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----- Original Message -----
From: "John Barnes" <barnes@sensors.com>
Newsgroups: lugnet.robotics.rcx.legos
Sent: Tuesday, May 07, 2002 2:23 PM
Subject: Re: Powered Sensors
> 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
Ok... lets see if I understand this....
For passive sensors it looks a bit like the following:
+5V
|
|
<
< 10K
<
|
|____________ 10 bit A2D
|
|
<
< Resistive Sensor
<
|
|
Ground
In this case (passive) The RCX converts the divided voltage into a 0 ->
1023 (10 bit) sensor reading.
That seems to make sense.... now what your saying is that an active sensor
looks like this:
+5V (Read) +7 (Power)
|
| Current
|_________>_________
To Active Sensor
_________<_________
|
|___________________ 10 bit A2D
|
<
< 10 K
<
|
|
Ground
So in this situation (active) the RCX supplies power (7 volts) and then
reads (5 volts). When it reads it measures the current by reading the
voltage across the 10K resistor? Something doesn't seem quite right about
this.... assuming that the active sensor had 0 impedance... the most
current that could be supplied would be (8 - 1.4)/10K. where 1.4 is
approximately the voltage drop on the diodes of the actual active sensor.
Maybe there is an H-Bridge or something similar inside to switch between
powering and reading? Does anyone know of a schematic that shows how this
works? I am in electrical engineering and would like to start building
some of my own sensors, but I do not want to damage my expensive little toy
:)
Thanks.
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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 (...) (22 years ago, 7-May-02, to lugnet.robotics.rcx.legos)
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