Matrix user forums

Hi to all.
I am trying to add a light sensor to my board so I can dim the LCD display when it is dark in order to save power.
There seem to be many options but some seem complicated.
I do not need high precision but am looking for something simple.
But I discovered that an LDR is hard to find those days.
Anyone who can recommend something.
Best Regards:
Uli

Hi Uli,
I noticed on a different post you use US Digikey.
Take a look at:
http://www.digikey.com/product-search/e ... 23?k=photo

They have a got LDRs but under a different name.

I believe LDR is by far be the easiest method.

Martin

Thanks very much.
So they call them Photocell.
What do you recommend should I take the one with the lowest resistance and make a simple voltage divider?
I see the resistances are quite high , would I also need to put a condenser between the Input into the ADC and ground to get good readings?
Best Regards:
Uli

You don't need a capacitor for a simple version using voltage divider method.
If it was me, I would choose an LDR with say a dark resistance of around 1Meg, and in the light about 10K = 20K ish.
It's not that critical.
The other part of potential divider, I would choose a resistor around the similar value of LDR when light is hitting it. e.g 10 - 20 K
Then you can use ADC input for LDR/Resistor potential divider.
If you want ADC voltage to increase when light hits LDR then LDR is connected between ADC and 5V, resistor is connected between ADC and GND

Another alternative to LDR is a Phototransistor. see:
http://www.digikey.com/product-search/e ... rs/1967049
That will require a higher value resistor to limit current when lit.

Thanks
I have no Idea what 4-11kilo ohm at 10 Lux means, does it mean Bright light?
I see there is one with 300K in darkness is that even better?
If I would use a photo transistor could I also use a voltage divider?
What are the advantages of a photo transistor?
Best Regards:
Uli

Hello Uli

[quote="Creative25"]
I have no Idea what 4-11kilo ohm at 10 Lux means, does it mean Bright light?

Lux indicates how well the surface area of an object, wall or floor is illuminated at some distance away from the source

10 lux is very dark a typical office is between 500 and 750 lux

i have used a ORP12 for light sensing before with good results

http://pdf1.alldatasheet.com/datasheet- ... ORP12.html


Hope this helps

Steve

Creative25 wrote: I have no Idea what 4-11kilo ohm at 10 Lux means, does it mean Bright light?
Uli

Creative25 wrote: I see there is one with 300K in darkness is that even better?

It depends on your application.
My guess is the higher the resistance between light and dark then the more sensitive photo Transducer is.

I have a suggestion.
MatrixMultimedia use a TEPT5700 in there latest Sensor boards.
Here is Digikey part:
http://www.digikey.com/product-search/e ... s=TEPT5700
The description is AMBIENT LIGHT SENSOR.
Datsheet:
http://www.datasheetarchive.com/TEPT570 ... sheet.html
On the data sheet it states

Applications
Ambient light sensor for control of display backlight
dimming in LCD displays and keypad backlighting of
mobile devices and in industrial on/off-lighting operation.

For circuit that uses TEPT5700 see:
http://www.matrixmultimedia.com/resourc ... 3-30-4.pdf
The advantage in using TEPT5700 over LDR is in dark conditions TEPT5700 will only draw approx 3nA which is ideal for saving power.
If you use an LDR then in the dark its drawing approx 250nA (20M)
If you use the 300K version (not recommended) then dark current is approx 16uA
So TEPT5700 saves power for battery applications.

Hope this helps.

Martin

Thanks for this excellent information,
now I start to get an understanding about LDR's.
Those information I could not get from the datasheets.
That is why I appreciate this forum so much.
Best Regards:
Uli