Light sensor (Part 1)

Part 1, 2

Sensing light is probably one of the most common sensing and probably one of the simplest measurement to start with.

Light sensors are used to detect light or brightness in a manner similar to the human eye. They are most commonly found in industrial lighting, consumer electronics, and automotive systems, where they allow settings to be adjusted automatically in response to changing ambient light conditions. By turning on, turning off, or adjusting features, ambient light sensors can conserve battery power and provide extra safety while eliminating the need for manual adjustments. (as per Vishay’s documentation)

Here is a list of various lighting conditions

10-5 lux Light from Sirius star, the brightest star
10-4 lux Total starlight, overcast sky
0.002 lux Moonless clear night sky with airflow
0.01 lux Quarter moon
0.27 lux Full moon on a clear night
1 lux Full moon overhead at tropical latitudes
3.4 lux Dark limit of civil twilight under a clear sky
50 lux Family living room
80 lux Hallway / toilet
100 lux Very dark overcast day
320 to 500 lux Office lighting
400 lux Sunrise or sunset on a clear day
1000 luxOvercast day, typical TV studio lighting
10 000 to 25 000 lux Full daylight (not direct sun)
32000 to 130000 lux Direct sunlight

Sensing light is safe, simple and cheap. Many application notes describe the use of the dated LDR (Light Detecting Resistor), so that I will skip this sensor.

Some time ago I used the TEPT5600 ambient light sensor which is a photo-transistor. This versatile sensor can be used in various ways to address various applications exposed to variable brightness (Ev, in lux).

Photo-transistors are equivalent to photo-diodes used in conjunction with bipolar transistor amplifiers as shown below:

Typically, the current amplification, B factor, is between 100 and 1000. The active area of photo-transistor is usually about 0.5 x 0.5 mm2. The data of spectral responsiveness are equivalent to those of photo-diodes, but must be multiplied by the factor current amplification, B.
Next picture illustrates the linear luminescence to photo-current relationship:

Next is a cut view of the TEPT5600 sensor as per Vishay’s documentation

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More about physicis and technolgy involved in phototransistors > here <

…For a given irradiance, phototransistors may show lot-to-lot variability of the output current …. The lot-to-lot variability of photodiodes is significantly lower because it is caused only by the variability of the photosensitivity. Vishay offers its ambient light sensors with phototransistor output in binned groups. These groups cannot be ordered separately but each reel is marked with a label A, B, or C that will allow the user to select the appropriate load resistor to compensate for these wide tolerances. (as per Vishay’s documentation)

Next is a table showing the opto current flowing though the collector and the emitter (Ipce, in µA) biased with 5 V while exposed to a Ev of 20 lux.

Binned groupMin.Max.
TEPT5600 A2550.4
TEPT5600 B41.784
TEPT5600 C69.4140
TEPT5600 D113.4226.8

Next are a few exemples of use of the TEPT5600 sensor. For more information, read this publication.

Basic switch

The output goes high at Ev > 25 lux, Ipce: 10 μA, Vout: 2.0 V, Input Leakage Current: < 1 μA

Improved Switch featuring a level converter

Output Low at Ev > 10 lux, Ipce: 4 μA, Gate Threshold: 2.0 V, Input Leakage Current: < 1 μA

Basic Light Meter
As the photo-transistor acts as a current source directly proportional to the luminescence, a simple resistor in series with the sensor suffices.

EV: 10 lux to 1000 lux, Ipce: 4 μA to 400 μA, Vout: 16 mV to 1.6 V

Low Illuminance Light Meter

Ev: 0.1 to 10 lux, Ipce: 40 nA to 4 μA, Vout: 16 mV to 1.6 V . The output signal is amplified and buffered by an operational amplifier before the A/D converter

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