]> CDS Cell Light Sensor
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    One of the most common items used on small hobby or educational robots are light sensors. With even the simplest of uses, they enable the robot to perform tasks such as navigating towards a light, hiding in dark corners, following other robots, etc. More advanced uses permit following lines and detecting obstacles.

   The most common of such sensors fall into two categories: photoresistors and phototransistors. One more particular type of the former are made of Cadmium Sulfide cells. A picture is presented below. These are commonly available from Radio Shack or from any electronic components catalog pretty cheaply.


A CDS cell as commonly packaged.

Function and flow diagram of CDS Cell.

   CDS Cells are photoresistive light sensors. When no light is present their impedance is extremely high, and conversely very low when no light is present. In contrast to phototransistors, these cells generally don't seem to have as large a range of values between the two extremes of light and dark. These cells also have a much slower reaction time in response to changes in light as they have a large memory effect.

   These sensors are straightforward to wire. They're bidirectional, so simply connect one leg to your sensor input pin and the other to ground. Follow this link for a discussion on connecting phototransistors to a HandyBoard. These CDS Cells connect in the same fashion except you don't have to worry about which leg goes to which pin.

[Note: This page is a small demonstration of marking up pages about robots and their components. You can click on the "DAML Inside" link to see the markup, which corresponds to the function and flow diagram above. The diagram isn't truly necessary as it currently has no bearing on the text. I note that the markup is in a weird form---a class oriented view---which I think will be odd in practice. Instead, the device will probably be modeled as an instance in any actual system we develop. The markup's very simple currently. Some of the interesting things to add is the additional information present in the text, such as the note about its response time and notes on connecting it to other devices.]
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mailto:joe@plan.mcs.drexel.edu - Joe Kopena updated://thursday/9-19-02?11.19.28