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    Power Functions Battery Box & Mindsensors' NXTServo

     

    I recently got a Mindsensors' NXTServo and RC-Mini 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.


    March 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 them here.

    Opening 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 pack

    Now pull firmly the battery holder apart from the top cover.

     

    A 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 current

    Here is the complete schematics of the battery pack.

     

    Modifying the 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!)

 

    Connection with NXTServo

    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.

     

    WARNING:
    I shall not be held responsible if you burn your NXTServo or RC servos with incorrect voltage or bad polarity !
     

    NXTServo LDraw parts pack

    Here 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 battery.


     

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