Electronics – Page 2 – The Wheeler Model Railway

Train-Tech Signal Construction

There are many options available for colour light signals, however, many of the commercial solutions are let down by using LEDs that are either oversize or have unrealistic colours.

After investigating a number of solutions, I’ve found a simple method of fitting LEDs that look much more like real signals into a Train-Tech SK1 kit. The SK1 kits cost around £5, the LEDs and other bits around £1.

Train-Tech do sell kits with LEDs already fitted to a PCB, but the size and colours of these are poor. In addition, the PCB sticks a long way below the layout, making them difficult to use without modifications on all but the deepest base boards.

LEDs

After much research, I’ve found the OSRAM “TOPLED BLACK” range to be the best currently available at a sensible price. This is a surface mount LED, but the pads are large enough to solder wires directly to them. They are primarily designed to be stacked together for LED message boards.The front dome is the right diameter for a 00 signal, and “True Green” LEDs are available, which are a much more realistic colour than standard (‘yellowish’) green LEDs.

Colour	Part Number	Wavelength	Resistor
Red	LRT64F-BADA-1	625nM	22K
Yellow	LYT64F-BBDA-35-1	590nM	10K
True Green	LTT64G-DAFA-29	528nM	150K

You will find similar part numbers listed; these generally refer to devices with different average light intensities.

The resistor values shown, when used with a 5V supply, give an appropriate light output for a layout, with a reasonably similar intensity for each of the different colours. The 150K resistor for the ‘True Green’ LED is not a misprint; these LEDs are VERY efficient. Use higher values for a 12V supply; lower values for a multiplexed drive.

RS Components stock these LEDs, though only sell them in multiples of 25.

Construction

Solder a couple of lengths of 0.5mm tinned copper wire together, and bend them as shown.

Fit the LEDs (in the correct order and polarity) into the back of the signal. A bit of glue can help to stop them flying away. A 5V power supply with a 4K7 resistor in series helps to identify the colour and polarity of each LED. Solder one arm of the wire to the negative LED pads, taking care that the ‘Y’ of the wire is in the right place for the back to fit,. Make sure it does not touch the other pad of the red LED:

Now solder some very thin insulated wire to each of the positive pads:

Ensure the signal head base can slide over the wires, and check that the signal head back still fits. Don’t glue anything at this stage, but test it works electrically. Slide a piece of heatshrink sleeving over the tinned copper wire so that the post doesn’t become ‘live’, then slide the post up to the signal head:

The base can then be fitted. Put a kink in the thick wire to stop the signal coming apart, and solder wires to the various connections; I stagger the connections so they won’t short:

Cover the connections with another piece of heatshrink sleeving, and glue the signal head back and front together.

The signal is now ready for fitting to the layout. The ladders and other parts are best fitted after the main signal is installed. The connections and wire underneath can be gently bent to suit.

Whilst this example shows a three aspect signal, the process is the same for two or four aspect signals.

Ready Made Signals

To change the LEDs on ready made Train-Tech signals, first carefully remove the existing LEDs from the PCB:

Clean the old solder from the pads, but put a fresh blob of solder on the three top (common) pads. Then tack the new LEDs onto the common pads, checking that:

They’re flat on the PCB.
They’re the right way round.
They’re the right colour.
That the cover still fits.

When you’re sure everything is OK, solder the remaining legs to the pads.

I also used this method to replace the LEDs on a SK7 Dual Head signal kit.

Even More Layout Lighting

I finally got a chance to get some more wood and have constructed the last of the layout lights.These have all been made using “5050 Day White” LED strips stuck on to “Scotia Mouldings”…

A wire is soldered onto the strip, normally somewhere in the middle. This goes through the a hole in the strip and into the wall:

Once attached to the wall, the wire is invisible:

Note that it’s the wall that’s wobbly, not the lighting strips!

All the layout lights together take 42W of power – it’s a LOT of light for not much electricity. The next project will be a dimmer to turn them up and down as required.

Revised Power Supply

The railway power supply arrangements have been revised. As more electrical bits were added to the railway, the original solution was running out of ‘steam’.

The gold coloured box at the top is the main power supply for the layout. This will deliver up to 10A at 16v.

The silver unit on the left steps up the 16V to 24V to supply the point drivers; this has improved the point reliability now that the track has been ballasted.

‘Country’ Mimic Panel

A small mimic panel has been added to the ‘Country’ end of the layout to simplify controlling the points at that end.

As there were a couple of spare inputs on the control board for this, I also took the opportunity to enhance the TOTI (Train on Track Indicator) circuits at this end.

Track Block Circuit Protection

For our club exhibition layout, there was a requirement that a short or fault in one section of the layout didn’t stop the entire layout operating. Initially, this would be done by having four power districts, each fed from their own booster.

As this would have increased the cabling, connectors and overall cost of the layout, alternative approaches were investigated, initially using the old trick of wiring a 12V lamp in series with each track feed.

However, with modern components, a simpler approach is possible using PTC (Positive Temperature Coefficient) resettable fuses. They are sometimes called a Polyfuse or a Polyswitch.

These are low cost (20p) two terminal devices whose resistance increases dramatically as their temperature rises.

This has the effect of limiting the current under short circuit conditions. When the short is removed and the device is allowed to cool down, normal operation resumes.

A typical device, which we use for our 00 gauge layout, is the Bourns MF-RO50 device. The ‘cold’ resistance of this device is around 0.5 ohm, and the ‘tripped’ current when a short occurs is around 0.5A.

Other gauge layouts would need larger (or smaller devices).

These devices are also very useful for protecting various DC power feeds from short circuits.

Typical Connection

This shows the basic track connections for the PTC fuses. Within reason, the shorter each track section, the more protection will be provided.

MERG NB1B Booster Modification

When a short occurs, the initial current is limited by the booster, the layout wiring and the cold resistance of the PTC fuse (around 0.5 ohm for the MF-RO50). This current (peaking around 8A) is sufficient to trip an unmodified MERG NB1B booster, which defeats the whole point of the block protection.

However, if C5 on the NB1B is increased to around 220nF, LK2 is removed and the booster is set for 10A output, a short on any individual track will not cause the booster to trip. For this to work reliably, the power supply feeding the booster also needs to be capable of delivering the maximum short circuit current, albeit only for periods.

The NB1B current limiter (when mounted on a heatsink) is more than capable of limiting the current for the 200mS or so that the PTC fuse takes to warm up.

Monitoring

If an indication of a fault is required, a LED (with a diode and series resistor) can be wired across each PTC fuse. This will only be lit when a short or other fault occurs.

The other method is to touch the PTC fuse – it will be too hot to touch under sustained fault conditions.

Testing

Whilst in theory the current available from the MF-RO50 is insufficient to allow any DCC 00 gauge loco to operate, in reality, it works with the largest modern 00 loco we have tried – even with the sound wound up to the maximum. Empirical tests would suggest that this is because the loco current peaks are too short to increase the PTC temperature significantly.

Long multiple units are not a problem for our layout either, as the maximum length of track in any protected section is less than 4 feet and the power for a long train will be taken from multiple sections.

Note that if a short is sustained for a long period of time, then it can be up to 30 seconds before the device cools down enough to restore full power.

First Signals

At last, some signals have been added to the railway. These are driven by the CANSIG firmware that I developed a few weeks ago.

The signals change automatically depending on the position of the trains, and for some signals yet to be fitted, the setting of the points.

CANSIG – Automatic Signal Control for CBUS systems

The CANSIG firmware provides automatic signal control for MERG CBUS systems. The firmware can drive Semaphore or Multiple Aspect Colour Light signals, and can support multiple events and routing to control each signal aspect.

Each CANSIG module can control up to four signals; these may be two aspect (or semaphore) signals, three aspect signals or four aspect signals. CANSIG can also support two ‘routed’ signals which will have different aspects depending on a ‘route’ setting – this would normally be set from a point position. Routed signals can also display a ‘feather on’ or ‘feather off’ indicator to show the route set.

CANSIG has six Non-Volatile (NV) parameters that control its operation; these are configured with the MERG FCU program. The FCU is also used to configure the events used to control the signals.

The software works with the MERG CANACC8 or CANLED64 boards. It will also work with the FLIM IO range of boards.

CANSIG has several different operational modes. Some examples are shown here:

Plain Track, full TOTI Indication.

Configure the events as follows:

TOTI-1 -> Red A On
TOTI-2 -> Yellow A On
TOTI-3 -> D-Yellow A On

With no trains on the track, the signal will be Green. When the train reaches the TOTI-1 section, the signal will change to Red. When the train reaches the TOTI-2 section, the signal will change to Yellow. When the train reaches the TOTI-3 section, the signal will change to Double-Yellow. When all the TOTI sections indicate clear, the signal will change back to Green.

Incomplete TOTI indications

This mode can be used if there are insufficient TOTI sections for full signal aspect control.
Configure a single event as follows:

TOTI-1 -> Red A On

Also configure Red-Green Timer Mode to 10 and Yellow-Green Timer Mode to 15.

With no trains on the track, the signal will be Green. When the train reaches the TOTI-1 section, the signal will change to Red. After it passes this section, the TOTI-1 section will be inactive, and the signal will change to Yellow for 10 seconds, then change to Double-Yellow for a further 15 seconds. Whilst a timer is active, if any events arrive that specifically change the signal aspect, timer mode is automatically cancelled.

Simple Junction

This example shows the use of CANSIG with a simple junction.
Configure events as follows:

POINT-1 -> Signal 1 D-Yellow D/Alt On
TOTI-1 -> Signal 2 Red A On
TOTI-2 -> Signal 2 Yellow A On
TOTI-3 -> Signal 2 D-Yellow A On
TOTI-4 -> Signal 1 Red A On
TOTI-5 -> Signal 1 Yellow A On
TOTI-6 -> Signal 1 D-Yellow A On

With no trains on the track, the signal will be Green. If the point is set, the feather will light, and TOTI-1, TOTI-2 and TOTI-3 will control the signal aspect. If the point is clear, TOTI-4, TOTI-5 and TOTI-6 will control the signal aspect.

For full details and other examples, see the CANSIG Operation documentation (pdf), updated 21-Apr-13

Download CANSIG_v2b Firmware for CANACC8 and CANLED64 boards (hex files)
Download CANSIG_v2a Firmware for CANACC8 and CANLED64 boards (hex files)

CANTOTI

The CANTOTI firmware is an extension of the CANACE8C v2g firmware and is designed to run on the MERG CANACE8C board. FlimConfig V14510 or later is required to configure the CANTOTI firmware.

Whilst the firmware is primarily designed to be used with TOTI (Train On Track Indicator) hardware, it is also suitable for monitoring other inputs, particularly if inversion or input conditioning is required.

Unlike previous versions of CANACE8C firmware, this code samples each input every 10mS, and will generate an event when each input is the same state for more than a configured number of samples.

Each input can be optionally inverted to allow for an “Active High” input.

To allow for noisy signals being connected to the CANTOTI, optional delays can be enabled on an input by input basis.

If “Delayed input” is enabled for an input, the input must be ON for at least the “On time” before an ON event is generated, and must be OFF for at least the “Off time” before an OFF event is generated.

The “On time” and “Off time” are set in units of 10mS, with a minimum time of 10mS and
a maximum time of 2540mS.

If “Delayed input” is not enabled for an input, the input must be ON for two 10mS samples before an ON event is generated, and must be OFF for two 10mS samples before an OFF event is generated.

The “Extended Configuration” value is not currently used; ensure this is set to 0 for
future upgrade compatibility.

The current CANTOTI firmware defaults to an input “On time” of 100mS and a “Off Time” of 500mS.

In addition, there is a build for the CANACE8C itself. This has the standard CANACE8C module ID, and defaults to to an input “On time” of 10mS and a “Off Time” of 10mS.

Download CANACE8C & CANTOTI_v2j Firmware

TOTI Hardware

A wide variety of circuits can be used to detect trains. One of the simplest is this one:

This is for a DCC layout only; the diodes would be connected in series with the DCC feed to the track to be monitored. This requires an opamp with ground referenced inputs such as a LM324, and can easily be powered from the track itself.

I’m using three prototype DCC-TOTI boards (designed by Trevor Stockhill), each attached to a CANACE8C running the CANTOTI firmware: