SRT LED: Interactive Motion Sensor LED Installation

Kendra Hills + Eli Ericson + Will Friedrichs

Our project, SRT LED, is an interactive public art installation that will illuminate poorly-lit pathways and underused spaces along Philadelphia’s Schuylkill River trail (SRT). Sensor-activated LEDs respond to visitors’ movements with dynamic lighting effects, contributing to placemaking and engagement along the city’s riverside.

Our motivation  

The Covid-19 pandemic has shed light on just how important outdoor public spaces are to urban communities, and they thrive when people feel safe while using them. Lighting can help reduce perception and fear of crime, improve walkability, and has the potential to have a positive impact on local economies by increasing activity after dark. Lighting can also be a powerful tool of beautification, and our project aims to create an interactive public light installation to create more inviting and active open spaces after dark. 

In many cities with large rivers, including Philadelphia, riverfront spaces are often former industrial sites, with ports, warehouses, and factories more recently giving way to urban highways that block access to the riverside. Activating these spaces as enjoyable and interactive places can benefit residents and city economies, prioritizing residents’ quality of life and their access to public space. With strategic and adaptable lighting solutions, we hope to create fun and inviting spaces along all parts of the Schuylkill River Trail, beginning with parts of the trail adjacent to North Philadelphia’s historically disinvested communities.

Our site

This project was inspired by the dark, uninviting pathway pictured below — an offshoot of the SRT north of the Philadelphia Museum of Art. Unlike the riverfront trail in affluent Center City farther south, this trail segment is currently unlit, discouraging residents from using the trail after dark and limiting nighttime activity along the river. Our project has the potential to make this space significantly livelier and more inviting to all Philadelphians. Starting our modular and expandable SRT LED system at this site could help counter inequities between more affluent and lower-income areas along the SRT, creating an attraction to the trail’s usual nighttime activity hub in Center City.

Our solution

Our LED installation combines a photoresistor, ultrasonic distance sensors, and LED strips to line the trail with interactive lights that respond to human movement. The photoresistor detects the ambient light level and sends power to the LEDs only when the light is low, an energy-conserving measure that ensures the lights turn on automatically as the sunsets. The ultrasonic distance sensors detect pedestrians or runners passing on the trail and trigger interactive lighting effects that respond to the location of the activated sensor, the time spent in front of the sensor, and the direction of movement along the path. 

Demo

In the video, you can see the lighting effects in action in our prototype!

How it works

To create our interactive light display, we wired one photoresistor and five HC-SR04 ultrasonic distance sensors to a one-meter Adafruit NeoPixel addressable LED strip with 60 RGBW (red, green, blue, and white) LEDs. We used a breadboard and exposed components for our prototype, but an outdoor implementation would use the more rugged components described later in this post.

The diagram shows a simplified version of how the installation works:

Sensing ambient light to turn on LEDs at dusk

As you’ll see in the demonstration video, when the photoresistor detects that the ambient light level has dropped below a threshold, the lights turn on. Through our Arduino code, the addressable LED strip lets us create complex and dynamic light effects by modifying the hue, saturation, and value (or brightness) of each individual LED.

We chose a gently shifting rainbow gradient for our baseline effect, intended to draw visitors’ interest to the installation and serve as a visually interesting but not too chaotic backdrop to the interactive effects activated by the sensors.

Detecting passers-by along the path

Once the lights are on, the installation uses ultrasonic distance sensors to detect pedestrians and runners on the path.  The sensors measure the distance to the nearest object – or person – using an ultrasonic pulse that bounces when it encounters a barrier. (Our tutorial on how to detect movement direction with two HC-SR04 sensors talks about this in more detail.) If a sensor detects a distance smaller than the width of the path, as it would if someone walking on the path prevented the signal from reaching the other side, we set that sensor’s state to active. We set the sensor state to inactive if the distance detected is greater than the width of the path, indicating that the path segment in front of the sensor is empty.

To interface with the Arduino, each of the five distance sensors relies on two specialized pins in addition to power and ground: a trigger pin, which sends out the ultrasonic signal, and an echo pin, which receives the returning signal after it bounces. We were able to streamline the wiring and stay within the Arduino’s pin capacity by using the Gamegine HCSR04 library to allow the five sensors to share one trigger pin while keeping separate echo pins. The shared trigger pin sends an outgoing signal from all five sensors simultaneously, and the sensor-specific echo pins record a separate distance reading for each individual sensor.

Activating LEDs in response to sensor input

In our code, we manipulate the raw sensor readings to produce three interactive lighting effects that passers-by can experiment with by varying their movement:

• Activating a single sensor causes the LED strip segment immediately in front of it to produce a “rainbow sparkle” effect created by changing the color of a random sequence of LED “pixels” to a rainbow gradient set to cycle at a rate that contrasts with the baseline gradient. Using the timestamp of when each pixel activates, we let the contrasting pixels linger for a moment before returning to their original hue, creating a more subtle and layered effect than if they turned off instantly as the next set turned on.
  
• Staying in front of a sensor for a longer period causes the “rainbow sparkle” effect to intensify as the colors gradually shift from pastel to bright. We map the sensor activation time to the pixel saturation by recording the time when each sensor first triggers and using it to track how long the sensor has remained active.
  
• Activating more than one sensor in quick succession causes a segment of white light to travel down the LED strip in the direction of movement. The order in which the sensors activate lets us infer which direction someone is moving – and because movement in each direction is tracked separately, two people walking in opposite directions will each trigger the effect. In other words, if two people start at each end of the strip and move toward the center, two separate light segments will meet in the middle, cross over, and keep going to the opposite edge.

Over time, even more effects could be added, or existing effects modified, simply by uploading updated code without the need for any additional hardware.

Scaling up

We envision SRT LED as a modular installation that can be disassembled or moved for maintenance or testing at other sites. Any section could be removed for cleaning or repairs without needing to disassemble the entire installation. Each section will sit partially embedded into the ground, and will consist of a metal enclosure topped by solar panels with the ultrasonic sensors, battery, and LED components inside. 

The outdoor installation would use water-resistant components, including waterproof ultrasonic distance sensors, along with anticorrosive coatings for protection from the elements. A small opening in front of each sensor is necessary to allow the ultrasonic signal to pass through, but each opening will be covered with acoustically transparent wire mesh to block interference from debris or tampering. The ultrasonic sensors will be angled slightly upward to capture the body mass of average-height individuals using the trails.

The sensors will be powered by solar panels on the top panel of the enclosure. The collected solar energy will be stored in batteries inside the enclosure to power the LEDs overnight. The batteries will also power the programmable microcontroller wired to the network of LEDs and ultrasonic sensors to collect sensor input and respond with LED patterns.

Like the single-strip prototype, the full LED installation would work as one system, with the light patterns disseminating throughout the entirety of that system. The programmable nature of the LEDs would allow new behavior, interactivity, and aesthetics to be added with a single code upload. For example, lights could be color-coordinated in celebration of a holiday, or even in anticipation of a sports game. When implemented at large scale, we hope our system can contribute to Philadelphia’s cultural identity and draw both resident and tourist visitors to the waterfront year round.

Prototype component list

• Arduino Uno or equivalent generic microcontroller
• Adafruit NeoPixel RGBW LED strip
• Five HC-SR04 ultrasonic distance sensors
• Photoresistor
• 5V 2A power supply
• 2.1 mm jack to screw terminal block DC power adapter
• 1,000 uF capacitor (to use with LED strip)
• 10K ohm resistor (to use with photoresistor)
• Female-to-male DuPont wires
• Male-to-male jumper wires
• Breadboard

Resources and references

• “Adafruit NeoPixel Überguide” for information about powering, wiring, and programming addressable LED strips.
• Adafruit NeoPixel library for controlling the LED strips via Arduino.
• Gamegine HCSR04 ultrasonic sensor library for controlling multiple ultrasonic distance sensors.
• “Solar Powered RGB LED Magic Pathway” for inspiration and tips for building and waterproofing an outdoor solar powered LED installation.
• “Methods to conceal an ultrasonic sensor” for information about acoustically transparent materials compatible with ultrasonic distance sensors.