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help with ldr


Moderators: Ajay, Junied , abbas1707, Arun Kumar V, pdi33, Shailesh NAYAK, ۞ TPS ۞, shyam, sashijoseph

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rahuljin

Mon May 26 2008, 02:49AM

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i have some ldr and i want to make photo sensors with them !!! i tried to google but not found any circuit !!!

i have small type ldr !!! please anybody, provide a circuit diagram with some details to use it with microcontroller(89s52)

[ Edited Mon May 26 2008, 02:51AM ]

Arun Kumar V

Mon May 26 2008, 12:08PM

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Hi,

i don't know your skill level with micros but you can try reading the LDR value with ADC and micro, display the value on lcd - simple light/lux meter

Arun

pdi33

Mon May 26 2008, 02:07PM

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note that an LDR responds in an extremely non-linear way to the light intensity. The resistance of a LDR aries from a few meg-ohms in dim light to a few kilo ohms in bright light (maybe even a few ohms depending upon the light intensity and LDR used.). So I would suggest that u first connect the LDR as shown below:

VCC ------ LDR ------- 1K resistor -------- GND.
and plot the voltage across the !K resistor w.r.t different light intensities on the LDR.
then connect this voltage output to a ADC via a simple non-inverting op-amp ampilifier and connect the ADC to the uC as Arun suggested.

U may require to make a lookup table in the coding part to convert the readings to a LUX value.

* inspired to develop,developing to inspire *

rahuljin

Mon May 26 2008, 06:35PM

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thanks to both of u !!! i will try this

electronicfreak

Tue Aug 05 2008, 09:30PM

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pdi33 wrote ...

can i connect directly to micro other than connecting to GND ,like this
VCC ------ LDR ------- 1K resistor -------- uC(ADC)

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Arun Kumar V

Wed Aug 06 2008, 02:39AM

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hi,

No,you cannot connect LDR that way, you would be measuring Current and not Voltage.

you need to connect one end of 1K resi to Gnd and connect the junction of LDR and other end of 1K resi to ADC, this way the analog voltage is referenced to Gnd.

Arun