Electronics Assembly

Assembling the electronics for a basic LoCoRo is no more complicated than plugging a few wires onto some pins. Of course, it is very important to connect the right wires to the right pins. Wiring the wrong pins may destroy the one or more of the electronic parts.

If you have already done so, please read the Intro to Electronics page before continuing.

The initial documentation only covers the plug-n-play version of the electronics.

Clicking an image will display a larger version which may assist with documentation details.

Basic Parts List:

The electronics components for a plug-n-play LoCoRo include a Raspberry Pi 3, an I2C PWM servo controller board, a USB battery with dual ports (or two single port USB batteries), a micro USB cable, a bare-wire USB cable, a few jumper wires, and a USB 5V charger.

It may also be helpful to have a small project board.

The list of items are readily available from various online sources. An example shopping list is included on the Intro to Electronics page.

When connecting directly from the PWM board to the GPIO pins, you use male-to-male jumpers. If you wish to use a project board, then you will need double the jumpers and they will be male-to-female. One advantage of using a project board is the easy to add additional sensors - especially more I2C devices.

Electronics Layout:

Determine the layout for the electronics. It is helpful to keep space around the perimeter of the Raspberry Pi so the ports are accessible.

When using a 3D printed basic electronics plate (available from the 3D Prints page), there are small standoffs for both the Raspberry Pi and the I2C PWM servo controller board.

Raspberry Pi GPIO:

The Raspberry Pi 3 has a forty pin GPIO (general purpose input / output) connector. A basic LoCoRo uses 3.3V power source, a ground connection, and the I2C pins. More advanced versions a LoCoRo may use more of the GPIO pins for switches and sensors.

Use the included reference to insure you are working with the correct pins in subsequent steps.

I2C Connections:

Depending on which I2C PWM servo controller board you have, the jumpers may be vertical or horizontal. The important connections are VCC, SDA, SCL, and GND. The VCC is a 3.3V source and should not be confused with the V+ connection which is used to power the servos. The SDA and SCL pins are the I2C interface.

The connections between the PWM board and the Raspberry Pi are as follows:

  • PWM VCC → GPIO pin 1
  • PWM SDA → GPIO pin 3
  • PWM SCL → GPIO pin 5
  • PWM GND → GPIO pin 9
Power Connections:

Recommended Method:

The micro USB cable provides power to the Raspberry Pi. The bare-wire USB cable is connected to the POWER terminals of the I2C PWM servo controller board. Be sure to observe the V+ and GND labels.

The 3D printed basic electronics plate has a large hole near the PWM board POWER terminals. Running the bare wire USB cable up through this hole keeps it away from the micro SD card slot of the Raspberry Pi.

Simplified Method:

The recommended method provides separate power to the raspberry Pi and to the PWM board. The reason for this is that starting/stopping and running several servo motors may create excess loads on the power. By running the Raspberry Pi on a separate power connection, it reduced the risk of a low-power condition for the CPU.

The basic LoCoRo only has two drive servos. The power requirements are unlikely to create an unusual load and cause a low-power condition for the Raspberry Pi. It is possible to eliminate the bare-wire USB cable and use a single port USB battery. In this configuration provide the 5V to the PWM board by adding a connector wire between PWM V+ → GPIO pin 2.

As the complexity of the LoCoRo develops and more servos are added, it is advised to switch the recommended method above.

Servo Connections:

The basic tricycle chassis LoCoRo uses two servos for the drive system. These may be connected to any of the available 16 channels of the I2C PWM servo controller board.

Some PWM boards do not have color coded connections. Be sure to observe the servo wiring. Typically, the darkest of the three wires - from the servo - represents GND.

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