[EMS Discuss] Light sensor refinement suggestions?

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Sat Aug 24 16:29:04 PDT 2013


Smoked glass could do the trick, but you can get more out of your sensor, as well:

If I am visualizing the circuit correctly, then perhaps the 10K resistor is too large.  At full dark, this voltage
divider reads .62 x 5V and then climbs from there as more light is added (and probably not linearly, either).  To make
best use of the analog in pin, you ideally want your signal to range from 0 to 5V.  You can't get that with two
resistors, but if you choose a 2K pulldown you could get a range of about (.25 x 5V) to around (.8 x 5V), which is
about the best you can do (actually, I think 1.7K is slightly better).  After that, then you can mess with the voltage
references for the analog pin to get closer to a full-range reading.  To do that, you need to supply a reference
voltage that is just slightly larger than your maximum sensor voltage to the AREF pin (use a fixed pair of
larger-value resistors as a voltage divider to provide the reference voltage).  Then follow the advice here and set
your reference to external:  http://arduino.cc/en/Reference/AnalogReference

That should get you the maximum resolution with the minimum effort.  (Although, if you are going the extra mile and
setting the external reference, you can eke out a bit more resolution by using an even smaller pulldown. Say 900 Ohms.
 I think.  Current through the two resistors is still just a few milliamps, so that's OK.)

-Darrell



> Put a piece of smoked glass over it?
>
> Sent from my phone.  No type good.
> On Aug 24, 2013 2:01 PM, "Greg Norman" <EMAIL HIDDEN> wrote:
>
>> I built a light sensor for a time lapse camera controller
>> but it goes into saturation very quickly when the sun rises.
>> It's made out of LDR (Light Dependent Resistor). I suspect
>> for a wider sensitivity I need to use a diode and a op amp
>> but I'd like to keep it simple.
>>
>> Anyone have a suggestion on biasing it as to respond to a
>> wider range of light? Currently it connects to a Arduino
>> analog pin and +5V with a 10K pull down resistor to ground.
>>
>> The LDR goes from 500 ohm with light to 6K ohm in dark
>>
>> The circuit is:
>>
>>   5V---(LDR)---(Arduino Pin)---(10K ohm)---GND
>> --
>> Posted from the Discuss forum
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