Smart Lighting – Automatic Dimming for Human Preference

Automatic lamp dimming is preferred in work environments such as factories and offices to ensure optimum illumination for workers to complete their tasks, while minimizing the overall energy consumed by lighting. Ambient light sensors (ALS) are effective in helping to determine the right brightness setting to achieve optimal illumination by combining electric lighting with natural light.

ALS Principles

ALS suppliers such as Vishay typically tune the spectral response of sensors to mimic the human eye using optical filters. In practice, this enables an electronic dimmer to adjust lamp brightness continuously to ensure the illumination always meets the needs of human workers as natural light levels vary during the day. Other sensor characteristics, however, can depend on factors such as detector size, lot-to-lot manufacturing variations, and whether the sensor has a phototransistor or photodiode output.

If the base of a silicon transistor is exposed to visible light, ionization occurs that produces charge pairs which have the same effect as an externally applied base current. If connections are made to the base and collector only, the device acts as a photodiode and a small current will flow depending on the intensity of the incident light and the photosensitivity of the chip. On the other hand, biasing the device as a transistor produces amplification of the photodiode current. In this case, the output current depends on the transistor gain as well as the photosensitivity of the chip.

Designing with Phototransistor Output

Ambient light sensors based on phototransistor and photodiode principles are available in the market. Phototransistor-based devices exhibit greater variability due to manufacturing tolerances because the output current is dependent on the transistor current gain as well as photosensitivity. Vishay offers phototransistor-based devices such as its TEMT6200FX01 and TEMT6000X01 in three BIN groups (A, B, C) according to the photocurrent produced under stated test conditions. This enables users to minimize sensor-output variability in their own applications by optimizing the value of the load resistor that must be connected to the emitter pin of a phototransistor sensor (Figure 1).

Figure 1: Load resistor in series with phototransistor sensor emitter.

The value of the load resistor is selected according to the illuminance expected or measured in the end application. For the TEMT6200FX01, the output current ranges from a minimum of 7.5 μA to a maximum of 39 μA at 100 Lux. Within this range the devices are split into three BINs that have minimum, mean and maximum photocurrent as shown in Table 1.

Table 1: Phototransistor BINs and corresponding photocurrent limits.

If the application detection range is to be from 10 Lux to 1000 Lux, a load resistor of 10 kΩ will give an acceptable output voltage range from 0.023 V at 10 Lux to 2.30 V at 1000 Lux. When connected to devices in bin B, a 10 kΩ resistor results in a mean output voltage of 0.18 V for a mean photocurrent of 18 µA, as in Table 1. Alternative resistor values can be calculated for use with devices from bins A and C, to achieve the same mean voltage of 0.18 V and thereby minimize the differences in response between circuits using devices from different bins.

Hence by changing the resistor depending on the BIN, the tolerance of the photosensor is reduced from a factor of 5 (7.5 µA to 39 µA) to 2 (7.5-15 µA, 12-24 µA, or 19.5-39 µA).

If the output of the sensor is to be converted by an analog to digital converter (ADC), an amplifier circuit may be required as shown in Figure 2, depending on the sensitivity of the ADC.

Figure 2: Amplifying the phototransistor ALS output for conversion by an ADC.

The example shown is optimized for illuminance ranging from 1 Lux to 1000 Lux. This may be suitable for an application such as automatic dimming of office lighting (see Table 2). Other applications may require the sensor to remain responsive at higher illuminance. Sensors such as Vishay’s TEMT6200FX01 and TEMT6000X01 provide a linear response over a wide range from 1 Lux to 100,000 Lux. The output current of the TEMT6200FX01 over this range varies from 180 nA to 18 mA. If a 1 kΩ load resistor is used, the output voltage will range from 0.18 mV to 18 V. Given a typical operating voltage of 5 V or less, 1 kΩ is clearly not a suitable value for R_L in this case. A lower value must be calculated, or the gain of the optional op-amp circuit may be adjusted, to achieve an output voltage range suitable for the ADC.

Table 2: Typical Lux values for common lighting scenarios.

Designing in ALS with Photodiode Output

Because the lot-to-lot variability of photodiodes is caused only by the photosensitivity variation, device characteristics are more closely matched than with phototransistor-based ambient light sensors. Tolerances are typically within 20% to 30%. On the other hand, the output of the photodiode is much smaller than that of a phototransistor-type ALS. A low-noise preamplifier circuit must be used, such as that shown in Figure 3.

Figure 3: A photodiode-type ALS requires a low-noise amplifier at the output.

However, when working with photodiode-based ALS, designers should also bear in mind that the output current may be influenced by the chip size. Larger chips obviously have a larger detection area and so generate greater photocurrent. The TEMD5510FX01, which contains a 7.5 mm² chip, generates double the current of the TEMD6010FX01, which has a much smaller chip with a sensitive area of only 0.27 mm².

Calculating window size

If the sensor is to be mounted behind a cover, a window is required. Whichever type of ALS is used, the size and performance of the window influences the magnitude of the sensor output. Ideally, the window material should be transmissive to visible light in the 400 nm to 700 nm range. Moreover, the window should be sized to permit adequate illumination of the sensor to generate an acceptable output current. If the sensor’s angle of half sensitivity is specified as ±60 degrees, the field of view can be envisioned as a cone-shaped region having an angle of ±60 degrees, positioned directly above the center of the chip. Based on this assumption, the two critical dimensions governing the design of the window are the distance from the top surface of the chip to the outside surface of the window, and the width of the window.

To ensure maximum illumination of the sensor, the required window width for a given distance from the sensor can be calculated using the diagram of Figure 4, where α = 60 degrees. Recognizing that the distance d may be dictated by constraints such as a limitation on the maximum overall height of the sensor, the dimension x can be calculated. When x is known, the window size, w, can be calculated as 2x plus the size of the sensor’s active area. In Figure 4 this is assumed to be 0.75 mm.

Figure 4: Calculating the window width and position for maximum illumination.

If a smaller window is required for any reason, the sensor may be positioned closer to the aperture resulting in a smaller value for d. It is worth noting that setting the value of α as 60 degrees gives values for the optimum window width. If necessary, the window width may be reduced in relation to the chip size and the distance, d, understanding that the sensor output current will also be lower than can be inferred from the datasheet. In this case, it is advisable to determine the minimum window width necessary to achieve an acceptable output from the sensor. It may be possible to adjust the resistor values or amplifier gain in the amplifier circuit to achieve a larger maximum output voltage with a smaller window.

Conclusion

Ambient Light Sensors are typically tuned to exhibit spectral response close to that of the human eye. Even so, designers have several choices open to them when selecting the optimum device for their application. Depending on the acceptable sensitivity tolerance as well as solution size, output circuitry component cost and design constraints, a photodiode type or phototransistor type sensor may be selected. Following device selection, the size and position of the protective window, if required, must also be determined to ensure the output has suitable magnitude at both the maximum and minimum illumination levels as determined by application.