Why ACI Built a 16-Channel Current Detector

Prototype hardware undergoing evaluation during the development of ACI's first 16-channel model railway current detector.

Some products begin with an idea. Others begin with a practical problem that needs to be solved.

The ACI 16-channel current detector is one of those products.

Every automated model railway needs one thing before it can make a decision: it has to know where the train is.

Without reliable feedback, a control system cannot know whether a section of track is occupied, whether a route is clear or whether a train has reached the point at which the next action should begin.

That challenge became very real during our work on Holgate, the large N gauge exhibition layout operated by Bridlington Model Railways Society.

As the layout evolved, so did the demands placed on its detection system. What began as a straightforward requirement to identify occupied track sections became a much broader challenge involving wiring, installation, maintenance and future expansion.

Although Holgate revealed those challenges on a large scale, the lessons apply just as readily to a home railway. Whether a layout has four detection sections or forty, the same requirements remain: reliable operation, straightforward wiring, clear indication and equipment that is easy to maintain.

We looked at the detection products and input hardware already available. Many worked well for the jobs they were designed to do, but none quite matched the way we wanted to organise detection, input expansion and local indication.

Rather than continually adapting the railway around separate pieces of hardware, we decided to develop a detector based on the lessons we had learned from building and operating a real layout.

The Problem Holgate Revealed

For an operator, knowing where a train is located is straightforward. We can see it moving around the railway.

A control system cannot see the train. It needs sensors to report when a locomotive or item of rolling stock enters or leaves a section of track.

On Holgate, that information was needed to:

• Identify whether a section of track was occupied.
• Change signals in response to train movements.
• Protect routes against conflicting movements.
• Manage trains entering and leaving staging areas.
• Provide feedback for automated sequences.

The initial requirement sounded simple: detect when a train entered a section and report that information to the layout-control system.

As the number of detection sections increased, however, it became clear that the detector itself was only one part of the problem.

Choosing the Right Detection Method

During the development of Holgate, several forms of train detection were considered and tested.

Each method had advantages, but each also introduced practical limitations.

Infrared Detection

Infrared sensors can detect a train as it passes over a sensor or interrupts a beam.

Their performance can, however, be affected by ambient light, sensor positioning, vehicle colour and the amount of infrared light reflected by different surfaces.

A system that performs reliably under workshop lighting may behave differently once installed on a completed layout or operated in another environment.

Hall-Effect Detection

Hall-effect sensors detect magnets fitted beneath locomotives or rolling stock.

This can provide accurate positional detection, but it depends on every train having a correctly positioned magnet.

For a layout using locomotives from several operators, fitting and maintaining magnets on every vehicle was not a practical solution.

Current Detection

Current detection monitors the electrical load being drawn from a section of track.

When a locomotive or suitably equipped item of rolling stock enters that section, the detector recognises the current being consumed and reports the section as occupied.

It does not rely on ambient light, the colour of the vehicle or a magnet fitted beneath the train.

For Holgate, current detection proved to be the most practical basis for dependable occupancy feedback.

The earlier stages of that process are covered in our related article: How Holgate Solved Train Detection for DCC-EX Automation .

Choosing Current Detection Was Only the Beginning

Once current detection had been selected, the next challenge was connecting a growing number of detection sections to the layout-control system.

Our original arrangement used separate detector circuits connected to separate input-expansion hardware.

During early testing, this worked well. As more detection sections were added, the amount of wiring, the number of boards and the number of interconnections also increased.

The issue was not that current detection had failed, nor that I2C could not be used. The problem was the growing complexity of the complete installation.

Every additional board meant more connectors, more wiring and more possible points to investigate when an input did not behave as expected.

Experience showed that the detector and its input interface needed to be treated as one complete system rather than as separate pieces of hardware.

Why ACI Decided to Build Its Own Detector

Some current detectors are designed for traditional block signalling, while others are intended for proprietary feedback buses or particular control systems.

Many provide only a small number of detection channels, which can result in several boards being installed together with separate input-expansion hardware.

We could not find a single product that quite matched the way we wanted to organise detection, input expansion and local indication.

Rather than redesigning our ideas around the available hardware, we chose to create a model railway current detector shaped by the problems we had encountered ourselves.

The finished product needed to be:

• Sensitive enough for small-scale locomotives.
• Capable of monitoring several track sections from one unit.
• Easy to connect to compatible control hardware.
• Simple to diagnose during installation and maintenance.
• Protected inside a proper enclosure.
• Suitable for both home and exhibition layouts.

Those requirements became the basis of the ACI 16-channel DCC current detector.

Designed Around Real Layouts

Holgate revealed the problem on a large scale, but the detector is not being developed only for exhibition railways.

A home layout may have fewer detection sections and shorter cable runs, but the same practical requirements still apply.

Easy to Organise

The ACI detector combines sixteen detection channels and the input interface within one unit.

This reduces the amount of separate hardware required beneath the layout and makes each group of detection sections easier to organise, connect and identify.

The detector communicates using I2C, allowing its channel states to be passed to compatible control hardware through a clearly defined interface.

I2C installations still need to be planned sensibly, particularly where longer cable runs are involved. Additional interface hardware can be used where the bus requires support over greater distances.

For most home layouts, positioning the detector close to the track sections it monitors helps keep track wiring shorter and the installation more organised.

Ready to Grow

A modeller may begin with only a few occupied sections and later add hidden sidings, block signalling or automated routes.

A single-channel or four-channel detector may be suitable for a small application, but the number of required inputs can grow quickly once more advanced control is introduced.

Using several smaller boards increases the number of mounting points, power connections, data connections and separate components that need to be installed.

Sixteen channels provide enough capacity to cover a useful area of a layout while keeping the unit compact enough to mount beneath the baseboard.

A smaller railway does not need to use every channel from the beginning. The spare capacity simply leaves room for the layout and its control system to develop over time.

On a larger railway, additional units can be installed in logical groups close to the areas they monitor.

Easy to Maintain

Installing a detection system is only one part of the job. It also needs to be understandable when something does not behave as expected.

Fault finding beneath a baseboard can be frustrating, particularly when several boards, connectors and software settings are involved.

Each detector channel therefore has its own status LED, allowing the modeller to see immediately whether a track section is being detected without first connecting a computer or opening configuration software.

This makes it easier to separate a track-detection problem from a wiring, communication or software issue.

When you are beneath a baseboard trying to trace a fault, small details like that matter.

Designed as a Complete ACI Product

The detector is being developed as a complete ACI product rather than as an exposed circuit board.

The current boards are still undergoing testing, and the enclosure has yet to be finalised.

The finished case will be designed to protect the electronics while keeping the terminals and status indicators accessible.

This is important beneath any layout, where exposed boards can be vulnerable to loose wires, tools, scenery materials and accidental contact.

The enclosure will also provide clearer channel identification and help create a more organised installation.

Our aim is to supply a finished product that can be installed and used, rather than leaving the modeller to source a board, create a mounting system and work out how to protect it beneath the layout.

How the Detector Fits Into a Layout

Each channel monitors an electrically isolated section of track.

Normally, one rail is divided using insulated rail joiners or carefully cut gaps. Power to that section is then supplied through one channel of the detector.

When a locomotive or another current-consuming vehicle enters the section, the detector changes the state of the corresponding input.

What happens after that depends on the layout and the control system connected to it.

The occupancy information could be used for:

• Block signalling.
• Hidden storage-yard occupancy.
• Automated route control.
• Computer-based layout control.
• Train-position feedback.
• Accessory and animation triggering.

Why This Product Matters to ACI

When ACI Model Railways began, our main focus was making DCC-EX easier to adopt through complete, ready-to-run command stations.

A command station is the centre of a layout-control system, but it is only the beginning.

As modellers become more confident, many naturally want to add signals, point control, train detection and automation.

That creates a gap between owning a command station and building a more complete layout-control system.

The current detector is the first ACI-developed product intended to help bridge that gap.

It represents more than another accessory. It marks the beginning of ACI designing its own model railway electronics based on practical operating experience.

The lessons behind this product came from years of helping to build, operate and maintain Holgate, along with the experience gained supporting other modellers through ACI.

Our aim is not simply to increase the number of products available on the ACI website.

It is to develop useful components that solve problems we have encountered ourselves and that we can confidently explain, test and support.

Over time, we intend this detector to become part of a wider ACI range of layout-control hardware designed to work together.

Although the detector was developed alongside our work with DCC-EX and EX-RAIL, it is an independently developed ACI product rather than an official DCC-EX product.

Currently Under Test

The first assembled detector boards have now arrived and are undergoing practical testing before the product is released.

Testing will include:

• Detection using N and OO gauge locomotives.
• Testing with low-current locomotive decoders.
• Operation with several occupied channels at the same time.
• I2C addressing and communication.
• Long-duration reliability testing.
• Testing with DCC-EX and EX-RAIL.
• Installation and terminal-access testing.
• Enclosure development and fit testing.

The final specification will be confirmed only after practical testing has been completed.

Once the case has been designed and the detector has completed testing, we will be able to publish photographs of the finished product together with more detailed installation information.

What Happens Next?

Once testing has been completed, we will publish further information covering:

• Full technical specifications.
• Track wiring diagrams.
• Detector sensitivity and supported scales.
• I2C addressing and configuration.
• DCC-EX and EX-RAIL examples.
• Enclosure and mounting details.
• Practical installation guidance.

The development process will also help shape the other layout-control products currently being planned by ACI.

Holgate taught us that reliable train detection is not simply about knowing where a locomotive is. It is about giving the layout the information it needs to make informed decisions.

The ACI current detector is our first step towards making that technology easier to install, easier to understand and easier to expand.

We built it because we needed it ourselves, and because we suspected other modellers were facing many of the same challenges.

We hope it helps solve the same problem on your railway.

We will continue to update this article as testing progresses. Once the finished detector and its product page are available, a direct link will be added here together with the final specification, installation guidance and ordering information.

You can read more about the layout that inspired the detector in: How Holgate Solved Train Detection for DCC-EX Automation .

For more information about ACI Model Railways and our current control products, visit our DCC-EX information and support hub.

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