via Bluetooth HFP
Ring command - ESP32 drives GPIO 13 LOW (SLIC RC pin)
Relay activation - SLIC RD pin goes LOW, activating external ring relay
Voltage switch - Relay disconnects -48V battery and connects 90V AC generator
Bell rings - 20Hz AC signal rings the electromechanical bell
Cadence control - ESP32 toggles GPIO 13: 2s ON, 4s OFF
Ring trip - If phone goes off-hook during ring, SLIC detects it automatically

SLIC Ring Control Interface

The HC-5504B SLIC provides ring control, not ring generation:

SLIC Pin	Name	Direction	Function
16	RC	Input	Ring Command - Drive LOW to activate ring mode
8	RD	Output	Ring Relay Driver - Goes LOW when ring active (drives relay)
3	RFS	Input	Ring Feed Sense - For ring trip detection

Important: The SLIC does NOT generate the 90V AC signal. It only:

Accepts the ring command from ESP32 (RC pin)
Drives an external relay to switch the line (RD pin)
Detects ring trip (off-hook during ringing)

Ring Generator Module

Selected Component: LT1684 Ring Tone Generator IC

For the Ma Bell Gateway, we use the LT1684 from Analog Devices (formerly Linear Technology) - a dedicated micropower ring tone generator IC designed specifically for telephone ringing applications.

Key Specifications:

Parameter	Specification	Notes
Output Voltage	Up to ±240V (90V RMS typical)	Adjustable via external components
Output Frequency	20 Hz (software controlled)	PWM input from ESP32
Output Current	Up to 200 mA	Well above 40mA ringer requirement
Output Waveform	Clean sine wave	2nd order MFB lowpass filter
Control Interface	PWM input	ESP32 controls frequency, amplitude, cadence

Component Details:

IC: LT1684 (~$8-15 from Mouser/DigiKey)
Datasheet: LT1684 Datasheet (PDF)
Application Note: The LT1684 Solves the Global Ringing Problem

Why LT1684:

Integrated solution - Purpose-built for telephone ring generation
ESP32 integration - PWM input allows software control of frequency, amplitude, and cadence
Flexible - Can generate various international ring standards (not just North American)
Cost effective - Significantly cheaper than commercial ring generator modules (~$200+)
Well documented - Detailed datasheet with complete application circuits

Additional Components Required:

The LT1684 requires external components per the datasheet application circuit:

2x Power MOSFETs (for H-bridge output stage)
Step-up transformer (for 90V output)
MFB filter components (resistors and capacitors)
Decoupling capacitors

See the LT1684 datasheet for complete schematic and component values.

Complete Ring Circuit

Block Diagram:

48V Supply ────┬──────────────────────────────────────────────────────►  To SLIC VB+
               │
               │     ┌─────────────────────┐
               └────►│  Ring Generator     │
                     │  Module             │
                     │                     │
                     │  48V DC → 90V AC    │
                     │          20Hz       │
                     └──────────┬──────────┘
                                │
                                │ 90V AC Output
                                │
                                ▼
                     ┌─────────────────────┐
                     │    Ring Relay       │
                     │      (DPDT)         │◄────── SLIC RD Pin (via driver)
                     │                     │
                     │  NC: -48V (normal)  │
                     │  NO: 90V AC (ring)  │
                     └──────────┬──────────┘
                                │
                                │ Switched Output
                                ▼
                          To SLIC Tip/Ring
                          (Phone Line)

Relay Driver Circuit:

The SLIC RD pin can sink current but may need a driver transistor for larger relays:

                                +12V (or relay coil voltage)
                                  │
                                  │
                                ┌─┴─┐
                                │   │ Relay Coil
                                │   │ (DPDT, 12V)
                                └─┬─┘
                                  │
                D1 (1N4148)       │
               ┌──────┤◄├─────────┤  Flyback diode
               │                  │
               │                ┌─┴─┐
               │                │ C │
               │          ┌─────┤   │ Q1 (2N2222 or similar)
               │          │     │ E │
               │          │     └─┬─┘
               │          │       │
SLIC RD ───────┴──[R1]────┴───────┴───── GND
(Pin 8)           1kΩ

Note: If using a small relay (< 50mA coil), the SLIC RD pin may drive it directly. Check the HC-5504B datasheet for RD pin current capability.

Relay Selection:

Type: DPDT (Double Pole, Double Throw)
Coil Voltage: 12V DC (matches SLIC VB+ rail)
Contact Rating: 1A @ 125V AC minimum
Example: Omron G5V-2, Panasonic TQ2-12V

Cadence Control

The ESP32 firmware controls ring cadence by toggling GPIO 13 (SLIC RC pin):

Standard North American Cadence:

2 seconds ring ON
4 seconds ring OFF
Repeat until answered or caller hangs up

The firmware implementation handles cadence timing, ring trip detection (when user answers during ring), and integration with the Bluetooth HFP call state.

For firmware implementation details including GPIO configuration and sample code, see SLIC Interface Monitoring.

Safety Considerations

Warning

90V AC is hazardous! The ring voltage can cause painful shock and injury. Follow proper safety practices when working with this circuit.

Electrical Safety:

Use properly rated components (125V AC minimum for relay contacts)
Ensure adequate creepage/clearance on PCB (2mm minimum for 90V)
Include fusing on the ring generator output (100mA fast-blow)
Never touch tip/ring connections when ring voltage may be present

Isolation:

The ring generator module should provide galvanic isolation between 48V input and 90V output
The SLIC provides isolation between high-voltage line circuits and low-voltage ESP32 logic

Protection:

MOV across relay contacts to suppress switching transients
Flyback diode across relay coil (required)
Current limiting in ring generator module

Bill of Materials

Component	Value/Type	Qty	Notes
Ring Generator IC	LT1684	1	Analog Devices (DIP-16 or SOIC-16)
Power MOSFETs	See LT1684 datasheet	2	H-bridge output stage
Step-up Transformer	See LT1684 datasheet	1	For 90V output
Filter Components	See LT1684 datasheet	Various	MFB lowpass filter (100Hz cutoff)
Ring Relay	DPDT, 12V coil	1	Omron G5V-2-H1 or equivalent
Flyback Diode	1N4148	1	Across relay coil
Driver Transistor	2N2222 or 2N3904	1	If needed for relay drive
Base Resistor	1kΩ	1	For transistor driver
Fuse (output)	100mA fast-blow	1	Ring circuit protection
MOV (optional)	150V	1	Relay contact protection

References

Ringer Signaling - Ring signal design alternatives
Power Implementation (48V Central Supply) - 48V power source for ring generator
Line Interface: HC-5504B SLIC - SLIC pinout and ring control interface
Analog Devices Design Note 134 - Telephone ring tone generation
Beta Dyne Ring Generator Application Note - Understanding ring generators