Switch Controlled Scalextric No3

This instructable shows how I further developed the a switch
operated scalextric controller (see Switch Controlled Scalextric No2) to allow
young people with a disability who cannot operate a traditional scalextric
controller to use a scalextric set. This method uses an off-the-shelf (or off
ebay in any case) Pulse Width Modulation (PWM) DC Motor Speed Control instead
of the variable resistor to allow pre-set precision control of the speed of the
car and stop and go to be operated via any jack plug connected switch. This
project was originally for use by the William Merritt Disabled Living Centre (www.wmdlc.org). This development was to try and overcome the heat dissipation issues of the last version without increasing the
wattage of the variable resistor to a point where it was unaffordable.

The main problem with a standard scalextric controller is that it requires
precision operation of a spring loaded trigger to allow the cars to run and
then to stay on the track. This rules out its use for many young people who
lack fine finger control. The purpose of this controller is to replace the
speed control trigger with a separate speed control adjuster and switch. It was
important that any type of single pole switch could be used with the controller
to allow its use by as many people with disabilities as possible (e.g. cerebral
palsy, muscular dystrophy, spinal injury and any other conditions that may
affect hand control).

The controller replaces the standard scalextric plug-in controller
and should work with any standard scalextric set. It will not however work with
the scalextric digital sets.

The standard scalextric controller consists of a high wattage
variable resistor connected in series with the power supply to one side of the
track. It adds in a "brake" facility by shorting a third wire across
the brushes of the car to allow instant braking. The controller I have built
does not use this brake function but still uses the third wire as the positive
16V supply to the PWM.

The circuit used is shown in the included figure.

The tools needed to complete this are:

A terminal screw driver

A small Posidrive/Phillips screwdriver

Pliers

Soldering Iron and fluxed core electrical solder

Electric drill

Drill bits to suit the mounting holes for the potentiometer used and jack
socket

Components needed are:

1 * Pulse Width Modulation PWM DC Motor Speed Control (about £4 from any one of the Hong Kong based electronic supplies on eBay). Make sure you buy one with an inbuilt fuse though otherwise
I would recommend fitting a 1A fuse in line with the power to the PWM controller.

3.5mm chassis mounting jack socket

3.5mm stereo jack plug (it is vital that it is a stereo jack and not a 2 pole
mono due to the connections in the scalextric power supply)

3 core cable to connect between the jack plug and the box (I used 3 core 1A cable as it was all I had but 0.5A would be better as it is easier to get the flex into the jack plug cable sleeve).

0.5A single core insulated cable

1 * cable tie

2 * M3 flat head screw and nut

1 * Sticky space arrangement (to lift the circuit board of the base of the box)

1 * Project box big enough to fit the PWM controller and jack socket. The PWM I used is about 80 x 61 x 28 mm
( length x width x height)

1 * 3A screw connector (from
standard connector block

If no fuse is fitted to the PWM controller:

1 * 20mmm fuse holder (chassis mounting type)

1 * 20mm 1A (max) slow blow fuse (its important its slow blow as sometimes the instantaneous motor start up current is far greater fro a very brief time instant). I have used a 1A but in reality 500mA would probably do.

This project requires basic soldering skills but nothing too difficult.

All of the components are available from Farnell, Maplins or eBay

The essential part is knowing how the scalextric controller
connects into the power supply module (the one I used is the standard
scalextric powerbase). This is wired in a way that slot car enthusiasts call
"negative wiring". This means that it is the negative return to the power
supply that is switched or has the resistance applied. The positive supply goes
straight to one of the rails of the track. Most scalextric controllers using a jack plug
will connect as per the connector diagram shown. The jack contact nearest the
cable outlet end/plug body is the negative and this is connected to one side of
the jack socket for the switch. You will see in the photograph I have used a
connector block as it was easier than trying to solder the 3 core cable
green/yellow core to the jack socket directly. The other connection on the jack
socket is then connected to the PWM terminal labelled "Power -" . The
centre contact of the jack plug is connected to the "Motor-" terminal
on the PWM terminals. The cable connected to the tip of the jack plug is the
positive (normally goes to the brake in the standard hand controller) and this
is the 16V supply that goes to the PWM terminal labelled "Power +".
It is for this reason that a stereo jack plug is essential for the connection
to the scalextric. If a mono jack plug is used then the brake (+) and wiper (-)
connections will be shorted and the car will not move and the power supply
could be damaged. However the jack socket used to connect the switch is a mono type. If you have a different type of controller connector (such as the old type banana or xlr connector type) then this page here might help you http://www.slotcar.org.uk/control/index.htm

Some experimentation with the potentiometer setting will be required to get the best balance of speed and staying on the track.

A hole for the potentiometer shaft to pass through the box needs to be drilled in the side of the box. I used a 10mm drill bit. I mounted the circuit board inside the box using M3 screws and some 3M sticky blocks as
spacers. You would be better using purpose designed adhesive M3 PCB mounting posts nut I didn't have any available at the time.

When it all works, drill some holes in the box to keep it all cool!