Laser Target Finder Sensor
I originally built this sensor to be able to
perform triangulation navigation. I needed a way to locate three
targets from some distance, so a laser pointer seemed to be
tailored for the job. To keep things simple, I wanted to have
passive targets, and used catadioptric reflectors to send back
laser beam to the sensor.
Now I needed to be able to detect this reflected
light. I first thought of integrated circuits used as infra-red
remote control receivers, but these devices are sold as modules
integrating the IR sensitive element and a filter eliminating
visible light. I was out of luck: inexpensive laser pointers
are red, a wavelength well out of the sensitivity range. I then
found Plessey SL486 device, an
old remote control receiver chip that did not include photodiode
nor filter, so it can be used with red light. Unfortunately
this chip is too old: no longer manufactured, it is hard to
find, and its price reflects its scarceness!
So I went on searching the ideal integrated
circuit, and finally stumbled on a family of photo-sensitive
devices made by Hamamatsu. Some of them had no visible light
filter, provided a modulated output to drive a LED - or a laser
-, and included a photodiode with a synchronous demodulator.
Perfect fit! Well... almost. Hamamatsu products are not available
through electronics distributors (at least not in France), and
I had to buy a bunch of chips (directly from Hamamatsu France)
to reach minimum order amount...
October '04 update:
Hamamatsu S6986 chips are now available
(internationally) from Mark
III Robot Store.
Have a look
to their other
products too: they have lots of
interesting (non-LEGO) robotics stuff.
October '05 update:
If you replace the laser with an infra-red
LED the same circuitry can be used to
build an IRPD (Infra-Red Proximity Detector).
But the Sharp IS471F sensor, more
easily available and equipped with an
IR filter is better fitted to that job
information to build such a sensor.
Don't forget to visit his Mindstorms
The first purpose for building this sensor is
not yet fulfilled... triangulation navigation requires fairly
sophisticated software that I have not yet written: as I often
say, my favorite programming language is the soldering iron!
But this sensor can be used in many other applications, such
as my Barrel Collecting Robot.
Thanks to the S6986, the circuit is very simple
and requires few components. D2, D3, D5 and D6 forms a bridge
rectifier allowing to plug the sensor connector brick in any
direction. C1 filters power supply, it must be connected near
to IC1 to bypass it properly. IC1 is the Hamamatsu S6986 sensor,
the "heart" of this sensor. LED output drives the
laser with a 1/16 duty cycle, and the included photodiode collects
reflected light. Internal processing circuitry correlates the
received light to the emitted light, this enables very reliable
detection even with high ambient light.
When S6986 receives enough reflected light,
its output goes to the low level (R1 limits the current), and
the RCX sensor input becomes "true". Note that this
sensor only returns a boolean value, "true" if the
target is in laser beam, "false" otherwise. No distance
information is provided.
Diodes used are general purpose small signal
silicon type, such as 1N4148. C1 is a 10µF 10V tantalum
capacitor. D7 is the laser diode that I scavenged from a laser
pointer sold as a "toy" for a few Euros. I already
explained the choice of S6986, here is its datasheet.
Note that the S4282-51 can be used too, only the package is
The easy part... I used pieces of 40mm PVC pipes,
covered with retro-reflecting tape normally used on security
clothes, motorcyclists helmets...
These photos shows the catadioptric effect: the
left one, photographed with diffuse light, looks
normal, while the right hand one is photographed
with a flashlight and seems to glow because the
flash light is directly bounced back to the camera.
Note that the sensor may detect other reflective
surfaces (mirrors, metallic tubes...) but cat's-eye
reflectors are much more reliable. Mirrors must
be perpendicular to laser beam, while retro-reflecting
tape works with a wide range of orientations.
the laser target finder sensor
shows the main components of the sensor, and how they are mechanically
The electronics is simple, but mechanical construction
is not easy!
The cheap laser pointer I used.
The laser module is extracted from the pointer.
I cut out the switch and battery contact to trim
the size. The 47 Ohm resistor, mounted in series
with laser diode was replaced by a 68 Ohm, as shown
in the schematics above (R2), to compensate for
the higher supply voltage when powered from RCX
(8V instead of 4.5V). Note that about 30mA are required
by the laser to work properly. This is well above
the current that a RCX sensor port can deliver (14
mA). But the S6986 drives the laser with a 1/16
duty cycle, so the average current is only about
2mA, well within RCX sensor port capability.
the Hamamatsu S6986 includes a 50 mA constant current
generator to drive the laser, don't remove R2
! I did that on my first prototype, and fried
the laser in less than one minute !!!
The whole sensor is built in a box made from
four 2x4 LEGO bricks. The laser is placed at the
bottom of the first compartment.
On this image, you can see also the RCX interface
board, which contains the rectifiers and R1.
Before gluing the laser module in place, you
must carefully align the beam with brick structure.
I built this bench to do that. The white target
can be moved near or far from the laser, the beam
spot must remain in the same place.
A second tiny board supports the S6986 and C1,
the filtering capacitor placed next to the Hamamatsu
The laser beam must be as close as possible to
the S6986. This one is placed just above the laser
The lens in front of the sensor collects the
light and concentrates it on the sensitive area
of the Hamamatsu sensor. Lens and S6986 centers
must be aligned. Without the lens the working range
would be below 1 meter, while a 5 to 10 meters is
easily reached with it (depends on laser quality)
I used a plastic lens scavenged from a disposable
camera. Its focal distance is about 30mm, the lens
of a LEGO brick, and it can be drilled to let the
laser beam pass through undisturbed.
You will have to fine tune the position of the
sensor (carefully bend the leads) so that returned
laser spot is perfectly centered on integrated circuit
sensitive area. Place a target
1 or 2 meters away, aim it with sensor laser beam
and adjust S6986 position.
The first version I built had the laser and detector
side by side, about 15mm apart.
A brick wall separates detector from laser, avoiding
reflections or any direct coupling.
Unfortunately, the optical axises are too distant:
if the sensor is placed at the right position
for distant target (left image), the light spots
falls completely outside the sensitive area for
close target (right image). This first sensor was
definitely long-sighted and couldn't detect targets
closer than 50cm. Note that the long distance sensitivity
was very good for this prototype, it can spot targets
more than 10 meters away. The reason is that I got
a much better laser unit this time (brighter and
better collimated). These ultra-cheap laser pointers
are only worth their price !!!
Some more images of the first version...
RCX interface board
This sensor needs no test program... used as
a light sensor, it returns 100% if a reflecting target is in
laser beam, 0% otherwise.
Sensor Test Vehicle (LTV)
This little vehicle goes to a target and
turns when bumper hits it. Several targets creates
a circuit and LTV travels endlessly between
You can see the red laser spot on the target.
The rear assembly is a trimmer used to adjust
the laser horizontality.
of LTV (130K). This movie is 5x accelerated.
This robot detects barrels up
to 3 meters away, goes straight to them, grab
them with its arm and collect them in a basket.
See it here.