The circuit shown above was devised to provide a method of fine-tuning the experimental vertical antenna.  This remotely-controlled tuner has enabled me to change frequency much more quickly than previously, when I only had the larger variometer with which to make such adjustments. (Not easy with an upstairs shack; and the loading coil/variometer positioned in the garden!)

The potentiometer (having a 1/4 inch spindle) is mounted at one end of the axle and acts as both an end-bearing, and a detector of variometer position.  The driven end of the axle passes through the former of the outer inductor and is glued to a large pulley wheel.  Short pieces of cord are tied to the pulley wheel to limit the rotation of the spindle to +/- 90 degrees, thereby protecting the slider of the potentiometer from damage.

The pulley wheel (actually, it is a plastic wheel from a discarded push-chair) is belt driven by a 'Lego' motor and reduction drive built by my son, Trevor.

Built into a plastic storage box measuring 435 x 445 x 210 mm, the unit includes a wideband matching transformer having a 50 ohm primary winding and secondary taps at 50; 75; 100; 130; and 160 ohms. 

The variometer provides an inductance swing of 135 - 300 uH.
The inner coil (94 mm diameter) has two windings of 16 turns, each having a winding length of 32 mm.  The two windings are spaced 16 mm (to allow space for the axle). 
The outer coil (165 mm diameter) has two windings of 10 turns, each having a winding length of 18 mm.  The two windings are spaced 30 mm to allow space for the axle and potentiometer.

At 136 kHz, with a 12 m antenna, a tuning range of about +/- 500 Hz was achieved when it was positioned alongside my old Mark I loading coil - more than enough to cope with short-term variation in antenna resonance.  When used underneath my new Mark II loading coil, I now obtain a tuning range of +/- 1000 Hz.

Click on the thumbnails below for a more detailed look at the construction.

The belt-driven variometer

The Lego drive unit

Step-by-step construction of the Lego drive unit

The base is made up of 'plates' (which is about one-third the height of a standard   lego brick).  The Technic plates with holes allow for a 3-point screw fixing to a firm base.  The components shown below are:
4 off:  3709 Technic Plate 2 x 4 with holes (shown in black)
2 off:  3023 Plate 1 x 2 (shown in red)
2 off:  4477 Plate 1 x 10 (shown in yellow)
2 off:  3738 Technic Plate 2 x 8 with holes (shown in yellow)

An underside view of the base is shown below.

Next add the following components:
2 off:  3710 Plate 1 x 4 (shown in black)
2 off:  3623 Plate 1 x 3 (shown in red)

Now add the following components:
1 off:  3010 Brick 1 x 4 (shown in red)
1 off:  3005 Brick 1 x 1 (shown in black)
1 off:  3009 Brick 1 x 6 (shown in yellow)
1 off:  3008 Brick 1 x 8 (shown in red)

The following picture illustrates some of the parts that you will be using during the next stages.

Now add the following components:
2 off:  3702 Technic Brick 1 x 8 with holes (shown in yellow)
1 off:  3023 Plate 1 x 2 (shown in black)
1 off:  3023 Plate 1 x 2 (shown in yellow)
1 off:  3673 Technic Pin (shown in grey)

Now it's time to add the following axles, bushes, and gears:
1 off:  3707 Technic Axle 8 (2 1/2 inch long) (black)
2 off:  4519 Technic Axle 3 (7/8 inch long) (black)
1 off:  3706 Technic Axle 6 (1 7/8 inch long) (black)
3 off:  3647 Technic Gear 8 Tooth (shown in grey)
5 off:  3713 Technic Bush (shown in grey)
2 off:  3648 Technic Gear 24 Tooth (shown in grey)
2 off:  3650a Technic Gear 24 Tooth Crown (shown in grey)
1 off:  7/8 inch pulley wheel (shown in grey).  This part is not critical: it is used to stop the drive belt slipping off the bush on the driven axle!  Any crown gear or standard gear will work just as well.

And here is another view of the assembly so far.  This is probably a good time to study the two pictures of the completed Lego drive unit (above).  Check that the axles rotate freely.

Now add the following components:
1 off:  3023 Plate 1 x 2 (shown in red)
1 off:  3021 Plate 2 x 3 (shown in black)

Next, mount the motor and hold in place with the following cross member:
1 off:  6216 Electric Technic Motor  4.5V (shown in grey)
                                (alternative: 2838 Electric Technic Motor  9V)
1 off:  3894 Technic Brick 1 x 6 with holes (shown in red)

Now provide additional support for the motor using:
1 off:  3701 Technic Brick 1 x 4 with holes (shown in black)
1 off:  3673 Technic Pin (shown in grey)

For more information about the parts required, see:
http://guide.lugnet.com/partsref/