Current Sensing and Occupancy Detection for DCC

I’ve had several projects going on more or less simultaneously over the last 6 weeks in connection with the build-out of the 1st module of the L&NC. One project has been to further refine block occupancy detection in DCC to achieve 1 mA detection sensitivity—the sensitivity needed to detect rolling stock equipped with a 10kΩ resistor wheel set. High sensitivity occupancy detection will be especially useful in the Red Bluffs yard part of the layout I’m working on right now

This part of the L&NC is a Yard/Staging facility with a roundhouse & turntable, so it has lots of blocks

Commercial block occupancy detection systems for DCC typically use current transformers to sense current draw from locomotives or resistor wheelsets. The sensors are usually attached to stand-alone logic boards that turn outputs on or off to signal occupancy detection. Typically, these outputs are used to directly power signals. The December 2016 issue of Model Railroader Magazine included an article (Build a signal system with Arduino micro-controllers) where the author connected the outputs of RRCir-Kits BOD boards to an Arduino Mega, and ran his signals from there.

I’ve been avoiding solutions like that because of the high cost and inflexibility of commercial detection systems. All I need is a way to sense very low current flow; the Arduino can do all the rest of the work.Thinking about the issue further, I wondered why not just use current transformers as sensors that can be directly read by an Arduino?

Finding Experimental Sensors

Current Transformer

The type of current transformer useful in model railroading are designed to allow you to wrap a wire (e.g. track feeders) around it through the central hole making the wire the “primary” coil of the transformer. The secondary coil produces an A/C current, that you can measure, proportional to the A/C current running through the primary wire.

I had seen little about using CT sensors with Arduino until I bumped into a seller on Amazon of boards with current transformers marketing them as Gikfun DIY 5A Range AC Current Transformer Module for Arduino. The assembled boards were about $7.50 each, a bit expensive, but then I found a 5 sensor unassembled kit from the same vendor for just under $9. Less than $2 per sensor is my price point for BOD sensors, so I bought the kit to see what i could do with it.

Gikfun Current Transformer Kit

The kit consisted five current transformers, five 82Ω resistors, connector pins and five 19mm x 19mm pc boards on a break-apart strip, drilled and traced for the parts. The resistor is placed across the transformer leads, placing a load (I’ve seen it referred to as a “burden”) on the transformer enabling current flow. No instructions on how to use the sensor were anywhere to be found.

So I went with the methods I already know work with ACS712 sensors. I set up some feeders from the DCC system to my cleaning track, wrapping one of the feeders around the CT coil three times. I hooked leads from the sensor board to pin 0 and ground of my Mayhhew Labs Extended ADC Shield, then threw together a test sketch with the current reading utilities I’d previously used (adapted for the shield).

A Simple CT Sensor Test Rig

I “let her rip” and watched the readings go by on the serial monitor.

Oh My! Putting a DCC equipped locomotive on the track and turning track power on, I was rewarded with a clear reading even though the decoder was idling and all lights were off.  Bring up the lights and the readings jumped up appropriately. Then, taking the loco off the track, I tried shorting the track with a 10kΩ resistor: an immediate and clear response in the readings told me this was going to work.

Test readings in single ended mode using a Mayhew Labs 14 bit ADC Shield. Everything from a 10k resistor shorting the track, to different stages of locomotive operation are clearly shown by the sensor outputs.

I tried it again, this time connecting the CT sensor to A0 and ground on my UNO, and got different but workable results. Amazing!

The readings through the UNO’s built-in ADC are similar, especially at the low end. But notice that the scaling, which is spot-on correct in the first example, fails at higher current levels here. Evidently, the greater bit depth of the Mayhew Labs Shield (14 bits) vs the built-in ADC’s (10 bits) makes a difference where absolute accuracy is concerned. Our purposes are much cruder — we just want to know when a minimum current (1 mA) is flowing. For that purpose direct-to-UNO works fine.

Testing current sensing with and InterMountain EMD F3A

Wait! This Shouldn’t Work!

You see, I was using the ADC in “single-ended”, “uni-polar” mode so that it would read a single input in the range of 0 to +5 volts; this is the same way Arduino analog inputs are read by the built-in ADC. However, the signal from the CT is an A/C signal that the ADC (in single-ended, uni-polar mode) will not be able to read during the negative voltage part of the cycle. Nevertheless, it works with both the built-in and the external ADC, even though it shouldn’t.

I was puzzling as to why, apparently, I was able to cheat when it occurred to me that the difference from ordinary applications was probably the frequency of the signal. Ordinary A/C is a 60 Hz cycle (60 cycles per second).  That is slow compared to the rate at which an Arduino ADC can sample, so a lot of samples would be at 0 during negative phase of the cycle. This would throw calculations off.

DCC is an 8kHz  (8000 cycles per second) cycle which is very fast. My Mayhew Labs ADC can sample at a maximum rate of 3kHz, which is somewhat faster than the Arduino built-in ADC but still slower than the DCC cycle. So even at the fastest rate every ADC read will span multiple DCC cycles, resulting in the ADC returning a positive reading every time.

Or something along those lines is going on.

Mayhew Labs Extended ADC Shield on an UNO, reading the test CT sensor in differential mode.

For hard core accuracy, a CT should be read in differential, bi-polar (+/- voltage) mode, requiring a 14 or 16 bit external ADC with that capability. In differential mode, each CT lead is attached to an ADC port, then the two ports are compared and their difference reported. This effectively captures the entire A/C cycle. Read in differential mode, CT sensors produce stable, high accuracy readings. During testing with my 14 bit ADC I was able to sense down to near 400µA in differential mode, the draw of a 20kΩ resistor at 11 volts. Now that is sensitive!

Same test sequence using the Mayhew ADC to read the sensor in differential mode. Notice the increased accuracy, stability and sensitivity.

Next Steps

The Gikfun 5 sensor kit is a viable sensor package where you will be watching no more than a few (up to the 6 or 8 analog ports on your Arduino or Mayhew Labs ADC shield) blocks. For small installations, buy these sensors and use the ADC in your favorite Arduino board to read them.

For larger installations, the external ADC shield is worth the cost because it is fast and more accurate.

But what if you are watching more blocks than you have analog ports for? That is a bit of a problem, especially if you don’t want to have to use a new microcontroller for every 8 blocks. The module I’m working with has this problem, because it is a yard zone (with turntable and roundhouse) with 24 blocks, and I want just one Arduino board to listen to them all and run signals for this part of the layout.

In the next post, I’ll go into a solution to the problem and install the system.


My experiment was complicated by the fact that I was testing different ADC’s and different connection methods. Below is some basic code for testing a CT sensor such as the Gikfun units discussed above by directly attaching one lead to an Arduino analog port, and the other lead to ground. This is the simplest way to use CT sensors.

The code is built around current sensing techniques originally developed for ACS712 sensors. For more in-depth discussion of that development process, see these posts – 1 , 2, 3 and 4. ADC’s (analog-to-digital converters) read the voltage produced by a sensor which, in the case both current transformers and ACS712 sensors, is proportional to the current it is measuring.

The challenge with sensing alternating current comes from the fact that at any given moment when a sensor is read, the current could be anywhere in its cycle with a voltage somewhere between -Vmax and +Vmax (the nominal A/C voltage). Since we are sampling a bipolar wave form, it is necessary to take multiple samples spanning multiple cycles, then calculate the current from the samples using the Root Mean Square algorithm. This is the generally accepted method for determining A/C voltage and current from digital samples.

In addition to solving the basic problems of sampling an A/C signal, the RMS algorithm cuts through signal and sampling noise, a significant problem with ACS712 sensors when measuring low current. Since CT sensors are far less noisy, we can get accurate readings with fewer samples; but we still need the RMS method to calculate current flow.

To make the process I’ve described accurate across an array of sensors, I developed a calibration routine. All ADC’s will produce some minimal reading for a sensor even when the sensor is not producing a signal. This is a form of noise that has to be filtered out. Also, some sensors (such as the ACS712) produce a signal in the absence of any current to measure — this too is noise that has to be filtered.

Each ADC port / sensor combination will produce a unique amount of noise. The purpose of the calibration routine is to measure and record the noise level — I call it ADC Zero — for each port/sensor. This is recorded both as raw mV, and as a calculated current in mA. Later, the measured noise is subtracted from each raw reading (in mV) to determine what current (if any) has been detected. Then, when current is detected, I use a multiple of the ADC Zero calculated current to establish an occupancy threshold. This method allows me to overcome sensor differences and manage the sensor system in a uniform way.

// CT Sensor test
// Using UNO built-in ADC to read the sensor

#define VERSION "1.006"
#define SYS_ID "CT Sensor Test - Direct to UNO ADC"
const int adcpin = 0;

// Sampling Parameters
const unsigned long sampleTime = 2000UL; 
const unsigned long numSamples = 100UL; 
const unsigned long sampleInterval = sampleTime/numSamples; 

#define SENSITIVITY 5000

// variables to hold sensor quiescent readings
float aqv;  // Average Quiescent Voltage; e.g. ADC Zero
float aqc;  // Average Quiescent Current; 

void setup()
  Serial.println(String(F(SYS_ID)) + String(F(" - SW:")) + String(F(VERSION)));
  Serial.print("\nCalibrating the sensor at pin ");
  aqv = determineVQ(adcpin); 
  Serial.print("AQV: ");
  Serial.print(aqv * 1000, 4);
  Serial.print(" mV\t");
  aqc = determineCQ(adcpin, aqv);
  Serial.print("AQC: ");
  Serial.print(aqc * 1000, 4);
  Serial.print(" mA\t");
  float sense = (aqc * DETECTION_MULTIPLIER) - aqc;
  Serial.print("Detection Sensitivity: ");
  Serial.print(sense * 1000, 3);
  Serial.println(" mA\n\n");

void loop(){
  float current = readCurrent(adcpin, aqv);
  float delta = abs(aqc - current);
  bool occupied = delta > ((aqc * DETECTION_MULTIPLIER) - aqc);
  Serial.print("Current Sensed: ");
  Serial.print(current * 1000,3);
  Serial.print(" mA\t");
  } else {
    Serial.println("Not occupied");

// Current Sensor Functions
float readCurrent(int pin, float adc_zero)
  float currentAcc = 0;
  unsigned int count = 0;
  unsigned long prevMicros = micros() - sampleInterval ;
  while (count < numSamples)
    if (micros() - prevMicros >= sampleInterval)
      float adc_raw = (float) analogRead(pin) - adc_zero; // sensor reading in volts
      adc_raw /= SENSITIVITY; // convert to amperes
      currentAcc += (adc_raw * adc_raw); // sum the squares
      prevMicros += sampleInterval;
  float rms = sqrt((float)currentAcc / (float)numSamples);
  return rms;

// Calibration
// Track Power must be OFF during calibration

float determineVQ(int pin) {
  float VQ = 0;
  //read a large number of samples to stabilize value
  for (int i = 0; i < CALIBRATION_READS; i++) {
    VQ += analogRead(pin);
  return VQ;

float determineCQ(int pin, float aqv) {
  float CQ = 0;
  // set reps so the total actual analog reads == CALIBRATION_READS
  int reps = (CALIBRATION_READS / numSamples);
  for (int i = 0; i < reps; i++) {
    CQ += readCurrent(pin, aqv);
  CQ /= reps;
  return CQ;

Adjusting the Sketch

The SENSITIVITY and DETECTION_MULTIPLIER constants are the main values you can manipulate to adjust the sketch.  The SENSITIVITY constant defines the conversion from raw mV to sensed current in mA; raise that number to reduce the current reading and decrease dynamic range of the sensor readings (lower it to do the reverse).  The DETECTION_MULTIPLIER determines when a current reading is high enough to mean that a block is occupied. Try different values for these items and see what happens. The interaction between these two constants is the primary determinant of how the sketch performs.

If you have having stability trouble with your readings, try increasing the number of samples taken on each pass (numSamples) and/or changing the sampleTime variable that controls the interval between samples.

Bear in mind that for low current sensing, the 10-bit ADC in an UNO is just barely sensitive enough to work with a CT. For low current sensing you’ll need to adjust the variables to get a minimum of false readings.  The values given in the sketch above are what worked for me with the Gikfun CT coil and a particular UNO to achieve stable detection.  Expect different readings and different variable values for different brands of CT coils, and different Arduino boards. Working with large numbers of CT sensors, I can say that a given Arduino board and a given brand/model CT, using the same value for the load resistor, will generally produce the same readings.



Happy Independence Day weekend to my US readers. Well its been several months where time for railroading has been limited. Top of the list, our nine-year-old beagle developed serious back/neck problems a few months ago that ultimately required surgery.

Lewis the beagle; 2 weeks after back surgery.

Lewis the beagle; 2 weeks after back surgery.

Little fellow is recovering and doing well with his rehabilitation program–exercises given to us by the specialty vet. Things like stairs are going to be a problem for him for a while, so he won’t be joining me in the layout room, as he usually does, in the near future.

Even so, I have some L&NC construction news along with updates on on-going projects.


ACS712 – How Low is Low Current

ACS712 Board

ACS712 Board

I’ve been exploring the limits of low current sensing with ACS712 sensors. Sparkfun markets a sensor board combining an ACS712 with an OP Amp that they characterize as a low current sensor. I had no luck calibrating and using that sensor; if someone reading this has used it successfully, please leave a comment explaining how you calibrate and use it.  Additional experiments with Op Amps, including creating a fairly stable instrumentation amplifier, have not been encouraging though I haven’t entirely given up and have a few possible circuit designs yet to try.

The fundamental constraint with ACS712 sensors is that their output noise inhibits current sensing below 20 mA (officially, Allegro says that you can’t detect below 20 mA without a specially designed version of the chip; which they will happily work on for a paying customer). At higher currents, the sensor works pretty much as advertised and can be used with or without filtering. At and below 20 mA it is difficult to tell the signal from the noise without some sort of mathematical filter; the RMS method I use can detect down to around 15 mA with an UNO’s 10 bit ADC.  With a Mayhew Labs 14 bit ADC, I seem to get a little more sensitivity, around 12 mA, using the RMS algorithm.

Below 12 mA noise takes over, at least so far as short term measurements go. So I’ve been looking at data collected over longer time periods and have a preliminary method that can discern a low current signal down to around 8 mA.  In essence, a low current change causes the long term output noise range to shift up or down depending on whether the current is increasing or decreasing. Capturing this range shift should improve practical detection sensitivity considerably. Its promising, but I still have kinks to work out of the system.

Is 1 mA sensitivity—the holy grail for detecting a single 10k resistor wheelset—possible with an off-the-shelf ACS712 sensor?  Probably not, and that may matter to people who want to use off-the-shelf 10k resistor wheelsets.

That does not make these inexpensive sensors useless. If you don’t mind making your own resistor wheelsets, then a single 1.5k resistor (assuming one resistor wheelset per car; two wheelsets at 3k—in parallel producing a net 1.5k resistance—one at each end of the car, would be a better idea for a bunch of reasons) will be detectable while still drawing under 1/10 watt (at 11.5 volts DCC n-scale standard; higher voltages allow for more resistance). 100 wheelsets would draw less than one Amp total current. More on that when I get the system operational.

Nevertheless, I’m curious whether a 16 bit ADC will help further resolve the low current signal. I’ll be giving that a try this summer so we will see.

Construction Progress

Laying Track-Bed at the Red Bluff Yard.

Laying Track-Bed at the Red Bluff Yard.

I finished drawing in the track plan, including some additional yard space, on the lower level modules and started in on laying roadbed and track.

Roughed In Turntable

Roughed In Turntable

Roughing in the turntable was fairly straight forward. After preliminary rough siting, I installed the roundhouse base and drew in the track center-lines. That located the turntable center (the intersection of the track center lines), enabling me to cut out an appropriately sized circle to accommodate a pit I’ve already created using a plywood base and styrene strips.

Building the Turntable Pit with Styrene Strips glued to a plywood base.

Building the Turntable Pit with Styrene Strips glued to a plywood base.

The turntable project is a major layout item in its own right and will be the subject of multiple future posts.

Laying Track for the Yard

Laying Track for the Yard

It took a few sessions, but I laid all the lower level track except for the turntable/roundhouse area (which I will do when I install the turntable) and joined it with the helix.

Tracklaying on Lower Level Done.

Track-laying on Lower Level Done.

Track Laid and Ready for Painting and Ballast.

Track Laid and Ready for Painting and Ballast.

For those interested in the nitty-gritty of track laying techniques, I used adhesives rather than mechanical fasteners for all roadbed and track. I use Liquid Nails for projects (water based) to adhere foam to wood, and anything else to foam. For the track I used DAP ALEX clear latex silicone caulk, deploying T-pins to hold the track in position while the caulk cures (just a few hours). What I like most about this method is that once cured the adhesive is completely invisible creating a nice base upon which to detail the track. Now that I’ve used clear caulk for track laying, I can’t imagine using a grey (and definitely not white) product unless the color closely matches the intended ballast color.

I don’t know if you can tell from the photograph of the whole lower level above, but the lighting system using Addressable LEDS that I’ve devised is working rather well (granted the wiring needs  tidying up; I’ll get to that eventually).


Block Occupancy Detection for DC and DCC, Part 4

Through the first three parts of this series (links to part 1, part 2 and part 3) I’ve experimented with and refined to a degree the use of ACS712 current sensors for block occupancy detection. The system works well in DC mode, lighting up whenever a running DC train enters or is powered up in a block; program logic “remembers” block states when power is off for control reasons. My DC locos typically start drawing 30 – 50  mA when the lamps come on.

In DCC mode, ultra low current detection becomes an issue and the off-the-shelf ACS712 sensors don’t meet every need. My BLI DCC/Sound Locomotive draws enough power to be detected with the current system in idle mode, all sound and lights off. However, some standard DCC decoders draw so little current in idle mode that they are not consistently detectable. Further, constant track power in DCC invites us to find a way to sense any object drawing some minimal increment of current — such as 1.1 mA for a 10KΩ resistor wheelset on an 11 volt (in N scale) feed. There are a number of obstacles to achieving 1 mA  sensitivity with ACS712 sensors.

The Challenges

First, there is a question of how low a current the ACS712 can resolve. Theoretically, because the chip responds in a ratiometric way to current input, any amount of current will provoke a proportional output response. In reality, the output of the chip is a little noisy, and the noise masks the low current response.

Allegro MicroSystems says that the best resolution with the optimum filter capacitor is 20 mA. There is some general agreement among internet cognoscenti that with some amplification of the signal from the ACS712, greater sensitivity should be possible.

Second, the 10 bit ADC built into most Arduino boards has a maximum resolution of 4.88 mV (5 volts / 1024 steps), limiting the minimum detectable current to 26 mA (4.88 mV resolution / 185 mV/A sensitivity). Even the 12 bit ADC found in the Due and Zero boards is not quite close enough, with a maximum resolution of 1.22 mV (5 volts / 4096 steps), resulting in minimum detectable current of 6.6 mA.

A Better Analog-to-Digital Converter

Of the two problems, this is by far the easier problem to solve by using an external ADC with a 14 or 16 bit resolution. A 14 bit ADC has a maximum resolution of .305 mV, which is a good resolution for consistently detecting current at the 1.6 mA level or higher; a 16 bit ADC would provide 76 µV resolution and current detection in the mid to high µA range.  For those wanting to detect a single resistor wheelset, 16-bit is the way to go.

For my purposes and goals, I’m going to see how it goes with 14 bits so I’m not pushing resolution past the point of usefulness at this stage. My thinking is that 1.6 mA should be sufficient to detect a pair of resistor wheels. Two 10kΩ resistors in parallel have a resulting resistance of 5kΩ, which at 11 volts will draw 2.2 mA (@16 volts, 3.2 mA). Assuming I can overcome the noise problem to the point where 14 bits of resolution provides accurate current detection, the final step would be to go to 16 bits.

I looked at several ADC products and settled on the Mayhew Labs Extended ADC Shield, which is also available at Amazon. I chose this for several reasons: it supports 8 inputs, comes in a shield format and (perhaps most importantly) offers enough bandwidth to be able to sample multiple sensors at a reasonable rate.

Mayhew provides a code library for reading a variety of sensor types through the shield. It is fairly straight forward to use, and the shield is a very good quality piece of gear. I have found other ADC boards, but none with the unique capabilities of this one.

Mayhew Labs Extended ADC Shield

Sparkfun Low Current Sensor

Sparkfun sells a breakout board as a low current sensor using the ACS712 sensor chip plus an operational amplifier to amplify the signal. This is the only commercial solution of its type I’m aware of, so I bought one to check out and test. The schematic for the board can be seen here.

SparkFun Low Current Sensor Breakout – ACS712. This image is from an older version; different trimpots are used in the current version.

Although the circuit is promising, this board is really an experimental item and not suitable for production use on a model railroad. Part of the problem is that it does not provide — and is not drilled for — standard screw terminals for attaching the input current source, adding additional challenges to deployment. On the input side (I+ / I-) it is drilled for a pair of header pins and two larger holes with contact pads. The control side (VCC, OUT, GND) is drilled for standard headers.

Secondly, because this is an experimental board, it has two trim-pots for configuring the Op-amp.  Sounds great, but in practice this setup doesn’t work well. Add sketchy instructions for setting and using the board and I think most will find this board impractical for large scale use, though an interesting experimental tool.

Learning By Failing

So when the board arrived, I soldered on some header pins and hooked up the sensor to an UNO with the Mayhew ADC Shield, then attempted to configure and use the board. Here are the setup instructions in their entirety:

“To calibrate, first set the output offset to the desired level (with zero current on the sense lines, read output with a DVM). Then with a known current input (a 100mA limited supply works well for this), set the output deflection with the gain pot. Sensitivity is then calculated as (Vref – Vdeflect)/(current input).”

The “output offset” is (presumably, though not literally stated) adjusted with the vref trim-pot.  But what “level” is “desirable” for this application?  The second instruction is even weirder, since you are not going to short out a power supply by directly attaching it to the sensor without a load, and its the load that determines the current flow. I’m sure they meant to say something like “supply a load with a known current draw.”

I tinkered fruitlessly with the trim-pots for hours, testing various theories about how it should work and getting nothing intelligible out of the sensor. The gain pot is straightforward enough, resulting obvious changes to readings as the trim-pot is adjusted. Its the vref adjustment that mystified me and seemed to have no logic. I  did notice that when setting it there was a point where the output would suddenly, but only briefly, drop to O. That zero point is so finely specific that it is essentially impossible to set the trim-pot at that level; it always ends up off one way or another. That was a clue, but I did not yet understand.

Back to School

I found a nice tutorial about Op-amp circuits and, after a while started to understand the circuit and comprehend the problem. If you don’t understand Op-amps (who does among those of us not trained in electrical engineering?), check out the tutorial before reading further here.

The Op-amp circuit Sparkfun uses is a “voltage subtractor,” the intent of which is to  subtract the input voltage from the reference voltage and amplify the difference. This is the right choice—I think—but the implementation is wrong, at least for my purposes.

Unique ACS712 Properties

Many sensors that one might use with an Arduino are straight forward linear output devices where the output voltage ranges from 0 to 5volts in proportion to an input (light, temperature, etc.).

What makes the ACS712 different is that it senses both polarity and current of the measured input.  The “quiescent voltage” we measure when calibrating a sensor is the sensor’s “zero” point: ideally VCC/2 — midway between 0 and VCC (nominally 5 volts).  When the polarity is one way the measured current is represented by a value above VCC/2; when the polarity is the opposite, the sensor produces a value under VCC/2.

That means that we are only interested in the offset between the sensor output and its quiescent value. The software calibration routine I’ve been using seeks to get the most accurate average mid-point value possible to enhance the accuracy of the offset measurement. Amplifying that offset should be the route to gaining sensitivity.

Shouldn’t vRef be VCC / 2?

It may well be that the vref adjustment has value when measuring DC current with a fixed polarity. However, I’ve seen no math or theory in support of that proposition. For measuring DCC current, which is an 8kHz alternating polarity wave form, there is only one correct reference voltage for comparison to the ACS712 output: VCC/2.  So, Sparkfun had the right idea using a trim-pot as a voltage divider circuit to generate the reference voltage, but the trim-pot inhibits setting vref to VCC/2 because it is too imprecise to set the two resistor legs to exactly the same value.

What Now?

The Sparkfun sensor experiment was a bust in that it did not produce a usable result.  But it did help me research and think about what it is I need an operational amplifier to do in order to successfully amplify the ACS712 signal. Sometimes we learn more from failure than we do from success.

The next step is the set up an Op-amp with a fixed reference voltage of VCC/2, then continue experimenting from there.  I have the parts. In a few weeks, if time permits (between work and our ailing Beagle, time is tight right now), I should have some sort of result.