Swallow Systems has made small
commercial robots (PIP, PIXIE) for
educational use since 1989. We have now
started to use this technology in
the IEE Micromouse Championships (DASH-1, DASH-2, DASH
FREE). This note is to help those
who want to know how we do our wall and
The S4282-51 is a photo IC that
contains nearly everything you need
for a simple reflected-light sensor. All
you add for the simplest sensor is
an LED to provide the light.
The S4282-51 looks like a tiny
transparent four-legged DIL I.C.. The
four connections are +VE and -VE supply
pins, LED constant-current drive
and digital output.
The S4282-51 pulses the LED at about
7kHz. Inside the device is a
photodiode that senses the light falling
on the device and passes a
corresponding signal to the rest of the circuitry. The
signal is filtered and only
signals that are effectively phase-locked to the
pulse drive generate a response.
This makes the S4282-51 very tolerant
of ambient light. Both DASH-1 and
DASH-2 will operate on a sunlit maze with shadows on it.
They are tolerant of conventional
fluorescent lights but may be upset by
modern electronic ballasted ones; I
haven't tested them in these circumstances.
One form of light that does cause
problems is from LEDs driven by
other S4282-51s. You will need to take
care of light pollution from
these. There may be other sources of
pulsed visible or infra-red light.
I imagine light shining through a
rotating fan could generate suitably
The S4282-51 operates from supply
voltages betwen 4.5 and 12V.
The first thing to get right is the
connection details. These are
shown in figure 1 looking from the top
(active) side. Pin 4 is the short
lead. It is a good idea to physically test that this is
connected to GND to check your
The simplest circuit is shown in
figure 2. I always put a 100nF capacitor
across the supply pins of the S4282-51 to
give some measure of EMC
protection. This is particularly
important on Micromice sensors as
they are typically mounted on longish
leads away from the main PCB.
The red LED can be an infra-red type
for the best match to the sensor
characteristics. I don't use these as I
want to see where the light is
going. I use a Hewlett-Packard HLMP4101. This is a bright
narrow-angle visible red LED. It
projects a beam only some 1-2 centimetres across
at a range of 10 centimetres.
I always add an indicator LED so that
I can tell when the sensor is on
without recourse to connecting test equipment. The
circuit for this is shown in
figure 3. The green LED can be a cheap commodity
type from any supplier. It does not need
to be shielded from the S4282-51
as it is not being pulsed.
When you build your first sensor it is
worth mounting the red LED on
extended leads. This will allow you to completely shield
the LED from the sensor to check
the off condition and also shine it directly
onto the active surface of the sensor to
check the on condition.
The first thing to check is the supply
current. Using the circuit in
figure 3, this will be less than 25mA if all is well.
There is a large tolerance on the
duty cycle and amount of drive current to
the LED so it may be much less than this
when the green LED is off.
The next check is that the red LED is
on. Then you can do a functional
check. If you have an oscilloscope you
can check that the LED cathode is
being pulsed. During the on condition it will be about
1.5V below the supply voltage.
During the off condition the voltage will be
partly determined by the ambient
light going through the clear case of the
LED as it behaves as a
The on time is about 8usec. the pulse
repetition period is about 130usec.
There is a large tolerance on these
The most likely problems are the
normal ones of dry joints, badly
cut stripboard tracks and missing
connections but there are a range
of problems caused by optical
You may have a problem with the sensor
not turning off. This can be
caused by stray light from the LED
reaching the sensor by unexpected
Although the LEDs that I use are
narrow-angle, they do emit quite a
lot of light out of the sides and the back.
I use two main techniques for avoiding
this problem. The easy one is a
suitable rubber grommet pushed over the LED to give very
effective shielding of the light
from the sides of the LED.
The other technique is to use Tipp-Ex.
This is simply painted on to the
LED to keep the light in. You will probably need several
layers and the best place to do
this is in a darkened room where you can
clearly see where the light is leaking
from. Unfortunately wet Tipp-Ex is
not a good insulator so you will need to be careful not
to blow up the electronics if you
paint it on when power is applied.
The sensor circuit can be built in a
very small space. The height of
the LED can be a problem. I have not found suitable
surface-mount LEDs so in one case
I drilled a hole through the stripboard and
mounted the LED through the board.
This saved me some millimetres.
In some circumstances you need a
sensor that will detect a wall over
a range of positions. On DASH-2 I need to
detect whether a wall is too close
over a range of some 0-35mm. At 40mm the wall is in
the correct position so we must
not detect it as "too close" there. We
needed vertical sensors working off the
top of the wall to give this kind
We could have used several sensor
circuits ORed together but at £5.11
per sensor this was too much.
The solution was to use several LEDs
driven from one S4282-51. Each LED
projects a small spot of light onto the top of the wall.
Provided that at least one spot is
on the wall at all positions that you want
to detect, the sensor will get enough
light to detect the wall.
The circuit I use is shown in figure
4. This uses 4 LEDs. The LED
output can be tailored to suit their
individual distances from the
sensor by altering the current limiting
It is worth running at the lowest
effective LED current both to increase
battery endurance and also to minimise
the amount of modulated light
sprayed around your mouse.
You should ensure that the sensor is
far enough away from the wall so
that it can "see" the reflected light. The S4282-51
responds well to light at up to 30
degrees from the normal.
Sometimes you need to detect a wall at
long range. Both DASH-1 and DASH-2
use long range horizontal sensors to detect a wall before
using short-range vertical sensors
to get the correct distance from the
I do this by driving the LEDs very
hard and using 2 or more LEDs aimed
in the same direction. A typical circuit,
actually used on DASH-2, is shown
in figure 5.
The full LED cathode drive current
drives the base of a PNP power
transistor. The transistor provides peak
currents of the order of 0.5A to
each LED. This overdrives the LEDs but I am happy with a
shortened operating life.
The 47uF electrolytic capacitor is
absolutely necessary. The large LED currents will cause
an large voltage drop on the leads
back to the battery if there is no local energy
storage. The transistor does not
need a heatsink.
With two HLMP4101 LEDs this circuit
gives a range of about 80mm on
DASH-2. You will get longer range by
adding more LEDs but the effect is
not at all linear.
Range approximates to the radar
equation where it varies as the fourth
root of the light power. This means that
doubling the range needs 16 times
the power. To put it another way, if 2 LEDs give a
range of 80mm, you would need 32
LEDs to get a range of 160mm.
Wih long-range multiple LED sensors,
you should take care to align the
LEDs carefully, It is no good having a long-range wall
sensor if it is pointing at the
The requirements for the schools
competition are somewhat different.
Here you need to detect the difference
between the amount of light
reflected from white masking-tape and
from the background black paper.
The contrast ratio is low but it is
sufficient if you take care to
avoid direct reflections.
You should arrange the LED so that if
the surface was a mirror it would
not shine on the sensor chip. This ensures that the
sensor works from the colour of
the surface rather than how shiny it is.
You also need to accurately adjust the
sensitivity of the sensors. This
can be done by setting the height of the sensor and LED
above the surface but this is not
Another method is to adjust the LED
current. Figure 6 shows how we do
it on DASH FREE. The 5K0 trimmer can reduce the LED
current to a level suitable for
You will need to set the threshold
accurately as the S4282-51 has
hysteresis. Typically the light level
must fall to 65% of the switch-on
threshold to turn the device off. This
takes up a lot of the available
Once you have experimented with making
sensors you will need to think
about the number of sensors needed and
where they should be placed.
DASH-1 uses 6 sensors. Therer are
three horizontal long-range sensors
to detect if a wall is present at each
side or in front and three
short-range vertical sensors to detect
that the mouse is at the correct
distance from that wall.
This is a simplified description as
the long-range side sensors are
also wide area sensors.
DASH-2 uses 10 sensors. There are
three vertical sensors on each side
corresponding to too far, just right and
too close for that wall.
There are two longer-range horizontal
sensors pointing sideways at the
rear of the mouse to detect its position relative to gaps
in the walls.
There is one vertical sensor at the
front to detect that DASH-2 is the
correct distance from a wall in front. There is one
long-rage horizontal sensor to
detect that there is a wall in front of the
If DASH-2 had more sensor inputs I
would have liked several more sensors.
When designing your electronics, you
should always allow for more
sensors than you can possibly imagine you
will need, as shortage of sensors
is a performance limiter for both DASH-1 and
DASH FREE, our schools competition
kit, has two sensors mounted at
the front. The particular problem with
schools Micromice is that the
spacing between the sensor and the mat is
critical. With our two-wheel
mice there is a rocking action as it
accelerates or decelerates and
this can cause a lot of
These notes are not a comprehensive
treatment of the subject but they
do outline how I tackle the problem. If
you have any questions or
comments, or better still from my point
of view, a better way of doing it,
please contact me via e-mail. firstname.lastname@example.org