PHYSICAL COMPUTING

Photoconductive Cell
250mW, 250V peak
12Kohm max light, 1Mohm min dark
Jameco Part # 120299

The photoconductive cell (photocell), effectively reads the amount of light hitting its surface, changing the resistance in its circuit. As the light gets brighter, the resistance lowers; darker light increases resistance. One thing to keep in mind is that photocells generally have a low reaction time (in the 100 ms), which is too slow for many applications (to access a range, you'll probably have to read its value several times over several seconds).

From Wikipedia:
A photoresistor is made of a high-resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance.

Photocells have a wide range of commercial and industrial uses, including street light control, room light control, contrast control in TVs, toys, camera light meters, clock radios, and security alarms. GE:CU (Germanium Copper) photocells are used in infrared detectors.

I used this photocell to generate sound in an installation via Arduino and Max/MSP. The Arduino Playground proved to be a good resource in opening the doors of communication between the photocell and Max. After downloading the SimpleMessageSystem library, I uploaded this code to my Arduino board:


#include
/* Analog/Digital inputs to MAX/MSP trigger
* ------------
* send serial values to MAX/MSP to trigger something
*/
char firstChar;
char secondChar;
void setup()
{
Serial.begin(115200);
}
void loop()
{
if (messageBuild()) { // Checks to see if the message is complete
firstChar = messageGetChar(); { // Gets the first word as a character
if (firstChar = 'r') { // Checking for the character 'r'
secondChar = messageGetChar(); // Gets the next word as a character
if (firstChar = 'd') // The next character has to be 'd' to continue
messageSendChar('d'); // Echo what is being read
for (int i=0;i