The iPhone 6, like its predecessors, used a standard 3.5mm TRRS (Tip-Ring-Ring-Sleeve) connector for its headphone jack. This connector provided stereo audio output, microphone input, and remote control functionality through a single port.

Physical Design:

  • Connector: The 3.5mm jack itself is a female connector, mechanically mounted to the iPhone’s chassis.
  • Contacts: Internally, the jack has four spring-loaded contacts that connect to the corresponding sections (tip, ring, ring, sleeve) of the inserted headphone plug.
  • Switches: In addition to the contacts, the headphone jack assembly typically incorporates mechanical switches activated by the insertion and removal of the plug.

Headphone Jack Detection Circuit:

The iPhone 6 utilized a combination of hardware and software to detect the presence of headphones. The detection circuit primarily relied on mechanical switches within the headphone jack assembly and the resulting change in resistance across specific contacts.

Here’s a breakdown of the likely components involved:

  1. Mechanical Switch(es):

    • The headphone jack assembly includes at least one, and potentially multiple, small switches activated by the insertion of the headphone plug.
    • When a plug is inserted, the tip of the plug pushes a switch, either directly or through a lever mechanism.
    • This action either closes a normally open switch or opens a normally closed switch, depending on the specific design.
  2. Resistor Network:

    • The switch(es) are typically part of a voltage divider circuit connected to the iPhone’s logic board.
    • When the headphone plug is not inserted, the switch remains in its default position, and the resistor network provides a specific voltage level to the detection pin on the logic board.
    • Upon insertion, the switch’s change in state alters the resistance in the network, thus changing the voltage level on the detection pin.
  3. Logic Board Interface:

    • The iPhone’s logic board contains a dedicated GPIO (general purpose input/output) pin assigned to headphone detection.
    • This pin is connected to the resistor network through a dedicated trace on the logic board.
    • The logic board constantly monitors the voltage level on this pin.
  4. Software Processing:

    • The iPhone’s operating system (iOS) is programmed to interpret the voltage level received on the headphone detection pin.
    • When the voltage changes to indicate the insertion of headphones, iOS recognizes this and triggers appropriate actions, such as:
      • Routing audio output to the headphones.
      • Deactivating the built-in speakers.
      • Enabling headphone controls (if present on the headphone cable).
      • Displaying the headphone icon in the status bar.

Simplified Illustration:

Imagine a simple scenario with a single, normally open switch and two resistors in a voltage divider.

  • No Headphones: When no headphones are inserted, the switch is open. The voltage divider, consisting of two resistors, provides a specific voltage (e.g., high voltage) to the detection pin.
  • Headphones Inserted: Inserting the headphones closes the switch. This changes the resistor network configuration, resulting in a different voltage (e.g., low voltage) at the detection pin.