GL5506 GL5516 GL5528 GL5537 GL5539 GL5549 LDRs

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This pages describes the characteristics of the GL55-series photo resistors.
Photo resistors or LDRs (Light Depended Resistor) are used to detect photons.
The more photons, the more luminous, the lower the resistance of this passive element becomes.
With an analogue port of an Arduino or other microcontroller the resistance can converted to a value and the rest is coding.

It is not always obvious but there are several types of LDRs.
The ones described here are cheap and easy available; the GL-5xxx serie with a diameter of 5 mm.

Often the LDR type is not mentioned and which one is received is a surprise.
The difference between the types is the resistance characteristics at different luminous intensities.
The resistance varies between a few hundred ohms in the dark to Mohms in full light.

To investigate the differences between the LDRs in an Arduino environment the setup was used.
I used the following LDRs bought as a set:GL5506 GL5516 GL5528 GL5537 GL5539 GL5549
Beside these LDRs I had GL5528 LDRs I regularly use in my designs.

Datasheet
The datasheet gives to following specs. Power is probably mW or uW but the unit is not mentioned

Connections Eight LDRs were connected via A2 on the PCB to analogue ports A0 - A7 on the Arduino as shown below with a 22 kohm or a 10 kohm resistor. The higher the resistance the less power is lost in the design. Because a 10K resistor is used in many designs this resistor may be preferred to avoid using many different parts in your design.

Software

unsigned long sensorValue[7] = {0}; // variable to store the value coming from the sensor
char sptext[80];
void setup() 
{
Serial.begin(115200);               // Setup the serial port to 9600 baud 
while (!Serial);                    // Wait for serial port to connect. Needed for native USB port only
sprintf(sptext,"g506 g516 g528 g528 g528 g537 g539 g549 Min Max ",
sensorValue[0],sensorValue[1],sensorValue[2],sensorValue[3],
sensorValue[4],sensorValue[5],sensorValue[6],sensorValue[7]);
Serial.println(sptext);
}

void loop() 
{
long NoLoops=320;                   // Max 32000 loops
for(int i=0; i<8; i++) sensorValue[i] = 0;
for(int n=0; n<NoLoops; n++) for(int i=0; i<8; i++) sensorValue[i] += analogRead(i); 
for(int i=0; i<8; i++) sensorValue[i] /= NoLoops;

sprintf(sptext,"%0.4ld %0.4ld %0.4ld %0.4ld %0.4ld %0.4ld %0.4ld %0.4ld 0000 1000",
sensorValue[0],sensorValue[1],sensorValue[2],sensorValue[3],
sensorValue[4],sensorValue[5],sensorValue[6],sensorValue[7]);
Serial.println(sptext);
}

Results
The first results shows that in one batch of identical LDRs there can be a great variation. With the 22k resistor the following bits were recorded: 524 749 733 747 746 861 823 856
GL5506 (top line)  GL5516 GL5528 GL5537 GL5539 GL5549
GL5506 GL5516 GL5528 GL5537 GL5539 GL5549 (bottom line)

The results in the pictures above were sampled for several hours until it was dark outside.
The left graph shows the results with a 10 kOhm resistor in the design and the right graph with a 22kOhm resistor
The GL5537 and GL5539 are so comparable that they switched from the expected position in the graph.
Two GL5528 LDR (gray and yellow lines) are plotted in the graph and they show similar behavior to luminously.

When using LDRs to control the intensity of LED displays your pupil becomes larger and adapts to the darkness.
Using the direct readings from the analogue port results in LEDs to bright in the dark and not bright enough in full light. 

Taking the square root of the measured_value * 62.5 gives a transformed result between 0 and 255
compensates this effect in my designs when using the GL-5528 with a 22K resistor successful. 

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26 jan 2021

Mail: Ed Nieuwenhuys