Light and proximity sensor: 2-in-1 vs 3-in-1

As sensors gain more popularity and systems become more complex, the idea of a plug-in module becomes more welcome. In terms of light sensors, these modules are referred to in terms of the number of functions incorporated in one package. As sensors become more popular and systems become more complex, the idea of a plug-in module is gaining popularity. In terms of light sensors, these modules are referred to in terms of the number of functions incorporated in one package. The first devices are simple ambient, or visible, light sensors. (I guess these would be called 1-in-1.) When proximity sensors were invented, they existed as a solo product but also soon joined the ambient light sensor, or ALS. This is called a 2-in-1.

Please note that a proximity sensor needs an accompanying IR LED to generate the infrared signal it uses to bounce off of a nearby object and detect its presence (figure 1). Therefore the next generation, the 3-in-1, is a device where the IR LED is co packaged with the ALS and the proximity sensor. While this seems like a natural and easy progression, let me assure you that the optics and mechanics involved in this process are quite challenging. Of course, challenge is what makes engineering fun, usually... And remember that customers pay in proportion to the value the device has to them, not in proportion to the amount of effort that might have been exerted to create it. All of that said, let's examine the trade-offs associated with using a 2-in-1 device versus a 3-in-1 device.

For this discussion, the 2-in-1 device will be the ISL29038. Like the circuit on its evaluation board, it will be paired with an SFH4650 IR LED (figure 2). For the 3-in-1 device, I have chosen the ISL29044 (figure 3).

Let's start with mechanical considerations. Most obviously, since the 3-in-1 device is a single package, it has the smallest bill of materials (BOM) and smallest footprint as seen in figure 3. This saves the manufacturer from both having to choose an IR LED and from deciding the best distance that should separate the IR LED and the sensor. On the other hand, the 2-in-1 sensor forces the user to choose an IR LED. The advantage of making this choice is that there are a wide range of IR LEDs available. The two most relevant issues are package size and half angle. Both will be dictated by the desired application.