Two weeks ago I shared with you the first part of this post about adding working lights to some of my HO Scale Union Pacific excursion train water tenders.  You can find the post here.  In this week’s post I will share with you the next step.

At the end of the previous post I had gotten as far as adding the power pickups to the 3D printed trucks and installing the DCC decoder as you can see in the image below.

The original idea was to simply hook the white and blue wires up to an LED which was mounted inside the tender shell.  Below you can see the chassis with the LED fitted and working.  I have also added some weight to the chassis which greatly improves the power pickup.  The weight is off-center to avoid the baffle in the center of the tender shell.

However upon testing the system I discovered a small problem.  Because the DCC decoder I had selected is a function only decoder, that is to say it has no motor controls, it works well when run on a DCC layout but will not function properly on a DC layout.  This is because a function only decoder doesn’t recognise direction.  A regular DCC decoder will recognise forwards and backwards, switching on the relevant wires to turn on the correct lights.  A function only decoder will simply respond to function key commands.  In the case of the Digitrax TL1 that I have used; F1 will turn the white wire on and off.  When the tender is running on a DCC layout the operator can consist the tender with the locomotive, then when they push the F1 button the tender headlight will come on as required.  But under DC control the headlight will only continue to do whatever the last command was under DCC control.  For example if the light was lit when running on DCC, the light will remain lit in DC irrelevant of direction and cannot be turned off.

As these two tenders are wanted for both DC and DCC operation I will need to come up with another solution.  One option would be to replace the function only decoder with a full motor decoder.  That way it would respond to directional control in both DC and DCC but there would still be the risk of it being removed from the DCC layout with the light off, then it would not work on the DC layout.  Also full motor decoders are more expensive.  A simpler option that I have decided to use is to add a switch to the underside of the tender chassis which will allow the operator to switch the tender from DCC to DC control.  That way the function decoders that have already be purchased will not be wasted.

The type of switch required is a double pole double throw toggle switch.  Double pole means that it can switch two separate wires at the same time; in this case positive and negative.  Double throw means rather than simply on and off it switches each pole from one input to another; in this case DCC power to DC power.

As I described in part one of this post each truck picks up power from a different rail.  Irrelevant to whether it is a DCC or DC layout, one rail is treated as positive and one as negative.  At this point the two power wires need to be divided so that power goes to the DCC decoder and to one side of the switch.  The output from the DCC decoder will then go to the other side of the switch.  From the middle of the switch will come the two wires that feed the LED.  When the switch is set to the DCC side the system will operate as it did before, responding to the F1 command on a DCC system.  With the switch set to the DC side the power will bypass the DCC decoder and go directly into the LED.  Because and LED is a diode, which means power can only pass through in one direction, the light will only come on when the train is moving in the direction the light is facing.

To find the right DPDT toggle switch I had to look at the available space on the chassis.  Although there is a lot of room inside the tender itself, there is not a lot under the chassis and I did not want something huge sticking out that looked unsightly or might foul things like turnouts or crossings.  A standard DPDT toggle switche would be simply too big, as would a sub miniature one; but an ultra miniature one would work well.  Below is an ultra miniature switch next to a miniature one.

The switch will be mounted to one side near the center of the chassis.  That way there is no chance of it interfering with the swing of the trucks.

Although this switch is very small the lever that projects out is still too long so I cut about two-thirds of this off with a disc cutting tool and filed the edges to remove any sharp bits. It is now fairly inconspicuous under the chassis.

A small corner of the air tank also had to be cut off to allow the toggle switch nut to be fitted but this will not be visible when the tender is on the track.  The switch will also be painted gray to match the chassis helping it blend in.

Next the switch needs to be wired up.  The red and black wires from the trucks go to the two terminals on the far side.  The DCC decoder is then also connected to the same terminals.  This is where the power is split.  The output from the DCC decoder, the white and blue wires, are connected to the near side terminals.  With DCC decoders the blue wire is the common function wire which is positive. Therefore it needs to be connected to the switch on the same side or pole as the red wire.  If the light was powered by a regular bulb this would not make a difference but because LEDs are diodes the polarity of the wires is important.

Next the LED wires are connected to the two center terminals as shown below.

Often DPDT toggle switches have three positions, the middle being off.  This can be useful as it ensures there is no chance of a short as the switch is thrown. However the ultra miniature switches only have two positions but as the tender needs to be removed from the track to throw the switch this is not a problem.

The LED I am using is a 2mm lighthouse style with a warm white color.  Regular white LEDs are often too bright and give a very cold light.  I should also point out that it is necessary to use a resistor with every LED otherwise they will draw too much power and blow immediately.  Below is a comparison between an ordinary 5mm LED and the 2mm lighthouse LED.

The light these warm white LEDs give off is perfect for recreating locomotive headlights as you can see from the images below.  For these photos the headlight has not been properly secured so it is pointing up a bit.  I need to finish the shell and add all the decals before I can fix the headlight on properly.

Taking photographs in the dark is not the easiest thing to do but below is a shot of the tender in low light, as you can see the headlight gives a nice beam.

In next week’s post I will share with you the final stage of fitting the LED into the shell.  I will have also finished the shells, completing the tenders.

My 3D printed tender kits for Union Pacific’s excursion trains are now available in several scales.  The kits come with 3D printed headlights, or more correctly, backup lights.  In this post I will share with you how I finished a set of HO tenders with working backup lights.

Only one headlight will work on each tender.  This will be the one at the rear and will only come on when the locomotive and tender or tenders are backing up.  The headlight on the front of the tenders will be a dummy.  I need the lights to work in both DC and DCC modes.  To do this I am going to use Digitrax single function decoder TL1.  This simple decoder only has four wires, two which go to the track and two which go to the lamp in the headlight.

The kit, as shown below, has several parts printed in two different materials.  All the high detail parts can be printed in Shapeways Frosted Detail or Frosted Ultra Detail materials.  The low detail parts such as the chassis are printed in Shapeways White Strong & Flexible material.  When the kit arrives from Shapeways the headlights are attached to the loop that joins the ladders together; in the image below I have cut the loop and removed the ladders.

The first thing to do is to work out how to collect power from the rails and get it into the body of the tender.  To do this I am going to use metal wheels with axle wipers.  One truck will collect power from the left rail, the other will collect from the right rail.

The trucks that come with the kit, as pictured below, are designed to take Proto 2000 33″ metal wheels from Walthers.  The trucks are fixed to the chassis with 3D printed bolster pins that simply push in.

Although the Proto 2000 wheel sets have metal wheels the axle is made from plastic which is no use for picking up power.  A good alternative are Intermountain’s 33″ wheel sets.  These have metal wheels and a metal axle and one of the wheels is electrically isolated from the axle.  However using these does cause a problem.  The point to point dimension of the Intermountain wheel sets is slighty smaller than the Proto 2000 ones.  This means that the wheel sets will fall out when the tender is picked up.  To solve this I have used small off cuts of solid copper wire superglued into the V grove below the axle sockets as pictured below.  When the trucks are the right way up and sat on the track the wheel set axles will be resting against the top of the axle socket; the copper wire simply stops them from falling out.

The next issue is how to transfer the power from the axle wipers up into the tender.  A flexible wire is the easiest way but the position of the wire can cause complications.  The further away from the bolster pin the wire is the more it will rotate as the tender runs around a bend. A crescent shape will need to be cut into the chassis to receive the wire.  The maximum radius that the tender will be able to negotiate will depend on the size of the crescent.  An alternative to this is to run the flexible wire up through the bolster pin.  Because the HO kit is a scaled up version of the N Scale kit the bolster pins are quite big.

I used a small drill bit in a pin vice to drill a pilot hole through the bolster pin.  The head of the bolster pin was printed with a hole through the thicker section to reduce material, which helps guide the drill through squarely.

Then I fitted the bolster pins into the chassis and used the larger drill bit to drill all the way through.  The size of the drill bit depends on the wire and wants to be just a bit bigger to allow the wire to pass through easily.

The bolster pins were then removed and any swarf was removed from the holes.

Next come the fitting of the wheel sets.  It is very important to note their orientation.  In the image below the wheel set on the left is electrically connected to the wheel at the top.  The wheel set on the right is electrically connected to the wheel at the bottom, you can see the plastic isolator where the top wheel joins the axle.

It is important to get all the axles in the same truck orientated the same way around. otherwise the truck will cause an electrical short.  Because the trucks are the same, just rotated 180° you can insert the wheel sets the same way round in both, then when they are fitted to the tender one will pick up on one rail and one will pick up in the other.

The trucks can now be fitted to the chassis as shown below.

Doing a rolling test at this stage is a good idea to ensure the wheels run freely.

To make the axle wipers I have used 1mm wide phosphor bronze strips about 45mm (1.771″) long.

The wiper strip is pushed through the axles.  It goes under the two outer axles and on top of the middle one.  That way it will utilize all three axles to collect power.

Then using my fingers I bent the ends of the wiper strip around the outer axles.  This will stop the strip from rolling out and moving as I work on it..

Using my soldering iron I tinned up the strip just above the hole in the bolster pin.

Then one of the wires from the DCC decoder was fed through the chassis and bolster pin.  A small section at the end of the wire was striped back and tinned with solder.  The end was then bent over by 90°.  In the image below this has been done to the black wire.

This hook can then be positioned over the tinned wiper strip and with a quick touch of the soldering iron they can be joined.

There is a risk with this configuration that the axle wiper will rotate and run diagonally across the axles.  If the wiper strip touched a wheel that is isolated from the axle it will cause a short.  There are two things that can be done to prevent this.

The first is to remove one of the end wheel sets, this will cause the wiper strip to fall out.  If you gently roll the ends of the wiper strip around a bit further between your fingers and then refit the wheel set not only will it be a tighter fit almost preventing the wiper strip from going diagonally, it should also improve the contact with the axles thereby improving the power pickup.  This can add drag to the wheel sets and you should check that the tender still rolls well once this has been done.

The second is to add a guide which the wiper strip will run between.  To do this I used some of the same solid copper wire I used in the V slots of the trucks.  I bent the wire into the shape shown below.

This bent wire shape fits down through the second hole in the truck frame as shown below.  Once I was happy with the fit they were superglued in place .

The forked ends protrude down on either side of the wiper strip giving some room to move but preventing it from going diagonal.  The other end is held in place by the wire coming through the bolster pin.

Below is a close up of a finished truck with a fixed axle wiper collecting power from all three axles.

This now completes the tender chassis, in next week’s post I will show you how the working tender headlights were fitted and how the tenders were finished.