External Power Supply Circuit for the RCX 2.0

DISCLAIMER:  This information is distributed on an "AS IS" basis, WITHOUT
WARRANTY OF ANY KIND, either express or implied. The author assumes no
responsibility for any property damage or personal injury resulting from
the use or misuse of this circuit.  Please exercise caution.

Here's a circuit to provide external power for your RCX 2.0.  The advantage
this circuit provides is that program memory is retained when external power
is removed.  This is accomplished by using five AA cells to provide a
memory backup supply of 6 to 7 volts to the RCX.

The electrical circuit requires no modification of the RCX and can be
removed at any time to allow normal battery operation.  The only
modification you might consider doing (and this is entirely optional)
is to place a notch in the dark gray battery box cover so that the
power cable can exit the battery compartment with the battery cover
seated snugly on the RCX.

The AC-DC adapter used is a 12v 800ma regulated supply.  Use of an
unregulated supply is not recommended because these supplies generally
provide a significantly higher than rated voltage when the load is small.
When using a regulated 12V supply, the 3 diodes drop the voltage to ~9V.
When operating the RCX at high current, the diodes can become very
warm as they dissipate power (heat) associated with the voltage drop
they provide.

CONSTRUCTION NOTES

Please refer to the rcx2ps.gif image for the power supply schematic.

If you look closely at photo b1.jpg, you can see that the clips that
attach to the RCX are actually just the diode leads bent in a "U".  The
clip attached to the long lead (which goes to the negative RCX terminal)
is actually a small piece of a bent paper clip soldered to the wire.

Photos b2.jpg - b7.jpg illustrate how the circuit attaches to the RCX.
Make the bend at the end of each wire wide enough that it fits snugly in
the appropriate recess in the RCX battery terminals.  Be sure to observe
proper polarity when installing the diodes.  The end of the diodes with
the colored band should point towards the RCX positive terminal.

The power supply I used is a 3-12V 800ma regulated adapter available from
Radio Shack in the U.S.  The part is #273-1667.  The plug/cable assembly
used for the connection to the diodes is part #273-1742.

Please email if you have any questions,

Mark Riley
markril@hotmail.com
http://home.surewest.net/markril/lego/index.html