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Light to I2C converter


I have seen interesting applications done with these passives, like for instance an “automotive feature” of automatically turning the lights on when it gets dark outside, on a Dacia 1300 car. But we have evolved since then and light conversion is now following the trend: there are now sensors on the market that offer not only digital output, but they also offer it via commonly used interfaces.

Although optoelectronics is not something new, it usually employs devices functioning in the infrared spectrum, which is not really visible to the human eye. Phototransistors, photodiodes and photoresistors were the first available devices sensitive to the same wavelengths like the human eye. Their response, however was basically an analogue signal, and back then it was not the time of microcontrollers and ADCs (electronics was much more difficult!).

Even today, due to the fact that electromagnetic fields and radiated emissions are much stronger than 20 years ago, it is preferred to transmit digital signals than analogue ones. As a result, the market was ripe for the appearance of digital light sensors.

The latest to appear is the ISL29020 from Intersil. Below, for comparison, is the spectral response of this sensor together with the typical sensitivity of a photoresistor.



Spectral response of the ISL2902



Sensitivity of a typical photoresistor


I was impressed with the capabilities of this sensor, as I received a few of them from a work colleague who was clearing his desk (he was clearing it for good, unfortunately – the automotive industry IS in recession).

Just at a first glance, the sensor is stated to offer close to human eye response, while rejecting the ambient IR and UV and reducing the flickering owed to the 50Hz or 60HZ artificial light sources. It operates on a very low current (65uA in active mode!) which makes it ideal for battery powered applications. The actual sensor is no more than an array of light sensitive photodiodes but the actual IC also packs the circuitry for transmitting the actual data read from the sensor via the common I2C bus. It can act as a common slave on such a bus, with a predefined address, which can be slightly modified by an address pin.

Very interesting, for an automotive engineer at least, is the temperature range for which this sensor is qualified: -40 to 85 Celsius, which would make it “almost” automotive qualified (there is, however, a long way to get the AEC stamp…)

Anyway, since it is not in our nature to let unused ICs that we have in our drawers go to waste (especially new ICs), I set on to make a small breadboard with a circuit that would enable me to easily test how the ISL29020 works.

For this, I only needed a tiny micro, the sensor, and an LED connected to the micro; the LED would be PWM driven, and it would always be in an inverse proportion to the reading of the light sensor (that is when the sensor detects a lot of light the LED would be off; when the sensor detects total darkness, the LED would be completely on; in dim light, the LED would be glowing a dim light too).

I had to struggle a little, as the tiny 2.1mm x 2mm package of the sensor makes it very difficult to solder the wires, but I eventually managed to put in together the following schematic:




I know the microcontroller is an overkill, but that is what I had available. I wanted to see how the sensor works, not experiment with software implementation of I2C protocol. Following the experiment, I concluded the sensor worked according to my expectations and the next project on my “to do” list is to try and modify my Golf 2 car (1986, 1.6TD) to have its lights turning on automatically when the ambient gets too dark.

Come to think of it, this tiny digital sensor is even smaller than a photoresistor, photodiode, or phototransistor that I ever laid my eyes on.

Read the Italian version: Convertitore da segnali luminosi a I2C

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