Servo motor RCX interface
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I put this project in my web site only because I didn't find a practical reference for this device anywhere and I decided to make a guide.

This device is by Ralph Hempel, so if you are interested on details about it, follow the link.  

 

 

I bought a servo for about 20 € (quite equal to 20$), that's about the price of a Lego geared minimotor: but with a servo you can control position of the shaft without external sensors.

 

 

Here's the internals of the device I built, making only made some modifies:

 

I removed a voltage drop diode before the servo because it was weak (too low alimentation) ,

     and for packaging reasons.

I replaced the giant 220uF capacitor, with a much smaller 47uF, that's enough.

As Hempel suggests, I polarized the driver to respond to positive polarity,

        removing two diodes from the  feeding bridge.

It was a challenge to fit all inside a 2x3 brick!

 

 

 

 

 

 

 

 

 

 

 

 

 

 

I'm not very capable on programmation, so, after trying learning PBForth - that allows complete control on Servos - and after sadly failing,

I returned to good old NQC, that I use through BricxCC:

with it, due to limitations of original Lego RCX firmware, I could control only the extreme positions, all right, all left.

It is enough to set motor power levels at 5 or 6 to obtain this positions ( also try 4 for an intermediate position): it's a pity it's not possible to reach the center with power level control.

So I came up to this code:

int dir;

void right (int time)
{
    dir=1; SetPower (OUT_A,4); On(OUT_A); Wait(time); Off(OUT_A);
}

void left (int time)
{
  dir=2; SetPower(OUT_A,6); On(OUT_A); Wait(time); Off(OUT_A);
}

void center ()
{
  if(dir==1){ //if on the right,turn left until center
    dir=0;SetPower(OUT_A,6);On(OUT_A);Wait(18);Off(OUT_A);}
  if(dir==2){ //if on the left,turn right until center
    dir=0;SetPower(OUT_A,5);On(OUT_A);Wait(13);Off(OUT_A);}
}

void center2 ()
{
  dir=0;SetPower(OUT_A,4);On(OUT_A);Wait(40);Off(OUT_A);
}


task main ()
{
  SetSensor (SENSOR_1, SENSOR_TOUCH);
  SetSensor (SENSOR_2, SENSOR_TOUCH);
  SetSensor (SENSOR_3, SENSOR_TOUCH);

  while (true)
  {
    if(SENSOR_1==1){ //go right and remind it
      dir=1;right(60);}
    if(SENSOR_2==1){center();}
    if(SENSOR_3==1){ //go left and remind it
      dir=2;left(50);}
  }

}

It is only a driving test, using 3 touch sensors to control left right and center, obtained simply with a timing procedure. It could be done better.