# Measure Light Intensity using Photoresistor

#### alexlucan3

Joined May 17, 2021
2
hello , im a student and i got a project to done until 22.05.2021
I need help ! im desperate !
I have to build in OrCAD a circuit that measure the intensity of light in a room .
I have to use 3 or more LEDs ( of different colors ) to measure some thresholds.
The circuit is supplied from a voltage source VCC and
the resistence of the light transducer varies linearly with the value of the measured illumination level.

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#### MrChips

Joined Oct 2, 2009
29,206
Moved to Homework Help from General Electronics Chat.

#### DickCappels

Joined Aug 21, 2008
10,067
Do you know any types of integrated circuits that can signal when a threshold has been crossed?

#### alexlucan3

Joined May 17, 2021
2
Do you know any types of integrated circuits that can signal when a threshold has been crossed?
as i said , im a beginner .
even without a threshold , i still dont undertand things...
i would like to make a circuit with a LDR , but in OrCad , it doesn't exist a component LDR . I think i have to build it somehow

#### MrChips

Joined Oct 2, 2009
29,206
In any CAD program substitute a resistor for an LDR.

#### neonstrobe

Joined May 15, 2009
189
This is an interesting project but measuring light intensity accurately is quite a challenge. Our eyes "see" light which is measured in lumens, but the sensitivity varies with colour. We see green more than red and blue for the same power level. I assume that you are allowed to assume that the LDR you plan to simulate will respond in a similar manner to the eye - which, fortunately, is approximately true for some LDR's. That reduces the problem, as MrChips said, to measuring a resistance!

A proper model of the LDR would have some sort of input to control the resistance. Is this what you are expected to do?
A model of a variable resistance with a voltage input (higher voltage=>lower resistance perhaps) is a little more difficult but would be a better solution I think. But it means knowing quite a bit about simulators and models.

If that seems too difficult then varying the resistance in your circuit would be a solution.

#### DickCappels

Joined Aug 21, 2008
10,067
Depends on how accurate you need it to be. For response by wavelength we can use the photopic sensitivity curve with its peak near 550 ns (green). The sensation of luminance is associated with the amount of light under the curve.

A CdS cell has a similar curve with its peak near 550 ns (green):

Rohm Semiconductor makes a color sensor that costs less than US\$1. It is surface mount and has an I2C interface. See the attached datasheet.

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#### BobTPH

Joined Jun 5, 2013
7,523
Lets not complicate the problem way beyond what was asked.

The question stated that the resistance varied linearly with the intensity.

Bob

#### Audioguru again

Joined Oct 21, 2019
6,151
Why doesn't the student ask the teacher to explain what is needed??

#### MrSalts

Joined Apr 2, 2020
2,767
Why doesn't the student ask the teacher to explain what is needed??
That is a common problem - students delay, professors don't want to help with last minute questions if the student haven't even started 24 hours before the project is due.

also, it sounds like the student is supposed to use an LED as the light detector. This works well snd the red LED will detect all room light a green LED will have a slightly larger bandgap to detect greens and blues, a blue LEd will detect blue (and all LEDs will detect the UV that is not absorbed by the polymer body of the LED. An LED will create a voltage up to the band gap and generate about 0.1 to 0.3 micro amps of current (bright light aimed at LED sensor). Passing the 0.3 microamps through a 300k ohm resistor gets you to about 90mV of signal. Add an op amp circuit with gain of 10 to get a rail to rail output of about 1V. Use a JFET op amp to minimize current into the op amp input.

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#### BobTPH

Joined Jun 5, 2013
7,523
to use an LED as the light detector.
Not the way I read it. The LEDs are indicators at 3 different thresholds.

I don’t see much ambiguity in the OP. I believe he is being asked to light one of 3 different LEDs based on the resistance of a photoresistor.

Bob

#### BobTPH

Joined Jun 5, 2013
7,523
Perhaps we should add a rule for homework help stating that the actual assignment be posted word for word, rather than the student’s paraphrase of it.

Bob