This is a fantastic project that combines basic electronics and Lego to build a functional underwater ROV.

We have used this project in a few workshops with children ( and adults ) with great success.

This Instructable will guide you through building the basic ROV, whilst this will work very well in the water, it can be easily enhanced with additional Lego - a fun project is to mount a magnet onto the ROV so that it can be used to recover magnetic treasure chests - see video in the last step.

Step 1: Bill of Materials

The Lego pieces

1 x Medium Stone Gray Plate 8 x 8 with Grille (Hole in Center) (4151) 1

3 x LEGO Plate 1 x 8 (3460)

1 x LEGO Plate 1 x 6 (3666)

1 x LEGO Technic Brick 1 x 4 with Holes (3701)

1 x LEGO Technic Brick 1 x 6 with Holes (3894)

2 x LEGO Beam 11 (32525 / 64290)

4 x LEGO Tile 1 x 2 with Perpendicular Beam 2 (32530)

4 x LEGO Technic Brick 1 x 14 with Holes (32018)

4 x xLEGO Technic Brick 1 x 8 with Holes (3702)

2 x LEGO Brick 1 x 2 with Pin (2458)

8 x LEGO Technic Pin with Lengthwise Friction Ridges and Center Slots (2780)

8 x LEGO Axle to Pin connector with Friction (43093)

4 x LEGO Beam Bent 53 Degrees, 3 and 7 Holes (16616 / 32271)

The Lego parts should be available in most online Lego part sites or eBay - we used BrickOwl to find them for our build - search the site using the number in brackets after the name in the list above.

The remaining parts

I've added links after each item to either eBay searches or to the place we purchase the items from - you could probably get them elsewhere as well though.

2 x 35mm film canisters ( eBay )

2 x small propellers ( AliExpress)

1 x large propeller ( AliExpress )

3 x self adhesive motor holders ( RapidOnline )

3 x 3v DC motors ( RapidOnline )

1 x 2.5 metre Cat5 Ethernet cable (stripped and split into twisted pairs) ( eBay )

6 x female crimp terminals - 2.54mm pitch ( eBay )

3 x 2 Pin Dupont Housing - 2.54mm pitch ( eBay )

4 x cable ties - hardware store

If you have difficulty finding anything then let us know and we may be able to suggest an alternative.

Step 2: Assemble the Body Sides

Take the four 53 degree beams and arrange them as in the first photograph, placing a Axle to pin connector in the ends of each piece. Next take two of the 1 x 14 bricks and attach the pins to the end pins so they bend inwards. Place a long beam at the bottom of each - take careful note of the location of the overhanging holes.

Attach the perpendicular beams to the edges of the two remaining 1 x 14 bricks and place the friction pins in the holes shown on the photograph.

Attach them to the vertical beams with the 1 x 14 bars on the same side of the beams as the top two beams.

Finally add two 1 x 8 bricks on top of each of the lower bricks, and the push the bricks with pins through the first lower holes - take a careful note of the location and side on the photograph - these bricks should be on what will be in the inside of the ROV for holding the vertical motor mount.

Step 3: Join the Sides

Next we'll join the two sides together - place the 1 x 6, 1 x 4 bricks and a 1 x 8 plate on top of each other.

The edges of the plate should go under the two long bricks, the top of the 1 x 6 brick will go under the two pin bricks and the 1 x 4 brick will sit in-between.

It is important that the two pin bricks overhang at the front of the ROV as we need a flat surface on the inside for the motor mount.

Finally place the 1 x 6 plate on the top to keep everything together.

Step 4: Add the Grill and Buoyancy Mounts

Finally place the 8 x 8 Grid on the top of the ROV in the middle - this will protect the vertical motor from catching on wires, fingers and other items in the water.

The two remaining 1 x 8 plates go on either end - we'll use these to attach the buoyancy tanks later on.

Step 5: Attach the Motor Mounts

Take two of the motor mounts, remove the adhesive backing and stick them to the sides of the ROV.

They will overlap the bricks slightly, you can either place them as in the photograph so that they are flush with the bottom of the bricks, or place them flush with the top and overlap at the bottom.

Both will work, however if you are planning on using a wider vertical propeller than we do then you should have the overlap at the bottom so that you have a clear gap for the propeller between the top bar and the side walls.

Take the remaining motor mount and stick it to the inside wall of the ROV - try to get it in the centre. - you'll see now why it was important to the get the pin bricks on the right side of this wall, if you have them the wrong way around you won't be able to stick this piece.

Step 6: Crimp One End of Each Tether

The tether is made by taking a Cat5 ethernet cable - we used about 2.5 metres. Strip the outer jacket and you will be left with 4 x twisted pair cables, you need three of these twisted pairs - one for each motor.

With each pair, crimp a 2.54mm pitch female dupont pin on one end and place the pair in a 2 pin holder - these will attach to our controller later.

Ensure the lighter / white cable is on the right hand side as in the picture - or make a note of which cable is on the right for each pair.

Step 7: Solder the Motor to the Tether

The easiest way to solder the wire to the motor is to place it in the grips of a helping hand.

On the bottom of each motor, next to one of the pins you will see a small circle with a plus in it, you will solder the white (or right hand cable in the crimped holder) to this pin.

Strip each cable and wrap them through the holes on the motor then solder them. If you wish you could add heat shrink to the cable to cover the join later for a more sturdy join.

Solder all three motors.

Step 8: Attach the Motors and Propellers

The motors fit in the mounts as per the first photograph. It's probably easier to push the propellers on to the motors before putting them in the mounts - it will be a tight fit for the propellers but don't be scared of putting them on a table and pushing the motor down on to them.

The two smaller propellers should go on either side of the ROV, the larger one goes in the middle vertical mount.

Step 9: Route the Tether

Pass the tether for the side motors underneath the motor mount and between the bottom beam and the long brick and then towards the back or the ROV.

When you have all three cables together at the back, route them up over the back buoyancy tank mount so that we can use the rear tank to hold them in place.

Step 10: Attach the Buoyancy Tanks

Place the two 35mm film cannisters on either end of the ROV, the rear one should go over the top of the tether cables and hold them in place.

Wrap two cable ties around each cannister and the Lego plate and pull them tight.

Finally snip off the excess cable tie so everything looks neat.

Step 11: Fine Tuning Weight and Buoyancy

The last step is to get the weight of the ROV correct.

Place the ROV in the water, it may take a few seconds for it to settle as water fills the motors.

Push the switch on your controller (see next part of this Instructable) to make the ROV dive. If it descends in the water then congratulations you have the weight correct. If it struggles to descend then you have two choices:

1. Add more Lego bricks to add weight to the ROV - be careful to spread them evenly so the balance of the ROV is maintained.

2. Open each of the film cannisters and add a small amount of water to increase the weight.

If your ROV is nose or rear heavy then you should do one of the above steps on the lighter side to try to balance things out.

Once you have the weight and balance right you should be able to dive and control the ROV underwater.

See part two of this Instructable to make the controller.