Detector Kits (Current, InfraRed, Light & Magnetic)       Code: RK-D2C, D2IR, D2L, D2M

A number of kits on this website need to be able to detect trains for their operation - these include

  • Crossing Controller RK-XCtlP
  • Analogue Shuttle Controller RK-ASCP
  • Model Railway Speedometer RK-MRSDP
  • Various Signal Controllers - RK-SIGCP and RK-SBP4

The kits described here support those kits by providing the detection required.

Detectors are offered using current, infrared reflection, light interruption, and magnetic sensing versions.

The light sensing device detects the interruption of light to a light dependent resistor (LDR) mounted between the rails caused by a train passing. It will work best if there is a reasonable and consistent level of light at this place. The LDR is mounted on a mini PCB. (See third down on right in the diagrams at left) It is installed through a hole cut through the baseboard in the centre of a sleeper so that the top of the LDR ends up level with the top of the cut sleeper.

The light detectors are the easiest to use and install, requiring a single hole between the rails. However, they may be subject to false triggering depending upon how the light may be interrupted at the detector site, and of course, will not work in the dark.

InfraRed reflection detectors work by emitting an infrared beam upwards from an infrared LED, and detecting any reflected infrared energy in an infrared photo transistor mounted immediately next to the LED. The detecting elements are again mounted on a mini PCB. These are only slightly more difficult to install, requiring an elongated hole between the rails. They will work in the dark, but require reflection from the rolling stock to be detetcted. A piece of aluminium foil fixed under the rolling stock adequately fulfils this requirement.

The current detector senses the current drawn by a locomotive in the detecting section. A short length of one rail only of the track, ideally about a locomotive length minimum, is insulated at both ends, and the feed wire to this section of rail is passed through the sensing transformer on the detector unit. When a locomotive enters this section of track, current passing through the sensing transformer causes the unit to operate. There is no actual connection between the track supply, and the circuitry of the detector. For convenience, the current sensing printed circuit boards (PCBs) have terminals to accept input from the track bus, and provide output (suitably passed through the sensing transformers) for connection to the track. The diagram (see left) shows the arrangement required for the crossing controller RK-XCtlP. Full details are given in the information which comes with kits.

This type of unit works best on DCC, and will continue to indicate the presence of a locomotive as long as it draws current in the section. (Pure DC will only be detected when there is a change in the DC current through the sensing transformer, that is, as the current turns on or off. Thus, for pure DC, the unit will theoretically operate only as the locomotive enters or leaves the section. However, there is usually enough "electrical noise" in most motors for the unit to detect continuously.) Current detectors may not work at very low speeds with DC control, depending upon the current drawn by the motor

For reliable current detection of a stationary locomotive in a DCC system, it may be necessary to put a resistance of a few Kohm across the wheels of the locomotive.

There is more involved in the installation of current detectors than other forms of detection, as this requires the cutting of one rail to provide isolation and the feeding of power to this section through the detector. Note however, that this does not interfere with the power supply - there is negligable loss of voltage in the detector. The current detector is very sensitive (down to a few milliamps) and will work without ambient light.

Magnetic detection requires drilling two small holes between the rails and cutting several sleepers to install a magnetic reed switch. This may be disguised by covering it with ballast.This form gives accurate spot detection, but does require the fitting of magents on all vehicles to be detected. It also will work without ambient light. Its main advantage is that it can selectively detect only certain vehicles - the ones with magnets fitted.

Current and magentic detectors can be effectively disguised, while there is some visibility of light and IR detectors.

Detector kits come as 2 way units - that is, 2 detectors per unit. For kits requiring four detectors, (RK-ASCP, RK-SBP4) two detector kits are needed. For RK-XCtlP for multiple tracks, one detector kit is needed per track.

The three kits apart from the current detecting kit use substantially the same main printed circuit board (PCB) with connections taken from that to the detecting elements (LDR, IR LED/transistor, or magnet.) Light and IR detectors have a sensitivity adjustment on the board for each detector.

Detectors may be mixed and matched - if a modeller is setting up a crossing controller say, and wishes to use say IR detection in a tunnel coupled with light detection on an open area of track, that is perfectly feasible.

Further information on all of these detectors is given in the instruction sheets provided with the kits.


Code: RK-D2C Dual Current sensing detector kit
Code: RK-D2IR Dual Infrared sensing detector kit
Code: RK-D2L Dual Light sensing detector kit
Code: RK-D2M Dual Magnetic sensing detector kit
All D2x Kits price: $22-50




Detector Relay Driver (2 channel) for D2 kits.         RK-DRD2



DRD2 is a kit which takes the output from any of the above detectors, and uses it to control a relay. Thus, external devices can be switched in conjunction with detection, up to approximately 50 volts, and 2 amps. The relay output is 2 pole, changeover contacts (DPDT)

The relay may be adjusted to stay "on" after the detector has ceased detecting for a period which ranges from a few to theoretically 128 seconds. The actual time depends upon timing accuracy in the microcontroller, and is set by means of trim pots (marked T2 and T1 in the diagram at left.)

The kit has 2 relays, and thus mates with any D2 kit to provide a 2 channel occupancy detection/control device.


Kit price for       RK-DRD2:$24-50



Click here to go to Aztronics web site

All kits available from :
Aztronics Pty Ltd       Phone 8212 6212
170 Sturt St,                     Fax 8212 4212
Adelaide, SA, 5000.                                
email:               sales@aztronics.com.au

Information:         David Crosby         david@railwaykits.net

Click Aztronics logo to go to Aztronics Web site, or Click to email     Aztronics     or     Kit designer



Click here to go back to Kits Summary Page

Last updated 3rd February, 2017.
(C) D. Crosby, August, 2010.