White Light is showing until march 26th at West Hall room 111 on RPI campus . The gallery is open on week days from 9-5pm. Please feel free to contact me for off hours viewing. pete [at] casperelectronics [dot] com
What it is
White Light is an interactive sound and light installation. It is part sculpture, part musical instrument and part social exercise. White Light invites viewers to become part of a complex sound and light display by manipulating the controls on one of 3 control podiums spaced evenly around a glowing orb.
Each podium can function independently but the true strength of the piece is realized when the viewers work together to control all three podiums at once.
What it does
As a viewer approaches a podium it begins emitting an audio drone and the ball in the center illuminates. The viewer may manipulate a series of controls on the podium which alter the characteristics of the audio signal and light display.
Light & Sound
Enabling the podium activates two independently tuned square waves. It also activates a light within the sphere which is linked to the sound.
These two elements are linked in a unique way.
The brightness of the light is relative to the phase alignment of the two oscillators. When two oscillators are slightly detuned it creates a cyclically modulating phase alignment. This manifests as a modulating volume or in the case of the lights, a modulating brightness. The rate of the drone is relative to the detuning of the oscillators. The closer they are together, the slower the drone or pulse. Guitarists use this trick when tuning their strings. The same drone signal is used to generate the sound as well as the light.
You are seeing what you are hearing and visa versa.
Each of the three podiums generates a different color in the sphere. Red, green and blue. The name White Light refers to the light generated by activating all three podiums at once, thus mixing red, green and blue light making white.
White light only occurs when all three colors are at equal intensity. Due to the nature of the light modulation control, each color is constantly pulsing in and out. This means that the total color of the sphere modulates as each of the three colors combine at constantly changing intensities. The result is unpredictable and ever changing without ever being random. It’s similar to watching a flickering flame, simple but hypnotic.
There are two levels of interaction with this piece.
One is as an individual viewer. Manipulating the controls on a single podium can create a range of sounds and accompanying lighting effects.
The second level of interaction is as a group. multiple viewers can manipulate the controls to create a wide range of audio and color mixing effects. White light is placed as a central concept for this piece as it symbolizes the perfect union of the three elements. This serves as a suggestion to the participants. An encouragement to work together and make something unified and harmonious.
How it works
There are a few different elements at work in this piece:
NOTE: Schematics are posted below.
The point of the drone circuit is to generate two oscillators which can be slightly de-tuned in order to generate the phasing pulse effect.
The oscillators are square waves generated using 555 timer ICs. The trick with this circuit is avoiding oscillator lock. This has been a big problem with various drone circuits I’ve been working on. When the oscillators are tuned at nearly the same frequency they want to lock up. This destroys the drone effect. I have minimized this by using low power ICs (part #7555), hefty power decoupling, and current limiting on the power supply.
This stage consists of a 15watt amplifier and a tunable audio filter.
The amp is pretty straight forward. It takes the combined output of the 555s and makes it nice and loud. The filter was added simply to make the podium more “playable” and give the viewer more control over the sound. It doesn’t have any effect on the light. The filter has two stages. First is a tunable resonant band pass filter. Next is a non-tunable low pass filter for increasing some of the low end. I fit both of these on a TL082 dual op-amp IC.
The light control uses the audio signal to control the brightness of the LED as well as responding to the proximity sensor which I’ll explain below.
Pulse Width Modulation (PWM)
The best way to modulate the brightness of an LED is by using a high frequency pulse width modulated power supply. This means that the power going to the LED goes on and off very quickly, too fast to be perceived. The “pulse width” in PWM refers to the length of time the pulse is on vs time it is off. As this ratio changes it results in an increase or decrease in LED brightness. At a 50% duty cycle square wave, the LED will be a half brightness. At 80%, meaning it is on 80% of a cycle, 0ff 20% means the LED will be much brighter (80% brightness).
The cool thing about a drone (in relation to PWM) is that two overlapping square waves create a single, varying pulse square wave. I have this illustrated in the image below at the bottom of the page. The problem is that it only makes a 50% spread meaning it only goes from 0% to 50%. What you really want is 0% to 100% to get the full sweep from the LED off to full brightness. This can still be very useful for certain applications.
Luckily by sending the square waves through an exclusive OR gate you are able to get a FULL 0% to 100% pulse width sweep (see the illustration below). by sending this signal to an LED driver I am able to get a dramatic brightness pulse.
I also want the light and sound to fade in and out as viewers approach and walk away from the podium. This requirs a whole other means of brightness modulation. I found that current limiting via a transistor network worked well. I’ll explain how this works in the next section.
The effect of this feature is that the podium activates by the viewers presence.
When the viewer stands close to the podium it will begin making sound and the light will turn on.
Different Kinds of Sensors
I looked into several different kinds of proximity sensors for this project. The three most common kinds are visual, audio and capacitive.
The visual and audio based proximity sensors both utilize an internal transmitter/receiver network. A signal is transmitted (UV light, ultra sonic sound) which bounces off of the viewer and is read by the receiver. My primary concerns with both of these methods are that A) they are very directional, if the viewer isn’t in the right spot it won’t read. And B) they require exposed components. I didn’t want the viewer to be distracted by the sensors. A proximity sensor is a trick, it’s magic. The podium turns on when you approach it. An exposed sensor would be like a badly performed magic trick.
So I went with the capacitive sensor. This sensor can read slight differences in ambient capacitance introduced by a persons presence. I found an IC intended for use in touch screen displays which does the trick. It has been pushed beyond it’s intended range, but seems stable so far.
Attack and Decay
The way the sensor works is that when it senses a person the output pin goes high (outputs 5 volts). When it no longer senses that person it goes low (0 volts). It’s ON or OFF. I found that switching the light and sound abruptly ON and OFF was jarring and I wanted to make them fade in and out.
I did this using some capacitors, transistors and a whole lot of trial and error. The output signal is sent through an RC (resistor/capacitor) network to make a voltage envelope. That means instead of going ON and OFF the volume and light fades in and out slowly. It’s shown in the schematics below.. That means that the light brightness is controlled using two methods, PWM and current limiting. It’s a slightly inefficient system, but all told it works well.