A CdS Light Sensor:

 

The Mindstorms Light sensor is based on a silicon photo transistor that is mostly sensitive to Infrared light. It has trouble with low light levels since the built-in LED light source leaks into the detector. Radio Shack sells a package of 5 Cadmium Sulfide (CdS) photoresistors, catalog number 276-1657, for only $2.29 that provide a more visible range of spectral sensitivity and also a wider range of photo sensitivity than the LEGO Light Sensor. The picture below shows my prototype constructed with the Dean Husby cut up LEGO Electric Plate method. The sensor is used by setting the sensor type to 0 and reading the Raw values from 0 to 1023. It may seem confusing, but the brighter the light the lower the Raw value.

The plot below shows how the CdS photoresistor is the most sensitive to visible light. The only disadvantage to CdS photoresistors is that they are relatively slow to respond to changes of light levels. They can take several seconds to change to a new value.

Here is a plot that tells you what RCX Raw values to expect to see given the light level. I took measurements with the CdS photoresistor held next to a photographic light meter. I related the light meter readings to units of light intensity or Foot Lamberts and common descriptions of the light conditions. Also shown is a plot relating the LEGO Light sensor reading to the CdS Raw reading.

 

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