I wonder if anyone could help with an insight into this situation...  
I'm out of my debugging depth at this point.

I'm working on a project with several remote sensors.  On each of 4  
flights of stairs there are mounted two Sharp GP2D12 IR distance  
sensors (pointed lengthwise to detect a stepping foot anywhere on the  
monitored stair) and a KC7783 IR motion sensor to detect activity at  
the doorways.  Both of these parts operate at +5V and are connected  
to to a 5V 5A power supply.  The distance sensors output an analog  
voltage (2V range), and the motion sensor TTL.   The distance sensors  
draw 33mA each and the motion sensor 400uA.

When the three sensors are all connected near (within a foot of) the  
power supply, they all work fine.
When connected at the end of about 60' of CAT5, the motion sensor  
refuses to work and I see low frequency noise on all three sensor  
outputs.
If I power the motion sensor right at the 5V supply, but leave the  
distance sensors at the far end of the long cable, they all work fine.
If I leave the motion sensor at the far end of the cable, but power  
the distance sensors right at the supply everything works fine.
If I leave all three sensors at the far end of the cable, power the  
distance sensors right where they're mounted using a 9V battery +  
LM7805, and power the motion sensor from the 5V supply at the near  
end of the cable, all three work fine.

The only non-working configuration is all three devices at the far  
end of the cable, with all three powered through the cable.

I'm wondering what the distance sensors could be doing to the power  
supply from the far end of the cable that would cause the motion  
sensor to crap out, but not have that effect on the near end of the  
cable.  Is there something about running the power to those device  
through the 60' of wire that would make such a difference?

Any suggestions much appreciated!
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