Why Does My Photoresistor Always Read 1023

Photoresistors

Table of Contents

1. Photoresistors
   1. Photoresistor examples
   2. How do photoresistors work?
   3. Photoresistor resistance vs. illumination
      1. Photoresistor spectral response graph
      2. Photoresistors lag
      3. Cadmium-Sulfide is classified as a hazardous material
   4. Measuring photoresistance with a multimeter
2. Making an LED dimmer with a photoresistor
   1. Initial auto-on nightlight circuit
   2. An improved auto-on nightlight excursion
3. Using photoresistors with microcontrollers
   1. What value should we make our fixed resistor?
      1. Using the "Axel Benz" formulation
4. Allow's make a uncomplicated auto-on nightlight with Arduino
   1. Make the circuit
   2. Write the lawmaking
   3. Workbench video
5. Exercises
6. References
7. Photoresistor uses

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In this lesson, you'll learn almost photoresistors and how to use them with and without microcontrollers.

Photoresistors

A photoresistor—sometimes chosen a photocell or light-dependent resistor (LDR)—varies its resistance in response to light. They are small, inexpensive, and easy-to-utilise. Consequently, photoresistors are popular in children's toys (see instance below), nightlights, clock radios, and other inexpensive gadgets. Nonetheless, they are not particularly accurate and then are best suited for measuring coarse-grain low-cal levels (e.g., the difference between a low-cal and dark room) rather than precise illuminance. Other common calorie-free sensors include phototransistors and photodiodes, both which are more than accurate and responsive.

Photoresistor examples

Equally ane example use case in consumer toys, this Melissa and Doug wooden fire truck puzzle uses embedded photoresistors to observe when each puzzle slice has been placed and when the puzzle is complete. There is one embedded photoresistor per puzzle piece location. When the all pieces are placed (all photoresistors accept been covered), the puzzle plays a rewarding "fire truck siren":

Each puzzle slice location has a respective embedded photoresistor, which is used to track whether a piece has been placed or not. When the puzzle is completed (and all photoresistors have been covered), the puzzle plays a burn truck siren. At that place are a few limitations to this sensing technique: while cheap, the "fire truck" siren can be triggered when the photoresistors are covered (either accidentally via an errant puzzle slice or mitt or on purpose), occassionally the siren will be triggered earlier the puzzle is really completed (simply when the last puzzle piece is hovering over the remaining location), and, of class, the sensing method cannot tell whether a puzzle piece is in the right position (which is fine if ane just needs to infer when the puzzle is completed and not, for instance, to aid guide a child in completing the puzzle).

Another example use example from inexpensive consumer electronics: an automobile-brightening nightlight from Full general Electric, which gets brighter every bit the ambient light gets darker.

An auto-brightness fading night low-cal from General Electric.

How do photoresistors piece of work?

Photoresistors are typically fabricated of Cadmium-Sulfide (CdS), which is a semiconductor that reacts to light. As Platt describes, "when exposed to low-cal, more charge carriers are excited into states where they are mobile and tin can participate in conduction. As a event, electric resistance decreases." Because they are fabricated from Cadmium-Sulfide, they are sometimes referred to as CdS cells.

Photoresistor resistance vs. illumination

To draw the relationship betwixt photoresistance and light level, we need a more precise definition of the how calorie-free levels are characterized. Enter: Lux!

The SI unit of illuminance is called lux, which is, formally, the "luminous flux per unit area". In photometry, lux is used every bit a measure of the intensity of lite that hits or passes through a surface as perceived past the human eye.

If you lot're unfamiliar with lux (as we one time were), it's useful to provide some examples (from Wikipedia).

Illuminance (lux)	Example
0.0001	Moonless, overcast night sky
0.05 - 0.3	Full moon on a clear dark
fifty	Lighting in a domestic family room
80	Office building hallway
100	Dark overcast twenty-four hours
400	Sunrise or sunset on a clear day
1,000	Overcast day
10k - 25k	Full daylight (not direct sun)
32k - 100k	Straight sunlight

While finding a detailed datasheet on photoresistors is hard, both Sparkfun and Adafruit provide low-quality graphs of photoresistor resistance vs. lux on their websites.

Graphs from Sparkfun and Adafruit. Both are in log-log scale.

Using a professional person light meter, David Williams at All Nearly Circuits likewise conducted their own experiments of photoresistance vs. illumination and found the aforementioned log-log human relationship. We've used this data to graph both a linear version (which is easier to understand) and log-log version with annotations. Same data, but the scales are different.

In short, a photoresistor is most sensitive to light differences at lower lux levels (i.e., in darker environments). This sensitivity drops exponentially as lux decreases. For example, photoresistance drops 65kΩ between \(lux=1\) and \(lux=2\) (~65kΩ per lux) and 54kΩ between \(lux=2\) and \(lux=20\) (~3kΩ per lux in this range). Between \(lux=900\) and \(lux=~1300\), however, the resistance simply drops 140Ω (2.8Ω per lux).

The Adafruit documentation emphasizes that each photocell will perform differently due to manufacturing and other variations and reaffirms that photocells should not be used to precisely measure light levels (and each photocell requires individual calibration).

Photoresistor spectral response graph

Photoresistors are also not uniformly responsive to all wavelengths of calorie-free. Their sensitivity peaks betwixt 500nm (greenish) and 700nm (red). See the relative spectral response graph below:

Graph from Adafruit.

Photoresistors lag

Photoresistors should also not be used to sense or measure rapid fluctuations of light considering of response latency. According to Platt also as others, photoresistors may have ~10ms to stabilize at a lower resistance when light is applied later total darkness and up to ane second to rising back to a stable loftier resistance after calorie-free'southward removal. Phototransistors and photodiodes are both more responsive.

Cadmium-Sulfide is classified equally a hazardous material

Cadmium-Sulfide is classified as a hazardous ecology chemical by the RoHS and are thus unavailable in Europe. They are, however, nevertheless available in the U.s.a. (and still used in toy manufacturing, yay!).

Measuring photoresistance with a multimeter

If you accept a multimeter, let'south use information technology to go your own empirical sense of a photoresistor'south resistance every bit a function of light. Hook up each leg of the photoresistor to the multimeter (with the ohmmeter setting): one leg to the red probe and the other to the black probe (either orientation works as photoresistors take no polarity). What do you observe?

Hither's the results of our own breezy experiments:

Lighting condition	Photoresistance
iPhone LED flashlight on full power directly against photoresistor	~50-100Ω
Desk-bound lamp on	1.6kΩ
Desk lamp off but some ambient light (eastward.g., from computer monitor )	10kΩ
Finger over photoresistor	~130kΩ
Very dark room (basement, no ambience lite)	1+ MΩ

And a video:

Our photoresistor ranges from 1.6kΩ with our desk lamp to 10kΩ with the lights off to over ~10MΩ when covered by a coffee cup.

Making an LED dimmer with a photoresistor

Let'south make something. How about a simple nightlight that automatically turns on (gets brighter) in the dark.

As before, we're going to explore this sensor showtime without a microcontroller to build upward familiarity.

Initial machine-on nightlight circuit

Like the FSR, the photoresistor is a ii-legged resistive sensor and is non-polarized. And then, you can connect them in "either direction" in your circuits.

Y'all might initially endeavor to hook upwards the photoresistor in the same way as the FSR: in-serial with the LED. As we measured a minimum resistance of ~fifty-100Ω when an ultrabright LED flashlight was pointed directly at the photoresistor, in both wiring diagrams, we include a backup current limiting resistor.

Try making this circuit. What happens?

Because the photoresistor resistance decreases with light levels, the LED gets brighter every bit the ambient light gets brighter. This is the opposite behavior of what we desire! Run across video below.

In this video, the photoresistor is in series with the LED. As the ambient lite level increases, the photoresistor resistance decreases, and the LED gets brighter. But we want the reverse effect? Remember, we are using the Arduino only for power here. Note: this video has no audio.

What should we practice? Well, the coder in u.s.a. wants to immediately claw the sensor upwardly to the microcontroller and solve this in code (which is a fine solution and, ultimately, what we volition do!). However, can we solve this in hardware also?

Permit's try it.

An improved auto-on nightlight circuit

We are going to create an inverse relationship between ambient light levels and LED effulgence by placing the LED in parallel with the photoresistor wired in a voltage divider configuration. Now, as the photoresistor resistance drops, the LED will get brighter. The key is in selecting an advisable fixed resistor \(R\).

If \(R\) is likewise minor, the LED volition still plough on even in ambient light. Through experimentation, we determined that \(R=four.7kΩ\) resulted in the all-time functioning: a 1.72V drop and 0.10mA through the LED with a desk lamp off and a 0.8V driblet and 0mA through the LED with the lamp on.

R	Desk Lamp Off LED Voltage Drop	Desk Lamp Off LED Electric current	Desk Lamp On LED Voltage Drib	Desk Lamp On LED Electric current
1kΩ	1.89V	two.9mA	1.85V	2.13mA
2.2kΩ	1.82V	1.23mA	i.78V	0.5mA
4.7kΩ	ane.78V	0.48mA	one.41V	0mA
10kΩ	1.72V	0.10mA	0.80V	0mA

So, while this circuit works, it doesn't work well. We are not able to sufficiently control the current through the LED based on lighting conditions. Yes, nosotros accept the general LED behavior nosotros want only 0.10mA is a very modest current, so the LED is non very bright (even in the darkest conditions). See video beneath.

This video shows a photoresistor wired in parallel with an LED in a voltage divider to inverse the human relationship between ambient calorie-free levels and LED brightness. Again, the Arduino is used solely for power. Note: this video has no audio.

Then, what should we do?

Two potential solutions:

1. We could go along a pure hardware solution and add together in a transistor like this video. This would exist the cheapest solution and the one EE's would advocate! :)
2. We could add in a microcontroller and solve this in software (a place where nosotros are more comfy simply it's always useful to consider a pure hardware solution, if possible).

Permit's pursue the latter option!

Using photoresistors with microcontrollers

Every bit a 2-legged variable resistor, nosotros tin apply the same voltage divider wiring every bit the FSR. Recall the voltage divider equation introduced in the potentiometers lesson: \(V_{out} = V_{in} \cdot \frac{R_2}{R_1 + R_2}\).

Beneath, nosotros prove two wiring options. On the left, the photoresistor is \(R_1\) in the voltage divider configuration then \(V_{out}\) will increment every bit low-cal levels increase. On the right, the photoresistor is \(R_2\) so \(V_{out}\) will increase as light levels decrease (a "darkness" sensor, if you will).

Either wiring will work. They are functionally equivalent but have contrary behavior.

And, of course, we could inverse the human relationship in software (rather than hardware). So, for example, if we wanted to make an LED brighter as light levels decrease with the left wiring configuration, we could do the following:

                          // In this code, nosotros brighten an LED inversely proportional to light level (every bit measured past                            // a photoresistor). We presume the photoresistor is R1 and stock-still resistor is R2 in the                            // voltage divider              int              photoresistorVal              =              analogRead              (              INPUT_PHOTORESISTOR_PIN              );              // read in photoresistor val              int              ledVal              =              map              (              photoresistorVal              ,              0              ,              1023              ,              0              ,              255              );              // catechumen to 8-bit range (0-255)              ledVal              =              255              -              ledVal              ;              // capsize so that LED gets brighter equally photoresistor gets darker              analogWrite              (              OUTPUT_LED_PIN              ,              ledVal              );                      

And I often like to simplify this even more than by relying on map for the inversion (notice how I flip the order of 255 and 0), so the code becomes:

                          int              photoresistorVal              =              analogRead              (              INPUT_PHOTORESISTOR_PIN              );              // read in photoresistor val              int              ledVal              =              map              (              photoresistorVal              ,              0              ,              1023              ,              255              ,              0              );              // changed human relationship              analogWrite              (              OUTPUT_LED_PIN              ,              ledVal              );                      

What value should nosotros make our fixed resistor?

I recollect, by now, we empathize how to claw up a ii-leg resistive sensor to a microcontroller: using a voltage divider! We covered this both in our potentiometers lesson and our force-sensitive resistor lesson.

However, one key question remains: how exercise we know what to use as the fixed resistor in the voltage divider?

Ideally, we would want to: (1) vary \(V_{out}\) beyond our entire ADC range (0-5V)—otherwise, nosotros're artificially limiting our precision—and (ii) focus our sensing range on the expected light levels of involvement (for example, practise we intendance more virtually vivid lights or darker environments?).

To assist answer this, nosotros tin can graph \(V_{out}\) as a function of various fixed resistors and a range of photoresistor resistances. We've also marked gauge resistances of the photoresistor based on ambient low-cal levels. Note that these graphs don't incorporate how the photoresistor'southward resistance changes in response to calorie-free: they simply graph the voltage divider output for \(R_1\) and varying \(R_2\). Both graphs show the same information, just the changed depending on whether the fixed resistor is \(R_1\) or \(R_2\).

A graph of \(V_{out}\) every bit a part of photoresistance values for various stock-still resistors. In this configuration, the fixed resistor is \(R_1\) and the photoresistor is \(R_2\), so this matches the "lightness" sensor configuration in the wiring diagram (the left ane). \(V_{out}\) decreases as the light level increases.

A graph of \(V_{out}\) as a function of photoresistance values for diverse stock-still resistors. In this configuration, the photoresistor is \(R_1\) and the fixed resistor is \(R_2\), and so this matches the "darkness" sensor configuration in the wiring diagram (the right one). \(V_{out}\) increases as the lite level decreases.

Let'south focus on the bottom graph for now (the "darkness" sensor configuration). Here'due south a table of \(V_{out}\) values for six different fixed resistors (\(R_1\)) and some various resistances for the photoresistor (\(R_2\)) extracted from the graph.

\(R_1\)	\(R_2=100Ω\)	\(R_2=1kΩ\)	\(R_2=10kΩ\)	\(R_2=50kΩ\)	\(R_2=100kΩ\)
100Ω	2.50V	4.55V	4.95V	4.99V	five.00V
1kΩ	0.45V	2.50V	4.55V	4.90V	4.95V
2.2kΩ	0.22V	1.56V	four.10V	4.79V	iv.89V
10kΩ	0.10V	0.88V	3.40V	4.57V	4.78V
50kΩ	0.05V	0.45V	2.50V	four.17V	4.55V
100kΩ	0.01V	0.10V	0.83V	2.50V	3.33V

From the graph and table, we tin can select an \(R_1\) best suited for our expected light level in our deployment environment.

For example, if we want more sensitivity for higher light levels, then a \(R_1=1kΩ\) may be suitable. Why? Note how with a 1kΩ for \(R_1\), almost our entire \(V_{out}\) range falls between the photoresistance \(R_2=100Ω\) and \(R_2=10kΩ\). So, a 1kΩ for \(R_1\) is useful if we want to discriminate between brighter calorie-free levels just not every bit useful for darker low-cal levels (indeed, from \(R_2=50kΩ\) to \(R_2=100kΩ\)—darker resistance levels—in that location is but a 0.05V difference across a 50kΩ range!).

In contrast, if we select a 100kΩ for \(R_1\), then at bright light levels (\(R_2=100Ω\) to \(R_2=1kΩ\)), our voltage only differs by 0.09V merely at darker light levels (\(R_2=50kΩ\) to \(R_2=100kΩ\)), the voltage differs by 0.83V. So, there is comparatively more precision in darker environments with \(R_1=100kΩ\) than \(R_2=1kΩ\)

Once once again, the handy 10kΩ for \(R_1\) may be a prissy compromise.

Using the "Axel Benz" formulation

To help select a fixed resistor value, both Platt and the Adafruit tutorial recommend the following equation: \(R_{stock-still} = \sqrt{R_{min} \cdot R_{max}}\) where \(R_{min}\) is the minimum photoresistance value expected in the deployment surroundings (i.eastward., resistance at highest light intensity) and \(R_{max}\) is the maximum resistance value expected (i.e., resistance at lowest light intensity). Adafruit refers to this equally the "Axel Benz" formulation but nosotros couldn't determine a reliable source for this.

Permit's make a simple car-on nightlight with Arduino

OK, now let's brand a simple automobile-on nightlight with the Arduino that inversely sets an LED'due south effulgence based on light level.

Make the circuit

Nosotros need to brand two independent circuits: one for the photoresistor and one for the LED. Dissimilar our non-microcontroller examples where the photoresistor was part of the same excursion as the LED, in this case, they are ii carve up circuits. Make certain you understand this conceptually because it's a common point of defoliation among beginning students! We have one input circuit (the photoresistor) and ane output circuit (the LED). They are not connected in anyway except via code!

Beneath, we've wired the photoresistor using a voltage divider with a fixed resistor of 10kΩ in the \(R_1\) position (so, the 'darkness' sensor configuration). The voltage divider output is connected to A0. For the LED circuit, we've wired the LED anode towards Pin 3 with a 220Ω current-limiting resistor.

Write the code

Endeavor writing the code earlier looking at our solution. Impress your analogRead values from the photoresistor to improve determine light/night thresholds to gear up your LED brightness. For example, your auto-on nightlight should be completely off when information technology'south "light" and fully bright when information technology's "dark" (only you tin can command these tolerances).

Our lawmaking:

Workbench video

Exercises

• map() assumes a linear mapping between two value ranges. What if yous wanted a logarithmic or exponential conversion? How would you implement this? How might this be useful for working with sensors?
• Just like we did for the FSR lesson, try hooking up a piezo buzzer (and be artistic about how it makes "music").
• Make the dark light brighter or multi-color (either with your RGB LEDs or individually colored LEDs)

References

• Photocells, Adafruit tutorial
• Designing a Luxmeter Using a Light-Dependent Resistor, All Well-nigh Circuits, David Williams
• Chapter twenty: Photoresistor in Platt, Make: Encyclopedia of Electronic Components Book 3: Sensing Light, Sound, Estrus, Motion, and More than, O'Reilly, 2016.
• Photoresistor, Wikipedia
• Photoresistor, Resistorguide.com

Photoresistor uses

Some interesting uses of photoresistors in HCI/UbiComp research and beyond:

• Chris Harrison and Scott E. Hudson. 2008. Lightweight textile detection for placement-aware mobile computing. In Proceedings of the 21st almanac ACM symposium on User interface software and applied science (UIST '08). Association for Calculating Machinery, New York, NY, USA, 279–282. DOI:https://doi.org/10.1145/1449715.1449761

• Azusa Kadomura, Cheng-Yuan Li, Yen-Chang Chen, Koji Tsukada, Itiro Siio, and Hao-hua Chu. 2013. Sensing fork: eating beliefs detection utensil and mobile persuasive game. In CHI '13 Extended Abstracts on Human being Factors in Computing Systems (CHI EA '13). Association for Computing Machinery, New York, NY, United states, 1551–1556. DOI:https://doi.org/10.1145/2468356.2468634

• Stacey Kuznetsov, Eric Paulos, and Mark D. Gross. 2010. WallBots: interactive wall-crawling robots in the hands of public artists and political activists. In Proceedings of the 8th ACM Conference on Designing Interactive Systems (DIS '10). Association for Computing Mechanism, New York, NY, The states, 208–217. DOI:https://doi.org/x.1145/1858171.1858208

• Preetha Moorthy, Michaela Honauer, Eva Hornecker, and Andreas Mühlenberend. 2017. Hello world: a children's touch and experience books enhanced with DIY electronics. In Proceedings of the 16th International Briefing on Mobile and Ubiquitous Multimedia (MUM '17). Association for Computing Machinery, New York, NY, USA, 481–488. DOI:https://doi.org/10.1145/3152832.3157811

• Jay Vidyarthi, Alissa N. Antle, and Bernhard Eastward. Riecke. 2011. Sympathetic guitar: can a digitally augmented guitar exist a social entity? In CHI 'xi Extended Abstracts on Human Factors in Computing Systems (CHI EA '11). Association for Computing Machinery, New York, NY, USA, 1819–1824. DOI:https://doi.org/ten.1145/1979742.1979863

• Photoresistor, Arduino, and Servo to Auto-Play Chrome Dinosaur Game link1 link2

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Source: https://makeabilitylab.github.io/physcomp/sensors/photoresistors.html

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