One thing I get asked about is point control,
and I've a bit of a bone to chew on where that subject is concerned.
The first step is to read Brian Lambert's
excellent guide. Find the section on point control and study it
carefully. There is no substitute for reading this carefully:
Points motors and moving them
He takes you through the basics of
using the Peco point motor (essentially two coils to produce a
magnetic force to move the point across). There's a variety of
methods shown for how to operate the coils - probe and stud from an
A/C supply, then onto using CDUs on either probe and stud or push
buttons, etc, which are both good options.
Then the next step is using an on/off switch,
single wire, single capacitor, and a pair of diodes - all very
effective and simple but it needs a decent power supply to drive it
properly. All good so far.
Now if you're using electrofrog points
and/or don't want to rely on the point blades for switching, then
you're probably using a Peco auxiliary switch which is stuck onto
the bottom of the points motor. In basic terms it's a sliding
contact on a section of PCB - much like the sliding switch on a
Seep-branded solenoid motor. Sliding contacts which are exposed to
the atmosphere don't last long before the emergence takes place of
high contact resistances due to dirt and oxidisation, and this plays
havoc with running trains over the point, etc.
It also requires some of the available
force to move the point across, which can result in a misfire. (If
you have had to clean the point blades then you'll already know what
I mean - if you've not cleaned them yet you'll have to at some
time!) There are also the Peco 'micro switches' which are commonly
used for double slips. These are better but, as they are also open
to the elements (even if those elements are in a centrally-heated
room in your house), again you can suffer high resistances. So,
ideally, we want to replace this with something better. Hold onto
Now, thinking about your control panel - its
nice to be able to see which way your points are set - you can't get
this without extra circuitry for the probe and stud method. That
just leaves Brian Lambert's switch and capacitor and diode method.
Use a rotary switch and it does just that - shows you where the
points are set. In addition, you could attach to this a relay to
manage the frog polarity, which will work just fine. (But remember
that this needs a decent power supply and is manual only. It cannot
easily be automated.)
I've moved on a step further. The group known as
MERG have developed a PD3 point driver - a very nice, professional
bit of kit using a lot more parts - which controls the point from a
single switch. This means that your control panel reflects the exact
position of the points. It also has sufficient contacts for the frog
polarity too. The only downside is you have to be a member of MERG
to be able to buy the kit, which is a bit of a shame as it's
Then a circuit appeared on the N Gauge Forum
(see the Links page), which
performs the same function as the MERG PD3 but with a lot less parts
and is an easy DIY build.
Basic schematic (relay point)
It works just as well with smaller power supplies
as it does with bigger ones. It's what I've fitted to all of Bridgebury's
points (with only one small adjustment to a resistor value), shown below:
Relay points schematic
Like the MERG PD3 and Brian's single wire method,
there is one common disadvantage - when you power up your layout the
points may not be in the state shown by the switch.
Don't worry! It's easy to deal
with! Switch each point over and back. This lines up the point with
the switch and it stays that way until you switch your layout off
again. It's not much of a hardship to do this at the start of every
operating session as it also shows you that each of your points is
So that's the Peco aux switch gone, a switch
added to the panel that reflects the position of the point, and the frog
polarity is more reliable.
Under operational circumstances, the point reflects
the position of the switch. With a bit of forethought you can have the
point set to whichever direction you want with the point switched off
(call this the 'normal' direction for the point if that helps).
Arrange the switch on the panel so its how you
want it - on or off for that particular direction. (Do not use
momentary switches - only on/off switches.)
Wire it up to the point relay and try it out. If the point
switches the opposite way to what you require then reverse the red and
black wires to the solenoid and re-test.
You should now have the point working the way
you want it. The next step is to connect the frog and running rail wires.
If you get a short circuit on the
track then reverse the running rail wires and re-test.
That's how complicated it isn't.
From there it is a very simple step to add basic
route setting (or slave points together). Add a diode to operate
each point you want to operate from a single switch to give you that
route (or connect the wires from two or more points to one switch
when slaving). The point is simply on/off controlled - now that's simple.
'Oh that's expensive', you say. Nope! A Peco aux
switch is around £2.50. A small section of stripboard, one relay, one
resistor, and two capacitors cost about the same on eBay.
Relays are less prone to high contact
resistances than the aux switch
Each point effectively has its own CDU
- you can operate as many points as fast as you like and they
will keep up with you
Can be driven from switches or any form
of automation - it's simple on/off control
Additional relays can be added to this
system to control signals, etc
No probe wire hanging around
Switches show which way your points are
Works on single slips, double slips, and
any form of solenoid point motor
Only control signals travel to the point
- the high current spike which operates the point is entirely
local to the point. You can use 7/0.2 wire to the point with no
problem at all
Slightly more complex to install - its your
As it causes a momentary short on the rails
it's not suitable for DCC without more tinkering (when the relay
operates to fire the point the momentary short will trip your
You cannot use one of these relays to do
two points - each point must have its own relay (unless you
dispense with the frog polarity side of things)
Isn't slow and realistic in the way that
servos might be, or hand controlled (wire-in-tube)
Point control relay 20V version
Point control relay 20V version diagram
Please note: the red X
marks a cut in the stripboard track (anything that cuts the track
creates and electrical break). The black + denotes the polarity
for the capacitors.
Failure to deal with either correctly results in
melting or popping! The resistor can be any value between 200P and 820R
- the lower the value, the faster the recharge speed. Capacitors are
2200uF 35V-rated for this '20V' version. Instead, you could have a
separate, higher voltage power supply, in which case reduce the value
of the capacitors. I use 1000uF 50V-rated on a 40V supply.
Point control relay 20V version reverse side when soldered together
Point control relay 20V diagrammatic breakdown
Four versions of the SPUD through the ages...
starting with bottom-left - 'components alone glued
together' - and proceeding anti-clockwise to 'strip board design',
then 'prototype board' and finally 'current
production version' top-left.
The stripboard design has been developed into a
printed circuit board by MalcC. Bare printed circuit boards, kits,
and even completed versions are now available from here:
Wiring diagram to help with the instructions
provided by MalcC
I've received my first batch of boards and
have even got the missus to test-build one in twenty minutes
(not bad, as she hasn't soldered before). In essence anyone
can now do this.
Building and installing your boards is now
very very simple!