For our streetcar design we wanted to :

• manually control things with a Web interface on a smart phone, and
• automatically have the streetcar stop at station, wait, then go in the reverse direction.

For this project we built everything in Lego, with Lego Mindstorm components. The programming was done in Node-Red which was running on a Raspberry Pi.

The Lego Mindstorms EV3 and NXT components can be wired into your Pi (or Arduino) projects by using some custom adapters. If you are brave you could cut one end off of the connector cables. We unfortunately didn’t have any extra cables so went the safe route with the adapters. There are a couple of different adapters that are available we used the breadboard version from Dexter Industries.

Using the following parts we were able to make an automated streetcar :

• 1 Lego Mindstorms motor
• 1 Lego Mindstorms touch sensors
• 3 Lego Mindstorms connector cables
• 3 Dexter Lego Mindstorms bread board adapter
• 1 small breadboard
• 1 Pi 3 (or Pi 2 with a network connection)
• 1 Pimoroni ExplorerPro Hat
• 2 long pieces of wire
• lots of Lego blocks

Streetcar Design

Our goal for the streetcar project was to have two stations, then have the streetcar automatically stop and switch directions at each station.

Station 1 had the motor, a touch sensor, and the Raspberry Pi.

Station 2 had a pulley wheel, a touch sensor and a small breadboard that wired back to the Pi at station 1. Our pulley string went above the streetcar. We needed to keep the string quite tight so the pulley would not slip.

The Wiring

All of the Lego Mindstorms wiring used the same pins on the left side of the connector. and they are labeled “ANG” and “GND”. We used an ExplorerHat Pro to connect the Lego Mindstorms Motor and touch sensors to the Raspberry Pi. As we mentioned earlier the station 2 touch sensor was wired into a small breadboard and then from there two wires connected it to the ExplorerPro. For our setup station 1 touch sensor was wired on Explorer Pro input 1, and station 2 was on input 2.

Node-Red Installation

To check that Node-Red is installed and working, go to a Terminal window and enter:

node-red-start &

If you are able to run Node-Red, the next step is to install the Pimoroni ExplorerHat node and a web dashboard node. Depending on your installation you might need to load the Node Package Manager, npm :

sudo apt-get update
sudo apt-get install npm

Then to install the added libraries:

\curl -sS get.pimoroni.com/explorerhat | bash
 cd ~/.node-red
 npm install node-red-contrib-explorerhat
 npm install node-red-dashboard

After you install these library nodes you will need to restart your Pi. Once Node-RED restarts, you use a web browser to build applications. If you are working directly on your Pi, enter 127.0.0.1:1880 in the URL address box of your browser.

Node-Red Manual Web Control

To make a web based manual control program, drag three button nodes from the left node panel onto the center panel. Then wire the button nodes to an ExplorerHat output node.

Double click on a button node to open an edit window. The edit window allows you to configure the dashboard, labels and button actions. To control the first ExplorerHat motor the topic is motor.one. The payload is between -100 and 100, and this corresponds to full reverse and full forward, with 0 being stopped.

After you have finished all your Node-Red configure click the Deploy button at the top right of your browser window. Deploying will run your program and enable the web dashboards. To access the web dashboards enter: http://your-ip-address:1880/ui. If you are unsure of your IP address go to a terminal window and enter : ifconfig.

Node-Red Automatic Control Program

We found the automatic control logic to be a little tricker than the manual logic. Our first step was to see if we could read the Lego Mindstorms touch sensors. For this we connected an ExplorerHAT input node to a debug node. We manually pushed each touch sensor and we used the debug tab to check the results.

To catch when the streetcar hits the touch sensor at station 1 we needed to create a function that looked at the topic explorerhat/input.1. If this topic’s playload is 1 or pushed then the ExplorerHat motor can be stopped by sending a message with the topic of motor.one and the payload=0. A similar function is created for the touch sensor at station 2 except the topic explorerhat/input.2 is monitored.

After the streetcar hits a touch sensor we wanted it to pause, and then move in the opposite direction. A delay node was used for the pause. Following this a function node is used to send a message with a payload of the new speed and direction. For our project station 1 restarted with a speed of -70, and station 2 restarted with a speed of 70 (or +70).

Our complete logic with both manual and automatic control is shown below: