Building the Bell Ringing Relay Module

The Goal: Ring the Bell Without Breaking the Phone

The original phone uses a hand crank magneto to generate AC voltage and ring the bells. That signal travels directly through the bell coils.

To replicate that behavior digitally, I need a way to:

• Inject an AC ringing signal into the bell circuit
• Keep the original magneto wiring intact
• Avoid sending any high voltage into the Raspberry Pi
• Maintain full electrical isolation between systems

This module solves the switching problem.

Why a Relay Is Used

A relay acts as a physical switch controlled by a low-voltage signal.

In this design:

• The Raspberry Pi or Arduino outputs a 3.3V GPIO signal
• That signal drives a transistor
• The transistor energizes the relay coil
• The relay contacts connect the bell to an external AC source

The relay becomes a bridge between two worlds:

• Low-voltage logic (safe)
• High-voltage AC (unpredictable)

Because the relay is mechanical, the two sides remain electrically isolated.

The Transistor Is a Relay for the Relay

One realization that clarified the design is this:

The transistor is effectively acting like a relay for the relay.

The GPIO pin cannot supply enough current to activate the relay coil directly. Instead:

• The GPIO controls the transistor
• The transistor allows current from a separate power source to flow
• That current energizes the relay

So the system becomes layered:

• The transistor is a small, fast, electronic switch
• The relay is a larger, physical switch handling higher power

This creates a chain of control where a tiny digital signal ultimately controls a much larger electrical system.

Why a DPST Relay Instead of a Single Switch

The bell circuit consists of two wires. To properly inject AC power, both sides must be switched.

A DPST (Double Pole Single Throw) relay allows:

• AC_A → BELL_A
• AC_B → BELL_B

This ensures:

• Full isolation when inactive
• Clean connection when active
• No shared ground or unintended feedback paths

Switching only one side could lead to strange behavior, noise, or false detection in other modules.

How the Transistor Drives the Relay

The transistor acts as a low-side switch.

• GPIO → 1k resistor → transistor base
• Transistor emitter → GND
• Relay coil → +V and transistor collector

When the GPIO goes HIGH:

• a small current flows into the base
• the transistor turns on
• the relay coil receives current
• the relay closes

Importantly, the transistor is not supplying power—it is simply allowing current from the relay’s power supply to flow.

Why a Flyback Diode Is Required

The relay coil is an inductor. When power is removed, it generates a voltage spike.

A 1N4001 diode is placed across the coil to absorb that spike.

• Cathode (striped end) → +V
• Anode → transistor side

Without this diode:

• the transistor could be damaged
• electrical noise could affect the system
• the Raspberry Pi could behave unpredictably

This small component prevents many potential problems.

Why a 1k Resistor Is Used on the GPIO

The resistor between the GPIO pin and the transistor base limits current.

At 3.3V:

• the GPIO safely drives the transistor
• the transistor still fully turns on

A 1k resistor is a common, reliable value for this purpose.

It protects the GPIO while ensuring proper switching behavior.

Keeping the Systems Electrically Separate

This module is designed with strict separation:

Low-voltage side:

• GPIO signal
• transistor
• relay coil

High-voltage side:

• AC generator (future module)
• bell coils
• magneto wiring

There is no direct electrical connection between these domains.

The relay is the only point of interaction—and it is mechanical.

What This Module Does (and Doesn’t Do)

This module does not generate AC power.

It only:

• connects the bell to an AC source when activated
• disconnects it when inactive

This means:

• the bell can still ring from the hand crank
• the system can later inject its own ringing signal
• additional logic can decide when to ring

A Known Limitation

As with the crank detection module, this introduces a new interaction:

If external AC is applied to the bell, it may still be detected by the crank detection circuit.

This is expected.

A future module will:

• generate the AC ringing signal
• suppress or ignore crank detection during active ringing
• ensure that hand cranking and automated ringing don’t conflict

Design Philosophy: Modular, Not Destructive

Rather than modifying the original phone, this project builds around it.

Each module:

• plugs into existing wiring
• avoids permanent changes
• can be removed without damage

This relay module is the first piece that allows the system to act—not just observe.

What Comes Next

With detection and switching in place, the next step is generating the ringing signal itself.

That will involve:

• producing an AC waveform suitable for the bell
• controlling cadence (ring… pause… ring…)
• coordinating with the crank detection module

At that point, the phone will not only function as it did in 1916—but will also respond to a modern digital system layered on top of it.