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<\/a>This part of the L&NC is a Yard\/Staging facility with a roundhouse & turntable, so it has lots of blocks<\/p><\/div>\n
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<\/a> included an article (Build a signal system with Arduino micro-controllers<\/strong><\/em>) where the author connected the outputs of RRCir-Kits BOD boards to an Arduino Mega, and ran his signals from there.<\/p>\n 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?<\/p>\n Current Transformer<\/p><\/div>\n 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.<\/p>\n 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<\/span><\/a>. The assembled boards were about $7.50 each<\/a>, a bit expensive, but then I found a 5 sensor unassembled kit<\/a> 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.<\/p>\n Gikfun Current Transformer Kit<\/p><\/div>\n The kit consisted five current transformers, five 82\u03a9 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.<\/p>\n So I went with the methods I already know work with ACS712 sensors<\/a>. 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<\/a>, then threw together a test sketch with the current reading utilities I’d previously used (adapted for the shield).<\/p>\n A Simple CT Sensor Test Rig<\/p><\/div>\n I “let her rip” and watched the readings go by on the serial monitor.<\/p>\n 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.\u00a0 Bring up the lights and the readings jumped up appropriately. Then, taking the loco off the track, I tried shorting the track with a 10k\u03a9 resistor: an immediate and clear response in the readings told me this was going to work.<\/p>\n 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.<\/p><\/div>\n I tried it again, this time connecting the CT sensor to A0 and ground on my UNO, and got different but workable results. Amazing!<\/p>\n 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.<\/p><\/div>\n Testing current sensing with and InterMountain EMD F3A<\/p><\/div>\n 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.<\/p>\n I was puzzling as to why,\u00a0apparently, 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).\u00a0 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.<\/p>\n DCC is an 8kHz\u00a0 (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.<\/p>\n Or something along those lines is going on.<\/p>\n Mayhew Labs Extended ADC Shield on an UNO, reading the test CT sensor in differential mode.<\/p><\/div>\n 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 400\u00b5A in differential mode, the draw of a 20k\u03a9 resistor at 11 volts. Now that is sensitive!<\/p>\n Same test sequence using the Mayhew ADC to read the sensor in differential mode. Notice the increased accuracy, stability and sensitivity.<\/p><\/div>\n The Gikfun 5 sensor<\/a> 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.<\/p>\n For larger installations, the external ADC shield is worth the cost because it is fast and more accurate.<\/p>\n 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.<\/p>\n In the next post, I’ll go into a solution to the problem and install the system.<\/p>\n 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<\/a> 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.<\/p>\n 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<\/a> , 2<\/a>, 3<\/a> and 4<\/a>. 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.<\/p>\nFinding Experimental Sensors<\/h4>\n
<\/a><\/a><\/a><\/a><\/a><\/a>Wait! This Shouldn’t Work!<\/h4>\n
<\/a><\/a>Next Steps<\/h4>\n
\nCode<\/h4>\n