DASH FREE faster
Once you have made your
DASH FREE micromouse, you are likely to
find that, although you can make it follow a straight
line very quickly the corners represent a bit of a
problem. Some ideas to improve the control of your
a) Minimise the moment of inertia of
the mouse about its turning centre. What this means is
that every part of the mouse, especially heavy items such
as the batteries, should be as close as possible to the
point midway between the wheels. This will mean that the
mouse can start turning very quickly and, probably more
important, stop turning quickly.
b) Improve the sensing system. When
the mouse reaches a corner, it will detect the corner as
soon as the inner sensor leaves the line. It will
continue to detect the corner even if the mouse goes
quite a long way over the line as long as it doesn't
reach the mark for the next line (white square on the
mat). This makes the mouse very good at detecting the
start of the corner.
Detecting the end of the corner when
the mouse should straighten up for the next straight is
more of a problem. The mouse cannot see that it should
straighten up until the sensor is back on the line. At
this time the mouse will be turning very quickly and its
momentum will tend to keep it turning. If it hasn't
straightened up before the sensor comes off the inside of
the line, it will keep going round in circles!
One technique is to make the sensor
"see" a much wider area. This can be done by adding
multiple LEDs to the sensor circuit. See
here for more information on the sensors.
Using an area coverage sensor, you can
make the mouse "see" the line over a width of maybe 100
mm (it depends on how you make the sensor circuit)
instead of the 50 mm width of the tape.
c) Another problem with making
DASH FREE go fast is the trimmers. It is
difficult to set these with great precision. You could
replace the speed setting trimmers with multi-turn
versions. Something like a 500R Bourns 3296Y (Farnell
part number 348-089) will make speed setting less fiddly.
DASH FREE 99s are supplied with
multi-turm pots as standard.
d) Braking. With the standard
DASH FREE there is no braking for corners.
The inner wheel simply free-wheels to a stop when the
corner is detected. As the inner sensor cannot be more
than 250 mm in front of the wheel because of the size
specification in the rules, this is a serious limitation
on the speed that can be used down the
One way of adding braking to the inner
wheel is to put a P channel power FET across
the inner motor connections. When braking is required the
FET can be turned on and this will give a path for the
current generated by the motor spinning. This current
will give a strong magnetic braking effect and cut the
braking distance dramatically.
I use a RFP10P03L (Farnell part
number 516-430). The source should be connected to
battery positive. The drain should be connected to the
collector of TR2 and the gate should be connected to the
drain of FET1. This will ensure that the FET is
turned on when FET1 is turned on; i.e. when the sensor is
You should remove diode D2 to avoid
the possibility of the brake coming on while the motor is
You should only do this braking
modification for the inner motor.
You should find that this reduces the
braking distances by a factor of at least 2 depending on
the weight of your mouse.
e) Batteries. You can add more cells
to give a higher battery voltage. I don't recommend that
you go much above 9 volts (6 cells) if you try this. You
can try using AAA cells instead of AA cells to reduce
One problem to address is the type of
cell chosen. Primary cells, especially in AAA size may
have problems delivering the high currents needed by the
inner motor when powering away from a corner. NiCd cells
may make a faster mouse despite having a lower cell
voltage as their internal resistance is low.
DON'T USE NiMH cells unless they
are rated at at least 4A discharge current (8A if you use
a 9 volt stack). The batteries must be rated for the
stall current of the motors.
f) Less voltage drop. The
complementary darlington TR1 and TR2, combined with the
drops across the steering diodes etc. mean that the motor
"loses" about 2.5V from the full battery voltage. This
can be reduced by changing the circuit a little and
adding a couple of resistors.
The original circuit used an
emitter-follower circuit where the voltage at the emitter
of TR1 was held about 0.6 volts above the voltage on the
base of TR1. TR2 collector could not go below the voltage
on the base of TR1.
This new circuit has two resistors
across the motor acting as a potential divider. The
voltage at TR1 emitter is now approximately a third of
the voltage across the motor. It is this voltage that is
held about 0.6 volts above the TR1 base voltage. TR2
collector can now swing virtually all the way down to
The resistors can be 1W types.
This modification should not be used
on the older DASH FREE kits unless the
multi-turn trimmer modification is also done or the
trimmers will be much too sensitive. It is all right on
DASH FREE 99s.
The modification will allow you to go
faster on the standard batteries. You may find that you
no longer need the weight of extra cells in the
Please email comments and your own
ideas for speeding up DASH FREE to:-