PRODUCT HOW-TO Article
As electronics seamlessly weaves its way into our lives, sensors playan increasingly important role. Light sensors are among the simplestand cheapest sensors, allowing their inclusion into numerous consumerproducts ranging from night lights to cameras.
There are many ways to sense light. This article looks at the arrayof available light sensors, and covers trade-offs in resolution,dynamic range and cost. Using integrated circuitry with light sensorsenables on-chip calibration, filtering and increased resolution. Theseadvances are discussed with respect to two applications: anambient-light sensor for a laptop and one for a handset display.
A spectrum of choices
A spectrum of optical sensors is shown in the Table 1 below. They are arrangedfrom left to right in order of complexity. In reasonable orderquantities, they are also arranged by cost. A discussion of thetradeoffs uncovers what advantages can be bought with a few extrapennies per unit.
|Table1: The array of available sensors offers various performance and costtrade-offs.|
Photoresistor. Thisis the simplest optical sensor. It can be identified by the meanderingchannel between two terminals. The low-end versions are made withcadmium sulfide; their more-expensive counterparts aremade with GaAs,which allows the inclusion of a photoresistor in an IC. The smallbandgap of GaAs (1.4V at 300K) allows the low-energy photons ininfrared light to free electrons into the conduction band. The datafrom the reference part is shown from 1lux to 100lux, but variousresistance values are available.
Photodiode. It is the next step in complexity. Photons thatbombard the junction produce current. For best use, the diode should bereverse-biased. The amount of bias directly translates into quality ofoperation, as larger reverse bias enhances speed and linearity whilealso increasing dark current and shot noise. Light will create forwardcurrent, subtracting from the reverse-bias current. External circuitrycan be added to linearize the diode's I-V curve, to amplify the signaland to allow a disable function.
Phototransistor. It exhibits the same general characteristics asthe photodiode plus amplification. It requires more bias current, butthe noise associated with the current forces a shift in the sensitivityof the sensor to a higher lux range of 1-100klux (instead of 7-50klux).Response time is similar and can be varied using the bias. Current willalso vary with the detected signal level. A phototransistor candetermine coarse environmental light levels like indoor/outdoor,day/night and bright light/shade. External circuitry is still needed tocalibrate the output signal and include an enable.
|Figure1: Shrinking device sizes has allowed the creation of hybrid devicessuch as the EL7900, which places a photodiode and transimpedanceamplifier in one package.|
Shrinking device sizes has allowed hybrid devices such as the EL7900to be created. It places a photodiode and transimpedance amplifier inone package (Figure 1 above) .
This combination allows for lead-length reduction and minimumparasitic capacitance on the amplifier inputs. This, of course, is theoptimal condition for minimum noise, high frequency response andconvenience.
The low-noise characteristics extend the sensitivity of the sensordown to 1lux, while keeping the upper limit of 100klux. The power drawnstill depends on the amount of light sensed, reaching 0.9 mA for1,000lux. To conserve power, a power-down pin is included. This deviceis suitable for many situations, not just digicams.
|Figure2: The ISL29001 family of devices is a packaged solution for lightsensing and calibration.|
The ISL29001 family of devices (Figure2 above ) is a packaged solution for light sensing andcalibration. The temperature-compensated light sensor is a pin diode.The output from the sensor is calibrated and fed through a currentamplifier before entering a high-pass filter to eliminate 60Hz noise.After the filter, an ADC and I2C interface deliver the output signal.
Two major benefits of using an ADC are constant power usage and15bit resolution. In fact, the current draw is less than for all of theother active devices. The ADC has an internal 327.6kHz clock that setsthe device response time to 100ms. Even with the increased delay, theserial 15bit output signal makes the sensor suitable for moreapplications.
Gauging amount of light
Light sensors are ubiquitous in modern society. Some applications,including barcode readers, laser printers and autofocusing microscopes,use reflected light with optical detection for position sensing. Otherapplications, such as digicams, cellphones and laptops, use opticalsensors to gauge the amount of ambient light. This article investigatesfurther this second group.
|Figure3: Ambient-light sensors are included in laptops to adjust the screen'sbacklight to comfortable levels for the viewer.|
Ambient-light sensors are included in laptops to adjust the screen'sbacklight to comfortable levels for the viewer. The range of thecomfortable levels depends on the room's light, with the relationshipshown in Figure 3 above.
It is understandable that a screen's brightness needs to increase asthe ambient light increases. What is less obvious is the need todecrease the brightness in lower-light conditions, both for comfortableviewing and to extend battery life.
<>In laptop design, ambient light sensors are typically placednext tothe speakers where the case has an opening for light. These audioportals are commonly covered by a cross-hatch pattern to protect thespeakers. Because of this protection and because the light sensor isnext to the speaker instead of on top of it, the light is obstructed.
|Figure4: For the accuracy needed in low-light conditions, the best sensorchoice is the integrated photodiode with an ADC|
The obstruction reduces the amount of light available to bemeasured, thus requiring a solution with good low-light accuracy. Forthe accuracy needed in lowlight conditions, the best sensor choice isthe integrated photodiode with an ADC (Figure4 above ). The inclusion of a high-pass filter minimizespower-supply noise from coupling into the backlight illumination.
Another common application is an ambient-light sensor used in acellphone, where every mA-hr saved translates into longer battery lifeand happier customers. The enable/disable function is equallyimportant for the battery- saving, power-down feature.
The extension of battery life is remarkable. With the light sensoradjusting the backlight illumination, battery life is increased by atleast a factor of six, assuming the backlight remains on full powerwithout feedback from a light sensor. Figure5 below shows a complete automatic backlight control circuit forcellphones.
|Figure5: With the light sensor adjusting backlight illumination, battery lifeis increased by at least a factor of six, assuming the backlightremains on full power without feedback from a light sensor.|
The EL7900 senses ambient light intensity and outputs a currentproportional to this intensity. The relationship between lightintensity (E) and output current (I ou t ) is shown by:
The light sensor output current is injected into the feedback inputof the white LED driver. In a bright environment, the light sensorsources more current into the feedback node. As a result, it reducesthe white LED's output current and output light intensity. Therelationship between ambient- light intensity and white-LED outputcurrent is shown by:
There are various optical sensors available in small packages atreasonable prices. Passive solutions have been serving consumers fordecades in night lights and digicams. Active solutions have increasedthe range and usefulness of ambient-light sensors. Typical activesolutions integrate a phototransistor or a photodiode with a currentamplifier.
Tamara Papalias is PrincipalApplication Engineer and Mike Wong is Director of ApplicationEngineering at Intersil Corp.