The two Lego motors are driven by an L293D motor driver IC (Integrated Circuit). This allows the motors to be powered by the 9V battery pack, but controlled by the 5V outputs from the Raspberry Pi. There are four outputs from the Pi to the motor driver, these control forward and reverse motion for each of the two motors. The rover is controlled by a Raspberry Pi Zero which is powered by the same battery pack as the motors via an L7805CV 5V power regulator. All the electronics components for the battery power, 5V power regulator, LED, motor driver, motor outputs and Raspberry Pi are connected on the breadboard by coloured jumper wires. I will explain below how to connect everything up to get it working. Although there are lots of wires on the finished breadboard, it doesn't look so bad when you see them added step by step. The labelled photo below shows the electronic components of the rover.
The power for the rover is supplied from the 9V battery pack that sits underneath the breadboard. The wires from the battery pack go into a 2 pin terminal block on the breadboard. The 9V power for the lights is also connected from this same terminal block. This is shown in the bottom left of the following labelled photo, which is actually at the back of the rover on the right hand side. The 9V power is connected on the breadboard to the input on the 5V power regulator (top left in photo) and to the 9V power pin on the motor driver (center). The 5V power regulator has three legs which are pushed into the breadboard. It also has a small round indent on the front left side of it. The leg nearest to this is the 9V input. The middle leg is connected back to the ground of the power supply. The remaining right hand leg is the 5V output. The 5V output and ground are connected to the power lines down the side of the breadboard which are used to connect the other 5V components and Raspberry Pi. The power regulator chip gets a bit hot after it's been on for a while, so be careful!
The L293D motor driver chip has 16 legs, 8 on each side, and a little round indent at one end to show which way round it goes (labelled with white arrow in photos). Breadboards are layed out so that you can put chips like this in the middle and the legs are connected to the numbered rows on either side of the board. Each leg is then connected up to the other components in the circuit by wires on the board. It's really important to connect everything up correctly here, because the 9V power supply could damage the 5V controller and Raspberry Pi The numbered photos below show the connections being added on the breadboard step by step. The instructions below the photos, explain each step in more detail.
The Lego power functions motors and lights each have a 4 wire cable. This is so that they can be switched on and off and reversed by the power functions switches. As the motors are going to be switched by the motor controller here, I don't need all 4 wires. And so I didn't have to cut the Lego connectors off the motors and lights, I bought some extra power functions extension cables and cut those up instead (except for one of the motors where the connector was already damaged). I have cut the square Lego power connectors off the ends of the cables, taped up the unused outer two wires with electrical tape, and stripped the inner two wires for connecting to the terminal blocks on the breadboard.
For connecting the 5V power supply to the Raspberry Pi, I have used a cut down old micro USB power cable (not a data cable). I read that it is safer to use the USB power input on the Raspberry Pi, rather than connecting power directly to the GPIO pins, because it has additional protection built into it's circuit. It also makes it easy to connect the power without any danger of getting it wrong and breaking something. I cut down the cable to the required length and there are two little wires inside (there would be more in a USB data cable). I think mine were white for power and black for ground, but this may vary on other cables. I connected these to some red and black wires for connecting to the terminal block on the breadboard and taped it up. I tested my power cable on an old bluetooth speaker, as well as checking it with a multimeter, before trying it with the Raspberry Pi. The terminal block is connected to the 5V and ground power lines down the side of the breadboard, so it is powered via the 5V power regulator. This is important as 9V would damage the Raspberry Pi. See section below on testing the motors before connecting up the power to the Raspberry Pi.
I added an LED on the breadboard to check that the 5V power was connected up reliably before connecting the power to the Raspberry Pi. The LED is just connected to the 5V power down the side of the breadboard with a 330ohm resistor. When the battery pack is connected to the breadboard (before connecting the Raspberry Pi power), this LED should light up. If it doesn't light up check it is the right way round and everything is connected properly. If it's super bright and gets hot, disconnect power and check you have connected the power from the 5V regulator, not directly from the 9V batteries.
Before connecting up the Raspberry Pi power and GPIO pins (step 4 above), I tested the motors and controller were connected up correctly and weren't going to damage the Raspberry Pi. Instead of connecting the 4 coloured jumper wires for the motor control inputs in step 4, carefully connect a wire between the 5V power (along the side of the breadboard) and one of the motor control inputs (2nd leg from either end, both sides of the chip). This should cause one of the motors to turn, so make sure to lift the rover up first, or take the tracks and wheels off, before it runs away! Check each of the control inputs, one by one (not all at once), by connecting to the 5V power (not 9V). You should see that each one causes one of the motors to run forward or backward. The program on the Raspberry Pi will turn these on and off to control the motors. If the motors don't work, go back through and check everything is connected properly and the power LED it on. Once the motors are working, connect up the GPIO wires to the Raspberry Pi (step 4). Then connect the USB power and check that you can connect to the Raspberry Pi via SSH after it has booted up. You can then move on to programming the rover.