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Track Occupancy Detection Fundamentals - Part 2 <-You are HERE

Handbook v3.0: CHAPTER 2



The above test is an effective way to accurately measure detector sensitivity. However by itself, if you are using the OD or the DCCOD, it does not guarantee that your detector is totally functional. This is because the primary purpose of the LED built into the detector is to aide in setting each detector to its maximum possible sensitivity without indicating that a block is occupied if it is in fact clear. To make this adjustment easy and quick to accomplish, it is necessary to position this LED, electrically speaking, ahead of the detector’s built-in turn-on and turn-off delay circuitry. For example, if the LED was placed after the built-in delays, then you would need to wait up to 3.5s each time you rotated the potentiometer a small fraction, to see if the LED came on or off. This would make setting detector sensitivity a very long and impractical process.

Because of the electrical location of the LED, if it functions properly you can be assured only that the front portion of the detector is working properly, i.e. the portion before the circuit delays. Therefore, to totally check that the overall detector is operating correctly, it is important to check its output. This is easily accomplished by using the clip lead assembly shown in Fig. 2-9.

Fig. 2-9. Clip lead assembly for testing detector operation

To perform operational testing, use the clip lead assembly in Fig. 2-9 and with your track power turned on (and set to 12Vdc if using conventional DC), execute the following steps:

  1. Attach the positive end of the clip lead assembly to the +5Vdc power and attached the remaining clip lead to the output terminal screw, Vout, corresponding to the detector to be tested.*
  2. Assuming the detector is still adjusted to its maximum sensitivity setting, then occupying the block should light the clip lead’s test LED, as well as the LED on the detector.
  3. Conversely, when the block is clear, neither LED should be illuminated.

*Alternatively, for situations where you do not have your +5Vdc supply handy, you can change the resistor value in Fig. 2-9 to the 1KΩ value [brown-black-red] and attach the positive clip lead to the +12Vdc terminal on the ODMB rather than to +5Vdc. However, to avoid possible damage to any C/MRI inputs when using 12Vdc, it is important to make sure that the Vout terminal on the ODMB is not also connected to a C/MRI input pin. Anytime you apply more than +5Vdc to a C/MRI input you will very likely damage the input buffer IC which will need to be replaced. Also, if using the 5Vdc, its supply ground needs to be connected to the 12Vdc ground, which is the case when using a surplus computer power supply.

Successfully passing the last two steps indicates that your detector is functioning properly. While performing the above test, it is also very easy to verify that the two delays, as built into the OD and DCCOD, are functioning correctly.

For example, when you make a block occupied, you should see the LED on the detector light immediately and then about .75s later the output LED should light. Similarly, when you make the block clear, you should see the LED on the detector go dark immediately and then about 3.5s later the output LED should go dark.

Advantages of Using a plug-in MODULAR Detector per Block

Some brands of detectors include multiple detectors on a single circuit board. Such an approach can be tempting at first with potential cost savings. However, the added card complexity, e.g. double sided versus single sided board construction, can easily cancel out any potential savings. Also the added complexity of multiple detectors per card introduces a significant loss in modularity and the ability for easy fault isolation and replacement, and complicates repair problems. Other detectors are sold to be hard-wired right into the railroad while others use edge tab connections to which you will need to add the cost of the edge card connector. Also the latter are not set up to handle the high amperage track currents associated with our railroads.

In my mind you cannot beat having a separate plug-in detector per signal block that uses a truly heavy duty Molex-style connector, like the DCCOD and the OD. Trying to locate a short circuit along the track, or within the track wiring itself, is greatly simplified by using separate plug-in replaceable detectors per block. This is especially true when each plug-in module contains a built-in indicator LED to show when blocks are drawing current, i.e. occupied.

For example, if you experience a track short and a detector LED is lit with a block unoccupied, you have almost instantly located the block with the short. If you experience a short during an important open house or operating session, simply pull each of the detectors that have their LED lit within the offending area. Doing so one at a time, quickly defines which block, or OS section has the short circuit. This approach works even with DCC and its fast acting circuit breakers because the detector’s LED will repeatedly flash for an instant each time the circuit breaker tries to reset.

This separate plug-in replaceable feature is especially handy for Command Control layouts that otherwise tend to be wired together in one large block. Also, if you ever happen to have a detector failure, or you suspect a problem, it is a snap to unplug a detector and plug in a spare. Having only a few parts per card and having the single IC in a socket makes repair easy to accomplish.

In my opinion, the modular features achieved by plugging detectors into the ODMB are a very great advantage. Because both the OD and DCCOD can make use of the same motherboard, we will cover this next in this chapter.

Optimized Detector Motherboard (ODMB)

Because applications require one DCCOD or OD per detected track section or block, it is convenient, as with the general-purpose I/O cards, to have an OD motherboard (ODMB) into which the detectors are plugged.

Each ODMB accommodates 12 detectors; if more motherboards are needed they can be parallel connected with each of the three bus traces wired from one board to the next. If you need less than 12 detectors at a particular location, you can cut the ODMB into smaller pieces for easy decentralization around your layout. Ready-to-assemble cards are available from JLC Enterprises and assembled and tested cards, as well as complete kits, are available from SLIQ Electronics. The photo in Fig. 2-5 shows the parts layout for the ODMB and Table 2-5 contains the parts list.

Table 2-5 Detector Motherboard Parts List

(in order of recommended assembly)





4-40 x ¼ Pan-head machine screws (Digi-Key H142)


4-40 Hex nuts (Digi-Key H216) (soldered to underside of board)



5-pin Waldom straight-headers (Mouser 538-26-60-2050 or use

breakaway section of Mouser 538-26-48-1241 per text)

Author’s recommendation for supplier given in parentheses above with part numbers where applicable.  Equivalent parts may be substituted.

Regarding the header connectors, you can save considerable cost if you elect to use the Mouser 538-26-48-1241. These come with 24 pins and are designed so that you can easily snap them into smaller sections as desired, in the ODMB case a grouping of 5 pins each. By doing so you will need only 2.5 of the Mouser 538-26-48-1241 (2.5 x 24 = 60 pins) for each mother board you assemble.

The ODMB assembly steps are as follows:

□ Terminal screws. Insert 4-40 screws in the connection holes from the top (or component side of the board) for the 12 track connections, 12 detector outputs, +12Vdc, COMMON and the -12Vdc used for OD applications (which is the logic ground connection for DCC applications). Add 4-40 hex nuts on the trace side, tighten firmly, and solder the nuts to the circuit pads. Use spade lugs later on when you attach wiring to these screws and you will have yourself a nice heavy duty connection.

□ S1-S12. Hold each header firmly against the board and solder the two end pins first. Then examine each header to make sure it is flat against the board with the pins perpendicular to the board surface. If not then reheat the end pin joints until the header is firmly against the board. Once you have determined that each header is installed correctly, then solder the other three pins.

□ Adjacent trace test. Using a VOM set on R x 100, check the resistance between the COMMON terminal and the +12V and -12V (or logic ground for DCC applications) terminals. It should be infinite, an open circuit. Then clip one lead to the +12V terminal and touch the other lead to each of the track terminal screws for each card slot. Each should also read open circuit. Then, for each card slot, touch one lead to the track terminal and the other lead to the output terminal. Again, the test should read open circuit. Any reading close to 0Ω indicates a solder bridge between adjacent pads somewhere along the two bus lines under test. Locate it, remove it, and retest to be sure.

□ Cleanup and inspection.

For mounting, use six ¼”-long standoffs. The ODs need a ±12Vdc power supply while the DCCODs need only +12Vdc. Actually, any voltage between 5 and 15 will be satisfactory, but the nearer the high end the better. For OD applications, the positive and negative values must be of the same magnitude. Each detector draws about 10mA (not counting external current on the open-collector output connection), so a 1.5A supply handles up to 150 detectors.

DistributED or Central Mounting Detectors

Because most wiring connections between ODs, or between DCCODs, are common, I group multiple cards together using a motherboard, the ODMB. Each ODMB mounts up to 12 detectors. The ODMBs can be distributed around your layout anyway you desire, with two or more boards piggybacked together for locations where you need more than 12 detectors. The ODMB cards also can be cut into separate pieces for wider distribution.

If you prefer, you can also forget the ODMB and connect the ODs, or DCCODs, directly to your layout wiring. There are at least four different ways this can be accomplished:

  1. Substitute a 5-pin Waldom right angle header in place of the normally installed Waldom side entry connector (S2) and then attach a 5-pin Waldom terminal housing connector to your layout wiring. This way you retain the flexibility of being able to plug and unplug your detectors without the expense of the motherboard.
  2. When assembling your detectors, solder a screw clamp-type terminal strip with the correct .156” spacing between pins in place of the recommended S2 connector. For example, Mouser part numbers 651-1727049 or 158-P02EK381V5, both with 3.81mm spacing (equivalent to .150” spacing), will do the job. With this approach you have screw terminals mounted right on each of your detectors.
  3. Solder the layout wire directly to the pads provided on the OD and DCCOD instead of installing the Waldom side entry connector (S2). This is best accomplished by inserting the wire from the component side of the board and soldering on the trace side of the board. Once soldered, trim the excess length of the wire so it can not bend over and short against any nearby traces. Optionally, for the DCCOD you can simply loop the track wire through the transformer coil rather that making their respective solder connections.
  4. For the case where you already have the Waldom side entry connector on the detector card you may elect to solder the layout wiring directly to the mating header connector that would have been used in the ODMB.

Methods 1 and 2 are the most popular approaches for those not wishing to use the ODMB.


Because connecting devices, such as LEDs, lamps, C/MRI inputs and relays, to detector outputs is identical whether using the OD or the DCCOD, I will cover making the connections in this chapter rather than repeating it in the next two chapters. Fundamentally, the output line from the OD and the DCCOD, namely Vout, is provided via an open collector transistor. Therefore, you are free to attach any load to the detector provided that it does not exceed the rating of the transistor, namely 40V at .3A.

Fig. 2-10 shows examples of different devices attached to the detector’s output.

Fig. 2-10. Typical connections made to detector outputs

Summarizing the situation, connecting devices to a detector output, either with the DCCOD or the OD, is identical to connecting devices to any other C/MRI output pin located on SMINI, DOUT32, DOUT and COUT24 cards. Therefore, if you are interested in more detail on this subject, consult Chapter 9, where we will look deeper into driving all types of different devices with C/MRI outputs, including the outputs from detectors.

Detection Summary: When To Use Which Detector

Table 2-6 provides a list summarizing the characteristics of the OD compared to the DCCOD.

Table 2-6. Comparison characteristics between OD and DCCOD




Layout applicability

Works on any layout, including all regular DC and all Command Control applications. Also works with AC applications such as Lionel and Marklin. With DCC applications wiring is a little more difficult than when using DCCODs and especially so if the DCC boosters are not optoisolated.

Works only on DCC-equipped railroads and railroads using other pulse-based CC systems such as CTC16, CTC16e, CTC80 and Railcommand. Wiring DCCODs into DCC-equipped layouts is easy and offers the best approach for layout owners using DCC without a prior investment in ODs.

Board size and modularity

1.87” x 3” with easy plug-in of one board per signal block. Test and repairability is easily accomplished with only 1 IC and 1 transistor per detector. Modularity makes locating and troubleshooting layout wiring problems and track shorts easier to correct.

1.75” x 2.85” with easy plug-in of one board per signal block. Test and repairability is easily accomplished with only one IC and one transistor per detector. Modularity makes locating and troubleshooting layout wiring problems and track shorts easier to correct.


Potentiometer adjustable up to about 1MΩ with 16Vdc across the track as most Command Control systems such as CTC-16, 16e and Railcommand™. Sensitivity somewhat less with regular DC applications.

Potentiometer adjustable up to about 150kΩ with six primary turns on the current-sensing pulse-transformer and 14V DCC power on track.

Maximum track current

3A using standard diodes and higher, e.g. 10A or more, if substitute different diodes.

Up to 20A using recommended current-sensing pulse-type transformer.

Maximum output drive capability

.25A at 40Vdc using open collector transistor.

.25A at 40Vdc using open collector transistor.

Built-in time delays

Approximately .75s turn-on and 3.5s turn-off. Values can be altered by substituting different resistors.

Approximately .75s turn-on and 3.5s turn-off. Values can be altered by substituting different resistors.

Built-in monitor LED

Included before time delays for easier setting of the sensitivity level.

Included before time delays for easier setting of the sensitivity level.

Power supply voltage requirements

Detector supply must have 3 output wires: +12Vdc, -12Vdc and ground. The + and - voltages must be of the same magnitude and be regulated. Supply voltages may be increased to ±15Vdc, as on the original SV, but ±12Vdc is more commonly used.

Detector supply uses only +12Vdc and ground connections. The voltage must be regulated and +15Vdc may be substituted if desired.

Can be used with ODMB

Yes, but optional as detectors can be distributed around layout without using the ODMB. If using the ODMB the bottom connection on the ODMB is to the -12Vdc supply.

Yes, but optional as detectors can be distributed around layout without using the ODMB. If using the ODMB the bottom connection on the ODMB is to logic ground and not to -12Vdc as with ODs.

Interchangeable with each other

No. The power supply connections are different between the OD and the DCCOD. Once a given ODMB is configured for either ODs or DCCODs all detectors on that ODMB must be the same.

No. The power supply connections are different between the OD and the DCCOD. Once a given ODMB is configured for either ODs or DCCODs all detectors on that ODMB must be the same.

Wiring and ground connections

Detector logic ground is the same as track ground. Requires series diodes added to all undetected blocks to equalize voltage drop.

All track wiring is totally separate from detector and signal logic wiring. The detection circuit ground, and hence the logic ground for the complete C/MRI system is isolated from track ground.

Approximate cost of circuit board plus user obtained parts and assembly

$10.36. Cost drops to $7.91 assuming an average 24% quantity discount on board and parts.

$11.47. Cost drops to $9.26 assuming an average 20% quantity discount on board and parts.


Typically, choosing the right detector is an easy decision. In most cases it resolves to the following:

  1. If you are using straight DC for train control, then choose the OD.
  2. If you are using DCC, CTC16, CTC16e, CTC80 or Railcommand and you do not have a prior investment in ODs, then choose the DCCOD.
  3. If you are using DCC, CTC16, CTC16e, CTC80 or Railcommand and you already have a significant investment in ODs, you can continue to use ODs. If your DCC boosters are optoisolated, as discussed in Chapter 6, then wiring with ODs is not much more involved than using the newer DCCODs.
  4. If you wish to use ODs with non-optoisolated DCC boosters, then you will need to include a separate detector power supply per booster track section and provide an optoisolator at the output of each detector, as covered in the Chapter 5.

If your situation is number 4, then it is desirable to give serious consideration to selling your ODs and replacing them with DCCODs. There is still a good market for ODs and typically, via the C/MRI User’s Group or via Ebay, you can sell your ODs for enough to cover most of the cost of building up the DCCODs you need.

In summary, starting from scratch on any DCC-equipped railroad, you are always better off using the DCCOD. If you are not using DCC, Railcommand, CTC80, CTC16e or CTC16 then you are better off using the OD. In the next two chapters we will examine each of these detectors and their application in detail. Feel free to skip the chapter that does not apply to your needs.