Subject:
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Re: 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 17:21:14 GMT
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Original-From:
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Malcolm S Powell <MSP@UMBRAstopspammers.CO.UK>
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Viewed:
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1262 times
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Pam Durham wrote:
> 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'm sorry if you have not been able to make my sensor work. However, the
ones I have built work just fine and I have had dozens of messages from
others who have built them successfully from my design. Guess you had
better check your wiring:-)
> 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.
You are correct in assuming that the RCX multiplexes power and data onto
the two wire connection to a sensor.
During the power phase of the cycle the RCX drives the connection in low
impedance mode so that there is very little voltage drop at the input
due to the current drawn to power the sensor, e.g. an IR emitter.
During the data input phase of the cycle, the RCX drives the connection
in low impedance mode so that any change in sensor current causes a
significant change in voltage drop on the input.
In my circuit, more current is drawn from the input when more light
falls on the photo transistor and this current is added to the current
drawn by the IR emitter thus changing the input voltage to the RCX.
During the power cycle, the additional current drawn through the photo
transistor drops very little voltage extra across the low impedance
source presented by the RCX.
As an alternative, the sensor itself may present a low impedance voltage
source to the input during the data phase in order to dictate the input
voltage. This allows more accurate control of the input voltage at the
cost of greater circuit complexity.
Malcolm Powell
--
Did you check the web site first?: http://www.crynwr.com/lego-robotics
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Message has 1 Reply:
Message is in Reply To:
 | | Do it yourself light sensors. How does RCX drive them?
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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 (URL) Unfortunately I haven't (...) (26 years ago, 13-Jan-99, to lugnet.robotics)
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