[Back to Home Page]
Using the Osram SFH7710 Optical Tilt Sensor
These are a very nice little SMD tilt sensor using digital optics
- 26th Oct 2009.
What is it?
The Osram SFH7710 is a TINY little SMD tilt sensor. It is normally
used in cameras and digital picture frames to rotate the image if the
appliance is rotated, ie portrait/landscape.
I decided to test this neat little sensor for robotics and security
applications, anywhere a tiny tilt sensor might be used to detect if
something has been tilted, rotated, knocked over etc. It can also be
used an an inertia sensor, to see if something has been bumped or if
a little robot has hit a wall etc.
How it works
The SFH7710 is an opto-mechanical angle sensor operated by gravity.
It contains a little ball (which must be as smaller than a pinhead!) that
slides inside a track and cuts off an internal infrared light beam.
This is basically the same thing as the old mercury tilt switch
(or the more modern ball tilt switches that don't use nasty mercury)
but it solves most of the problems with mechanical tilt switches!
It's MUCH smaller
The optical beam won't wear out like contacts do
The output is cleaned up by its digital filter and output circuit
Very high life (number of cycles before it wears out)
These things are tiny and difficult to solder, but fortunately you only
have to solder 3 of the 4 pins. Well, they are not actually pins which
is part of the problem... They are corner pads on a tiny double-sided PCB.
(See the top picture on this page). Ideally these would be soldered like
a QFN (no leads) package in an oven.
I couldn't be bothered etching a PCB and using my SMD oven so instead
I just "tacked" it on top of a SO14 adapter board with a fine point
I used 2 glass diodes (1N4148) to drop the 5v supply to 4v and used a
2.2uF cap across the power pins (GND and Vcc) of the device. Then since
the output pin is a pull-down only (open drain) I used a LED and 330 ohm
resistor from its 4v power pin to the Vo output pin.
Using it as a tilt sensor
It worked like a charm! I mounted the SFH7710 vertical on a protractor
with blu-tak, and then rotating the whole setup caused the LED to light
I tested the hysteresis zone to be about 15 to 20 degrees if the
device is rotated very smooth and slowly. If vibrated the hysteresis
zone was smaller as you would expect, because the vibration encourages
earlier ball movement.
Using it as an inertial sensor
As an inertial sensor it had both good and bad points. Very short bumps
and shocks (raps and taps) were not detected very well due to the internal
filtering and the devices 90mS update time. This is a deliberate design
feature to make the sensor operated by "orientation" rather than
But, with a longer more sustained acceleration or deceleration the device
worked fine. I rotated it's orientation until the light would just go out,
then sliding it across the desk and hitting a stationary object would
cause a reliable light-on event every time. That same internal filtering
seems to make a reliable inertial threshold.
It would be quite usable as a robot inertial forward collision sensor
provided the robot was on a smooth and level floor. I will attach it to
a robot later and test that application more completely.
Using it as a motorcycle alarm tilt sensor
I'm not sure it would be good for this application. The old mercury tilt
switches in glass capsules had a very small hysteresis of only a couple
of degrees. You can easily set one up to be reliable as a motorcycle
alarm sensor to detect when the motorcycle is lifted off the sidestand.
Even allowing for variations in angle caused by terrain.
But the high (15 to 20 degree) hysteresis would make this device extremely
fussy to set up especially if the motorcycle is not on level ground.
Although it is much better than a mercury switch in quite a few ways, the
hysteresis issue will mean this sensor is not suitable for some more
sensitive tilt applications.
(From the datasheet) Using 3 tilt sensors to make a dice or detect any
standard product orientation;
Just some ideas off the top of my head;
Tiny robot tilt detect where 3-axis accelerometer is not warranted
Tiny alarm sensors
Smart float switches
Inertial force detectors, collision detectors etc
High cyclic use (machinery?)
Human wearable computing, (finger movement sensors etc)
Using it (English)
- end -
[Back to Home Page]