Telephone Circuit

Part of Telephony

The complete electrical circuit of a telephone instrument — how all components interconnect to send and receive voice over a two-wire line.

Why This Matters

Understanding the complete telephone circuit means you can build a telephone from first principles, diagnose any failure point, modify the circuit for specific requirements, and explain to others how every component contributes to operation. Without this system-level view, individual component knowledge (how a carbon microphone works, what a hook switch does) remains isolated — you know the parts but not the whole.

The telephone circuit is a classic example of elegant analog systems engineering: a small number of components, each performing multiple functions simultaneously, organized so that a two-wire connection carries both transmit and receive audio in full duplex (both directions at once) with DC power superimposed on the AC voice signal. Understanding how this works is also the basis for understanding every subsequent telephone technology that built upon it.

The Two-Wire Full-Duplex Problem

A telephone must simultaneously transmit the local speaker’s voice down the line and receive the distant speaker’s voice from the line — all on the same pair of wires, at the same time, without the two signals swamping each other. This is the fundamental design challenge.

The solution uses the fact that the transmit signal (from the local carbon microphone) is a modulation of the DC line current, while the receive signal (from the distant party) arrives as an AC voltage variation superimposed on the DC. By using components with different responses to AC and DC, the circuit separates the two signal paths.

The carbon microphone has high DC resistance but changes resistance (and thus DC current) in response to sound. The electromagnetic receiver responds to AC current variations but ignores DC. The induction coil (transformer) is transparent to AC but presents high impedance to DC. These different behaviors allow the circuit to route transmit and receive signals to the correct devices.

Basic Circuit Topology

A complete telephone instrument circuit includes these elements connected as follows:

Hook switch: When open (on-hook), disconnects all instrument circuits from the line. When closed (off-hook), completes the subscriber loop and allows current to flow from the exchange battery.

Carbon microphone: Connected in series with the line (one conductor path). The microphone varies the DC line current at audio frequencies, which is the transmitted voice signal.

Induction coil (hybrid transformer): A transformer with primary and secondary windings. The primary carries the DC line current from the microphone. The secondary drives the receiver. The transformer isolates the receiver from the DC bias while coupling the AC voice signal. Additional windings in the induction coil provide the anti-sidetone function.

Electromagnetic receiver: Connected across the secondary of the induction coil. Responds to AC current variations (the voice signal from the distant party) and converts them to sound.

Ringer: Connected across the line through a series capacitor, separate from the voice circuit. The series capacitor blocks DC (preventing the ringer from loading the line current during conversation) while passing the AC ringing signal (90V at 20 Hz). The hook switch disconnects the ringer from the line when off-hook.

Anti-sidetone network: The balance network and induction coil arrangement that prevents excessive sidetone (the speaker’s own voice in the earpiece).

Signal Flow Analysis

Outgoing voice (transmit path):

1. Speaker speaks into mouthpiece — diaphragm flexes — carbon granules vary in resistance
2. Carbon resistance variation modulates DC line current from exchange battery
3. Line current variations travel down both conductors to the exchange
4. Exchange receives the modulated current and routes it to the called subscriber’s line
5. Called subscriber’s receiver converts line current variations to sound

Incoming voice (receive path):

1. Called subscriber’s microphone modulates current at their exchange
2. Modulated current travels to your exchange and down your subscriber loop
3. Current passes through your induction coil primary
4. Induction coil secondary sees the AC component of the changing current
5. Secondary drives your receiver — sound emerges from earpiece

Ringing (signaling path):

1. Exchange generates 90V AC at 20 Hz
2. AC passes through the ringer series capacitor (capacitor blocks DC line current)
3. AC rings the bell mechanism
4. Subscriber lifts handset — hook switch opens ringer circuit, closes voice circuit
5. Loop current increases — exchange detects off-hook, connects voice path

Common Circuit Variants

Local battery (LB) circuit: The subscriber’s own battery (1.5-3V) powers the microphone. The exchange battery powers only the signaling. The LB circuit requires two battery connections per instrument but works without exchange battery power on subscriber loops.

Central battery (CB) circuit: The exchange battery provides all power through the subscriber loop. The instrument contains no battery. Simplified subscriber instruments. Requires exchange to supply appropriate voltage and current for both local and remote subscribers simultaneously.

Magneto circuit: The subscriber has a hand-cranked magneto generator for ringing. The line is powered only during a call (by the local battery or exchange battery). Common in rural systems with local battery operation.

Building a Complete Circuit

For a practical community telephone instrument with central battery operation:

• Hook switch: DPDT (double-pole, double-throw), one pole for loop connection, one pole for ringer bypass
• Carbon microphone: 150-ohm resistance at operating current, mounted in handset
• Induction coil: 1:1 ratio transformer with anti-sidetone taps, 600-ohm characteristic impedance
• Balance network: 600-ohm resistor in series with 2 µF capacitor (for typical 1-3 km lines)
• Receiver: 600-ohm impedance, electromagnetic type
• Ringer: electromechanical bell with 0.22 µF series capacitor

Wire these components as described in the circuit topology above. Test by connecting to a bench power supply set to 48V DC (simulating exchange battery) through a 600-ohm resistor (simulating line resistance). The loop should draw 40-80 mA off-hook, <1 mA on-hook. Voice communication between two such instruments completes the system verification.