Functions Battery Box & Mindsensors' NXTServo
I recently got a Mindsensors' NXTServo
Servo. As the Battery
Holder with NXT Mount was not available at the time, I decided
to use a modified Power Functions
battery box to power the servos.
The main issue is that RC servos require 5 to
6V while PF battery box provides 9V. By using only 4 out of
6 batteries in the pack, the modified version can supply 4.8V
(with 4 NiMH cells) or 6V (with 4 alkaline cells).
You will find also some details on the battery
pack itself, and a LDraw parts kit for NXTServo.
2008 update: Dave Parker of NXTprograms
sent me photos of an other way of lowering battery pack voltage.
No solder required, no modification of your battery box! See
the battery pack
First remove the two screws at each
end of the battery pack...
...and the battery lids.
Make sure you
push the pack slider all the way at the opposite
of the PF connector. This is required
to avoid bending switch blades when opening the
Now pull firmly the battery holder
apart from the top cover.
bit of reverse engineering...
The slider makes several connections
on the PCB.
Both sides of the PCB. On top side,
from left to right, we see:
- D2, a power diode, preventing voltage
reversal on PF output connector
- The PF connector
- LED D1 and series resistor R1
- PCB pads for slider switch
- Resettable fuse R2, limiting output
Here is the complete schematics of
the battery pack.
The modification is
easy: just unsolder the black wire from point 1
and resolder it to point 2. This way, the two topmost
cells are no longer used, and battery voltage drops
accordingly. Take care to keep the channel (green
rectangle) free as a rib of the cover slides here.
Reassemble the battery
pack and put 4 AA batteries as shown here. Dont
forget to put a mark to identify this battery pack...
if you use it with your PF motors they will work,
but at a low power/speed, and you'll wonder why...
(I got caught with this one!)
Cut a PF extension wire,
and strip the insulator of outer wires. The two
center wires (C1/C2) are not used
Connect the cable to
the NXTServo this way.
I shall not be held responsible
if you burn your NXTServo or RC servos with incorrect voltage
or bad polarity !
LDraw parts pack
is the LDraw parts kit containing NXTServo PCB,
mini servo mounting plate, mini servo mockup and servo connector.
To use it, unzip it in your LDraw folder.
Danny modeled other servos types: you can get his parts here, and see a nice example
of servo use!
Dave's fake battery trick
If you replace some batteries in your pack with
fake batteries that deliver no voltage but conduct electricity,
you can reduce the voltage delivered (by 1.5V per replaced battery).
That's the trick used by Dave
Parker, out of wadded and rolled up aluminum foil that fit
in the space of an AA battery.
Power Functions battery
pack with fake battery inserted.
Close-up on the fake