Model Railway Scale Speed Display - Code:RK-MRSDP

This unit displays scale speed of a train, in either mph or kph. The time the train takes to pass between two detectors set into the track is measured, and the speed is calculated according to the scale and the distance between these. A DIP Switch on the main circuit board can be set up for distances of 250, 500, or 1000mm between the detectors, and the scale may be selected from Z, N, TT, HO, S, O or No.2. (Other scales may be accommodated by appropriately varying the distance between the detectors.)

A setup mode is provided to ensure the detectors are working correctly.

In addition, the unit may be used as to measure the time taken for a train to pass between the two detectors.

Readout is in 0.1 increments from 0.1-19.9, unit increments from 20-255, and 2 unit increments from 256-498

The accuracy of the readout is dependent on a number of factors.
  • Time is measured in increments of 1/256 of a second approx 3.9 milliseconds so there is the possibility of an error of that much. This will be more significant for large scale trains travelling at high speed.
  • Timing is based on the accuracy of the ceramic resonator, which is quoted at 0.5%
  • The distance between the two LDRs may not be exactly 250, 500, or 1000mm.
  • The speed calculation is accurate to 1 part in 256 in the worst case.
  • The actual readout will be to the nearest even number for readouts of 256-498, to the nearest unit for readouts of 20-255, and to the nearest 0.1 for readouts of 0.1-19.9.

For an N scale train travelling at 40kph between detectors spaced 502 mm apart, the error could be 1.2% plus kph readout error. The worst case error is for a No.2 gauge train at 500mph over 250mm, (with an error of say 5 mm) whence the error will be a possible 18% plus 1mph readout error. (There are only six 3.9 mS time periods in the calculation time.) To minimize error, use longer distances between detectors, and ensure the LDRs are accurately placed at the correct distance apart. The time increment error reduces at lower speeds, while readout error (as a percentage) increases at lower speeds.

The unit requires two detectors which are capable of accurate position detection. The kit RK-D2L is well suited to this, and the kit RK-D2IR will work provided there is a sharp distinction of position between when the IR beam is reflected by the passing train, and when it is not. RK-D2M may be used, again if there is accurate positioning detection.

The unit has two main printed circuit boards (PCBs). One is the controller board, while the other (SD7S) is the display board, which may be mounted remotely. One SD7S PCB with all display components and 1 metre of ribbon cable is part of the kit. Additional SD7S display units may be attached to the controller board, allowing up to three slave displays to be positioned around the model layout room.

The unit requires a 12 volt supply, and current draw averages less than 200mA.

The photo at right above shows a unit displaying a scale speed of 26 kph, while the one below shows a the same unit with an additional SD7S display unit attached, both displays showing 26 kph.


Kit price for RK-MRSDP: $64-50



Additional SD7S display extender units come as the kit RK-SD7SDX. This kit does not include the 20 way ribbon cable needed to connect the SD7S PCB to the MRSDP PCB - modellers will need to purchase whatever length they need to fit their requirements.

Kit price for RK-SD7SDX: $34-00 plus cablelength to suit.

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

Click here to go to Aztronics web site

Information:         David Crosby         david@railwaykits.net

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Last updated 3rd February, 2017.
(C) D. Crosby, December, 2011.