Come and see how to convert light intensity into an electrical quantity!

The determination of light intensity can be crucial, for example, when designing the lighting of a room or preparing a photo shoot. In the age of the Internet of Things (IoT), determining light intensity also plays an important role in so-called smart farming. In this context, a key task is to monitor and control important plant parameters in order to promote optimal plant growth and accelerate photosynthesis.

Therefore, light is one of the most important factors. Most plants typically absorb light in the red, orange, blue and violet wavelengths of the visible spectrum. Light in the green and yellow wavelengths of the spectrum is generally reflected and does not contribute much to plant growth. By controlling parts of the spectrum and light intensity during different growth stages, growth can be maximized, ultimately increasing yields.

Figure 1 shows a circuit design for measuring light intensity in the visible spectrum for experiments with plant photosynthesis. Three different colored photodiodes (green, red, and blue) are used here, which respond to different wavelengths. The light intensity measured by the photodiodes can now be used to control the light source according to the requirements of the specific plant.

Figure 1. Circuit design for measuring light intensity.

The circuit shown consists of three precision current-to-voltage converters (transconductance amplifiers), one for each color (green, red, and blue). The outputs of the current-to-voltage converters are connected to the differential inputs of a Σ-Δ analog-to-digital converter (ADC), which provides the measurements as digital data to a microcontroller for further processing.

Depending on the light intensity, more or less current will flow through the photodiode. The relationship between current and light intensity is approximately linear, as shown in Figure 2. The figure shows the characteristic curves of the output current and light intensity for the red (CLS15-22C/L213R/TR8), green (CLS15-22C/L213G/TR8) and blue (CLS15-22C/L213B/TR8) photodiodes.

Figure 2. Current vs. light intensity characteristics for red, green, and blue photodiodes.

However, the relative sensitivity of the red, green, and blue diodes is different, so the gain of each stage must be determined individually via the feedback resistor RFB. To do this, the short-circuit current (I _SC ) of each diode must be obtained from the data sheet and then the sensitivity S (pA/lux) is obtained at the operating point determined by it. R _FB is calculated as follows:

V _FS,PP is the desired full output voltage range (full scale, peak-to-peak); INT _MAX is the maximum light intensity, which is 120,000 lux for direct sunlight.

High-quality current-to-voltage conversion requires the bias current of the operational amplifier to be as small as possible, because the output current of the photodiode is in the picoamp range, and a large bias current will cause considerable errors. The offset voltage should also be small. ADI's AD8500 is an ideal choice for such applications, with a typical bias current of 1pA and a maximum offset voltage of 1mV.

For further processing of the measured value, the photodiode current is converted into a voltage and must be made available to the microcontroller as a digital value. For this purpose, an ADC with multiple differential inputs can be used, such as the 16-bit AD7798. The output code for the measured voltage is thus as follows:

in

A ₌ input voltage,

N = number of digits,

GAIN = Gain factor of the internal amplifier,

V _REF = external reference voltage.

To further reduce noise, common-mode and differential filters are used on each differential input of the ADC.

All of the components described are very power efficient, making this circuit ideal for battery powered portable field applications.

Error sources such as the bias current and offset voltage of the device must be considered. In addition, the amplification factor inside the ADC converter affects the signal quality (the offset voltage of the transconductance amplifier is multiplied by the gain inside the ADC, amplifying the error of the offset voltage), thereby affecting the final sampling result. The circuit shown in Figure 1 can relatively easily convert light intensity into electrical quantity for further data processing.