Conductor Plate
Another technique for attaching sensors to the RCX's inputs is based on special conductive plates. These plates are available as an accessory pack, #5037, from the LEGO Shop-at-Home service
(800-453-4652). The kit, which is $6.75US, comes with several plates to which you can attach your sensor electronics. This technique is fully described at:
http://www.akasa.bc.ca/tfm/lego_temp.
html.
Page 219
Passive Sensors
There are several simple passive sensors that attach directly to the inputs of the RCX. These are the simplest do-it-yourself sensors because you don't need any interface circuitry to make their
output comprehensible to the RCX.
A Peaceful Demonstration
To really understand passive sensors, it's helpful to see a diagram of one input of the RCX
in passive mode:
In passive mode, the sensor connected to the input is essentially a resistance. It forms a
voltage divider with R
input
. The A/D converter in the RCX converts the analog voltage to a
digital raw input value from 0 to 1023. The A/D converter itself has a resistance, R
ad
, but
it's so small you probably don't have to worry about it. The two diodes limit the voltage that
can be seen by the A/D converter; this makes it hard to damage the RCX by hooking
something up incorrectly.
Touch Sensors
Touch sensors are the easiest kind of sensors to make. Any kind of contact switch is appropriate, and no special circuit is necessary. All you need to do is attach the switch leads to the input
somehow.
Browse the pages of a catalog from Jameco (800-831-4242) or Digi-Key (800-344-4539) and you'll find a dizzying array of contact switches.
Another interesting possibility is using a mercury switch as a touch sensor. A mercury switch has a sealed bulb that contains a drop of mercury. When the switch is oriented the right way, the
mercury drop connects the two switch leads together. Figure 11-2 shows a photograph of a mercury switch; you may have seen ones with clear bulbs in your thermostat. The LEGO brick is shown
for scale.
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