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Ok, I have gotten myself confused again. 0-0.32mA represents a voltage
dropbetween 0 - 3.2 volts. Therefore the voltage that the RCX "reads" on
theA2D is between 5 - 1.8 volts (1023 - 368 RAW)? To get a reading of 0
(RAW)it seems as though you would need to sink 0.5mA through the sensor.
http://www.eece.maine.edu/~swinchen/images/poweredsensor.jpg is a picture
that shows what I believe to be happeningbetween the RCX and an active
sensor. This is a diagram from the "ExtremeMindstorms" book.
The voltage on the A2D will be:
Vad = 5 - 10e3 * I
The two diode drops add to 1.4 volts, so the voltage across the 11K "sensor"
is:
Vsen = 3.6 - 10e3 * I
Solving: I*11K = Vsen leads to a currnet of: 0.17143mA (3.2857 on the A2D
=> 672 RAW)
But from Gasperi, "For Light sensors: Light=146-RAW/7 within the range 0 to
100."
Using the RAW value I calculated leads to a reading a of 50. Not 61, like
the book claims.
Perhaps I made an error somewhere, if someone could just take a quick look
at it I would be gratefull.
First I am trying to see how the sensor behaves with a passive load, and
then I want to see what happens when I buffer the input, and start playing
around with voltages. I am confused as to how the diodes don't play a role
when using an analog input. It seems that they would throw the voltage
reading from the A2D off by about 1.4 volts. Also, I am trying to figure
out the role of that 1K resistor. Thanks for any help... and if this is
not the correct place to post such questions, could someone suggest a more
appropriate place?
> No, the RCX knows nothing about the diodes. That is why devices like the
> light sensor only send a current signal in the range 0 - 0.32mA back to • the
> RCX. This current then creates a voltage drop across the 10k reference
> resistor with respect to the 5v supply.
>
> By using a current signal rather than a voltage one, the unknown drop • across
> the diodes doesn't matter as long as it doesn't exceed a certain minimum.
>
> All the more accurate sensors mostly use this technique.
>
> JB
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Message is in Reply To:
 | | Re: Powered Sensors
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| No, the RCX knows nothing about the diodes. That is why devices like the light sensor only send a current signal in the range 0 - 0.32mA back to the RCX. This current then creates a voltage drop across the 10k reference resistor with respect to the (...) (23 years ago, 7-May-02, to lugnet.robotics.rcx.legos)
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