Subject:
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Do it yourself light sensors. How does RCX drive them?
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Newsgroups:
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lugnet.robotics
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Date:
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Wed, 13 Jan 1999 06:09:18 GMT
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Original-From:
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Pam Durham <pdurham1@ix.netcom.%spamcake%com>
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Viewed:
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1048 times
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Greetings,
I've been attempting to build my own IR sensor just like the one that ships
with Mindstorms. I've been to the various sites that describe such things,
and have even built the circuits described there (see
http://www.umbra.demon.co.uk/sensor1.html). Unfortunately I haven't been
able to make them work. I'd like some more insight in to how the light
sensor is driven by the RCX.
Here's my understanding so far:
The RCX time multiplexes the light sensor. For 300ms it drives it at 8v and
for 100ms it drives it at about 5v. The 8v cycle is for powering the light,
and the 5v cycle is for performing the measurement. Is this correct? More
details would be appreciated.
If so, none of the circuits I've seen on the various web sites should work.
It would seem that to make this work one would need a switching transistor
that shunted current to the light only when the voltage went above 5 volts.
This current shunt would then be used to charge a capacitor. During the 5v
phase the switching transistor would turn off and isolate the light and the
light would be driven by the charge in the capacitor (this is needed since
an led will instantly turn off, unlike an incandescent light. During the 5v
phase the RCX would only see the photo transistor.
The circuits I've seen hook the led and the photo transistor in to what
amounts to be parallel. Thus during the 5v measurement phase the RCX is
measuring the voltage drop across both the led and the phototransistor.
I've found that this makes the effective measurement range about 680 - 700
raw units, too small to be useful.
Am I correct in my assumptions and does anyone have a good circuit design to
build a IR sensor with an IR source? It would save me having to dig out my
college text books.
Why do I want to do this? I want to make a sensor where the light source is
in one brick and the light sensor is in the another brick. This way I could
stack them to make a sensor like the one that comes with Mindstorms, or I
could mount them across a gap to make a sensor that could detect when
something was in the gap, (a zero force touch sensor).
Thanks,
Doug Durham
--
Did you check the web site first?: http://www.crynwr.com/lego-robotics
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