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Cheap Alternative for Hard to Find CDS Light Sensor

cds_cell

CdS (Cadmium Sulfide) photo-resistors are commonly used for detecting light levels. Their resistance varies considerably depending on the intensity of light striking them. They are common, fairly cheap and easy to use. So what’s the problem? They are becoming hard to find. The reason is because of the RoHS directive. Since CdS cells contain cadmium, a toxic heavy metal, this important component is no longer stocked by major electronics distributors. The good news is that not only is there an excellent alternative, but it is more versatile and vastly cheaper.

If you guessed photo-diode you are close, but even better is the humble LED. If you are building an analog circuit, then you will be limited to photo-diodes and photo-transistors due to their greater sensitivity. If, however, you are building a micro-controller based circuit such as a PIC or Arduino, then you can use a simple LED to achieve the same end and even more.

Photo-diodes and LEDs

Both of these devices are essentially the same. Both are diodes enclosed in a translucent case. They will both permit a small amount of reverse current to flow through them when they are reversed biased. The difference is that the photo-diode … Read the rest

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The LED Lighting Revolution – A Primer

White LED

A lighting revolution is coming. LEDs will usher it in, but the road may be rocky along the way. Why? Because LEDs are a radically different type of lighting than what we are used to. For them to be used effectively, we need to change our way of thinking about lighting.

The Problem

LEDs are so different from Edison-style incandescent bulbs, that trying to create a drop-in replacement is problematic. This article in particular points out that current LED offerings have poor color spectrum, poor luminosity, and poor reliability. Ouch – they don’t sound very good at all, at least as screw-in replacements for a hundred year-old technology. While I think that they will eventually achieve a screw-in replacement that is fairly decent (look at this innovative offering from Phillips as an example), this pursuit is really the wrong approach. If you want to learn more about how LEDs work and how we can create some really special lighting that is hard or impossible to do with yesterday’s technology than read on. The best thing about LEDs is that the technology to solve our lighting problems is not in the future. It is available today. You just need to know … Read the rest

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