PICs, I2C and sensors

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Hello.

I am working on a fun little half-built device that is going to follow a
black trace line the floor, but due to my less perfect knowledge of
electronics this is starting to look like a huge task.. :/

Programming is no problem, using the PIC1F628 I've made it go round the
floor like a happy puppy, but without any sense of direction.
But the difficult part is combining the three little photointerrupters,
another PIC1F628 and the I2C-bus.

The photointerrupters are of simple analog type Kodenshi SG-2BC, and I
want to connect them to the PIC. But I am clueless what I need to do to
make the PIC utilize them? I suppose I can not directly solder them to
the PIC?

The whole point is to make the second PIC act as a slave on the I2C-bus,
take care of the input from the photointerrupts and deliver to the
master PIC upon request.


Thanks for any help :)



Re: PICs, I2C and sensors
Hi !

The photointerrupters are simple devices with two basic components: a) a
photodiode that emits an IR light and b) a phototransistor that is sensitive
to IR light.

Now, if you know how a transistor works, then a phototransistor works
precisely the same way, only that the IR light's luminancy acts as the "base
current", i.e. more light means more current, thus resulting greater current
via collector-emitter pins. In order to interface this device with any
microcontroller, you need to fathom some sort of a A/D converter. This would
consist of two parts.
First, you'd have the photointerrupter, and it's photodiode. The diode is
connected to a series resistor (small) and then to a 5V output on the PIC.
The other end of the diode is grounded. Now, when logic 1 is written to the
port, 5V's will cause a current through the diode, thus causing it to emit
IR light. To calculate the series resistor, use the data sheet's "forward
voltage" and "forward current" diagrams, making sure the current passing
through the diode doesn't exceed maximum ratings (i.e. break the diode)
Second part of the design has the phototransistor. The collector pin is
connected to a series resistor, and this resistor to a voltage source (not
the PIC, but some steady voltage source, such as 5V). When the transistor
detects IR light, it starts to allow current pass-through from collector to
emitter. The emitter is connected directly to an A/D converter, and a
parallel resistor is connected to the same pin. The converter input must be
as high-impedance as possible, so majority of the current will pass through
the parallel resistor to ground. Current passing through the parallel
resistor causes a voltage drop over the resistor. Thus, the A/D input pin
will have a voltage that is related to the amount of current passing through
the transistor (and indirectly related to the amount of IR light received).
Then you create an A/D conversion and depict a way to relate the amount of
voltage (distance of object) with the control logic.

If the robot is supposed to follow a black trace line, then you must place
the photointerrupters so that two of them will target both sides of the
line, and one will target the line itself. Whenever the two side
interrupters report max luminosity (they point to white floor) and the
center one reports low luminosity (the black line doesn't reflect IR that
good), then your robot is going to correct direction. If the center one
reports high luminosity, then you check which one of the side ones reports
low (i.e. where is the trace turning to) and adjust robot direction
appropriately.

This might or might not work, and it might or might not be complete. But
it's a start, nevertheless. You should first create a schematic that uses
the photointerrupter to measure a distance of some object, and interfacing
this with the A/D converter design. I don't know how PIC's A/D converter
works, so I can't give you a precise, working solution.

-Antti Keskinen


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Re: PICs, I2C and sensors
Hi Fred, Hi Antti,

Like to add just two comments.

i) The current through the IR-LED should not be more than
10 mA if you want to control the led by a PIC.

ii) Have in mind, that an IR phototransistor (the device
built into your "photo interruptors") uses infrared light,
not visible light. Therefore, a black line may not give
you enough infrared contrast to have a good bright/dark
distinction. Also, some floor coatings etc. are mirrors
for infrared light. To be sure I recommend using a
photocoupler with a red led. It has also the advantage
that you can actually see it working.

Happy tinkering,
Wolfgang

--
From-address is Spam trap
Use: wolfgang (dot) mahringer (at) sbg (dot) at


Re: PICs, I2C and sensors

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A minute with Google brings up this sensor circuit of another robot project.
http://murray.newcastle.edu.au/users/students/2001/c9806198/files/ddr.pdf

I wouldn't bother with doing it in analog. If a sensor sees the line then
change direction!
And why use I2C when you already have serial I/O built in?

Peter



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