Off-Hook Detection
==================

An **off-hook detection circuit** is required to sense when a telephone receiver is lifted—completing the subscriber loop between tip and ring—so that the system can recognize an attempt to place or answer a call.

.. note::
   **Safety:** The subscriber loop may carry up to –48 V DC and 90 V AC ringing. All detection methods must provide robust isolation from line voltages to protect downstream circuitry and users. Never connect microcontroller logic directly to line voltage.

Functional Role
---------------

Off-hook detection plays a critical role in managing call flow. When the telephone goes **off-hook** (receiver lifted), a closed current path is created across the tip and ring conductors, typically resulting in 20–60 mA of DC loop current. This signals intent to place or answer a call.

When the phone returns **on-hook** (receiver replaced), loop current ceases, indicating call termination or cancellation.

Telephone systems use this state change to:

- Initiate or answer calls
- Control dial tone and ringing behavior
- Manage call supervision, timeout, and billing
- Trigger switching or Bluetooth pairing logic in modern integrations

Engineering Approaches Over Time
--------------------------------

The methods used to detect the off-hook condition have evolved with changes in switching technology and safety standards. Historically, this detection occurred at the central office (CO) or within PBX systems using one of several approaches:

**Mechanical Relays (Early 1900s–1940s):**
- Early systems used relays in series with the subscriber loop.
- When the phone was lifted, loop current energized the relay coil.
- Relay contacts would activate the switching path.
- Reliable but bulky and prone to mechanical wear.

**Discrete Transistor Circuits (1950s–1970s):**
- As solid-state components became common, relay logic was replaced.
- Loop current flowed through a sensing resistor; the resulting voltage drop activated a transistor.
- Lower cost and fewer moving parts, but lacked electrical isolation.

**Optical Isolation (1970s–1990s):**
- Optocoupler-based designs provided safe galvanic isolation between line voltage and logic systems.
- Loop current was used to drive the internal LED of an optocoupler, with the output side connected to digital logic.
- Widely adopted in PBX systems, modems, and early embedded telephony.

**Line Interface ICs / SLICs (1990s–Present):**
- Subscriber Line Interface Circuits integrate loop current sensing with additional features such as:
- Battery feed
- Ring injection
- 2-wire to 4-wire audio conversion (hybrid)
- Over-voltage protection
- Off-hook detection is often integrated into these chips, especially in VoIP adapters, FXS ports, and telecom-grade line cards.

**Solid-State Relays (Recent Embedded Systems):**
- Some embedded systems use SSRs with built-in optical isolation.
- These can sense or switch based on loop current with no mechanical contacts.
- Leakage current and cost may be trade-offs depending on application.

Key Considerations
------------------

Regardless of the method used, all off-hook detection circuits must meet several design requirements:

- **Safe voltage handling** – Must tolerate and isolate –48 V DC and ringing voltages.
- **Reliable detection threshold** – Should trigger cleanly within the expected 20–60 mA range.
- **Fast response** – Should detect hook state changes promptly to support switching logic.
- **Long-term durability** – Especially in CO and PBX applications, components must handle millions of cycles.

Debouncing and Supervisory Logic
--------------------------------

Mechanical hookswitches may bounce when transitioning between on-hook and off-hook states. To ensure stable behavior:

- Implement **debounce filtering** in software (or optionally in hardware).
- Consider **timeout supervision** to detect calls left off-hook too long (e.g., abandoned handsets, stuck relays).
- Systems may respond by playing a reorder tone, logging an error, or releasing the line.

Signal Flow Overview
--------------------

The following diagram illustrates a generalized off-hook detection signal path—from the subscriber loop through the detection mechanism and into a logic or control system:

.. image:: /_images/off_hook_signal_flow.png
   :alt: Off-hook detection signal flow from phone line to digital control
   :width: 400px
   :align: center

Final Notes
-----------

Modern embedded systems often draw from historical practices when implementing off-hook detection, blending safety, isolation, and responsiveness. While discrete designs are still used for hobbyist and low-volume applications, commercial systems frequently rely on integrated line interface chips for robust and efficient detection.