tutorials:learn:sensors:cds.html
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tutorials:learn:sensors:cds.html [2010/10/07 19:08] daigo |
tutorials:learn:sensors:cds.html [2016/01/28 18:05] (current) |
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| ***Size: **Round, 5mm (0.2") diameter. (Other photocells can get up to 12mm/0.4" diameter!) | ***Size: **Round, 5mm (0.2") diameter. (Other photocells can get up to 12mm/0.4" diameter!) | ||
| ***Price:** [[http://www.adafruit.com/index.php?main_page=product_info&cPath=35&products_id=161|$1.00 at the Adafruit shop]] | ***Price:** [[http://www.adafruit.com/index.php?main_page=product_info&cPath=35&products_id=161|$1.00 at the Adafruit shop]] | ||
| - | ***Resistance range: **200KΩ (dark) to 10KΩ (10 lux brightness) | + | ***Resistance range: **200K Ω (dark) to 10KΩ (10 lux brightness) |
| ***Sensitivity range:** CdS cells respond to light between 400nm (violet) and 600nm (orange) wavelengths, peaking at about 520nm (green). | ***Sensitivity range:** CdS cells respond to light between 400nm (violet) and 600nm (orange) wavelengths, peaking at about 520nm (green). | ||
| ***Power supply:** pretty much anything up to 100V, uses less than 1mA of current on average (depends on power supply voltage) | ***Power supply:** pretty much anything up to 100V, uses less than 1mA of current on average (depends on power supply voltage) | ||
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| - | As we've said, a photocell's resistance changes as the face is exposed to more light. When its dark, the sensor looks like an large resistor up to 10MΩ, as the light level increases, the resistance goes down. This graph indicates approximately the resistance of the sensor at different light levels. Remember each photocell will be a little different so use this as a guide only! | + | As we've said, a photocell's resistance changes as the face is exposed to more light. When its dark, the sensor looks like an large resistor up to 10MΩ, as the light level increases, the resistance goes down. This graph indicates approximately the resistance of the sensor at different light levels. Remember each photocell will be a little different so use this as a guide only! |
| {{ http://www.ladyada.net/images/sensors/graph.gif?nolink&500x351 |}} | {{ http://www.ladyada.net/images/sensors/graph.gif?nolink&500x351 |}} | ||
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| |**300 - 500 lux**|Sunrise or sunset on a clear day. Well-lit office area.| | |**300 - 500 lux**|Sunrise or sunset on a clear day. Well-lit office area.| | ||
| |**1,000 lux**|Overcast day; typical TV studio lighting| | |**1,000 lux**|Overcast day; typical TV studio lighting| | ||
| - | |**10,000–25,000 lux**|Full daylight (not direct sun)| | + | |**10,000 - 25,000 lux**|Full daylight (not direct sun)| |
| - | |**32,000–130,000 lux**|Direct sunlight| | + | |**32,000 - 130,000 lux**|Direct sunlight| |
| ==== Testing your photocell ==== | ==== Testing your photocell ==== | ||
| - | The easiest way to determine how your photocell works is to[[http://www.ladyada.net/learn/multimeter/| connect a multimeter in resistance-measurement mode]] to the two leads and see how the resistance changes when shading the sensor with your hand, turning off lights, etc. Because the resistance changes a lot, an auto-ranging meter works well here. Otherwise, just make sure you try different ranges, between 1MΩ and 1KΩ before 'giving up' | + | The easiest way to determine how your photocell works is to[[http://www.ladyada.net/learn/multimeter/| connect a multimeter in resistance-measurement mode]] to the two leads and see how the resistance changes when shading the sensor with your hand, turning off lights, etc. Because the resistance changes a lot, an auto-ranging meter works well here. Otherwise, just make sure you try different ranges, between 1MΩ and 1KΩ before 'giving up' |
| [[http://www.ladyada.net/images/sensors/cdslitmm.jpg|{{ http://www.ladyada.net/images/sensors/cdslitmm_t.jpg?nolink&500x375 |}}]] | [[http://www.ladyada.net/images/sensors/cdslitmm.jpg|{{ http://www.ladyada.net/images/sensors/cdslitmm_t.jpg?nolink&500x375 |}}]] | ||
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| - | The way this works is that as the resistance of the photocell decreases, the total resistance of the photocell and the pulldown resistor decreases from over 600KΩ to 10KΩ. That means that the current flowing through both resistors //increases// which in turn causes the voltage across the fixed 10KΩ resistor to increase. Its quite a trick! | + | The way this works is that as the resistance of the photocell decreases, the total resistance of the photocell and the pulldown resistor decreases from over 600KΩ to 10KΩ. That means that the current flowing through both resistors //increases// which in turn causes the voltage across the fixed 10KΩ resistor to increase. Its quite a trick! |
| - | ^Ambient light like...^Ambient light (lux)^Photocell resistance (Ω)^LDR + R (Ω)^Current thru LDR +R^Voltage across R^ | + | ^Ambient light like...^Ambient light (lux)^Photocell resistance (Ω)^LDR + R (Ω)^Current thru LDR +R^Voltage across R^ |
| - | ^Dim hallway^0.1 lux|600KΩ|610 KΩ|0.008 mA|0.1 V| | + | ^Dim hallway^0.1 lux|600KΩ|610 KΩ|0.008 mA|0.1 V| |
| - | ^Moonlit night^1 lux|70 KΩ|80 KΩ|0.07 mA|0.6 V| | + | ^Moonlit night^1 lux|70 KΩ|80 KΩ|0.07 mA|0.6 V| |
| - | ^Dark room^10 lux|10 KΩ|20 KΩ|0.25 mA|2.5 V| | + | ^Dark room^10 lux|10 KΩ|20 KΩ|0.25 mA|2.5 V| |
| - | ^Dark overcast day / Bright room^100 lux|1.5 KΩ|11.5 KΩ|0.43 mA|4.3 V| | + | ^Dark overcast day / Bright room^100 lux|1.5 KΩ|11.5 KΩ|0.43 mA|4.3 V| |
| - | ^Overcast day^1000 lux|300 Ω|10.03 KΩ|0.5 mA|5V| | + | ^Overcast day^1000 lux|300 Ω|10.03 KΩ|0.5 mA|5V| |
| - | //This table indicates the approximate analog voltage based on the sensor light/resistance w/a 5V supply and 10KΩ pulldown resistor// | + | //This table indicates the approximate analog voltage based on the sensor light/resistance w/a 5V supply and 10KΩ pulldown resistor// |
| - | If you're planning to have the sensor in a bright area and use a 10KΩ pulldown, it will quickly //saturate//. That means that it will hit the 'ceiling' of 5V and not be able to differentiate between kinda bright and really bright. In that case, you should replace the 10KΩ pulldown with a 1KΩ pulldown. In that case, it will not be able to detect dark level differences as well but it will be able to detect bright light differences better. This is a tradeoff that you will have to decide upon! | + | If you're planning to have the sensor in a bright area and use a 10KΩ pulldown, it will quickly //saturate//. That means that it will hit the 'ceiling' of 5V and not be able to differentiate between kinda bright and really bright. In that case, you should replace the 10KΩ pulldown with a 1KΩ pulldown. In that case, it will not be able to detect dark level differences as well but it will be able to detect bright light differences better. This is a tradeoff that you will have to decide upon! |
| - | ^Ambient light like...^Ambient light (lux)^Photocell resistance (Ω)^LDR + R (Ω)^Current thru LDR+R^Voltage across R^ | + | ^Ambient light like...^Ambient light (lux)^Photocell resistance (Ω)^LDR + R (Ω)^Current thru LDR+R^Voltage across R^ |
| - | ^Moonlit night^1 lux|70 KΩ|71 KΩ|0.07 mA|0.1 V| | + | ^Moonlit night^1 lux|70 KΩ|71 KΩ|0.07 mA|0.1 V| |
| - | ^Dark room^10 lux|10 KΩ|11 KΩ|0.45 mA|0.5 V| | + | ^Dark room^10 lux|10 KΩ|11 KΩ|0.45 mA|0.5 V| |
| - | ^Dark overcast day / Bright room^100 lux|1.5 KΩ|2.5 KΩ|2 mA|2.0 V| | + | ^Dark overcast day / Bright room^100 lux|1.5 KΩ|2.5 KΩ|2 mA|2.0 V| |
| - | ^Overcast day^1000 lux|300 Ω|1.3 KΩ|3.8 mA|3.8 V| | + | ^Overcast day^1000 lux|300 Ω|1.3 KΩ|3.8 mA|3.8 V| |
| - | ^Full daylight^10,000 lux|100 Ω|1.1 KΩ|4.5 mA|4.5 V| | + | ^Full daylight^10,000 lux|100 Ω|1.1 KΩ|4.5 mA|4.5 V| |
| //This table indicates the approximate analog voltage based on the sensor light/resistance w/a 5V supply and 1K pulldown resistor// | //This table indicates the approximate analog voltage based on the sensor light/resistance w/a 5V supply and 1K pulldown resistor// | ||
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