Proximity Sensors

Introduction

Proximity sensors are designed to detect the presence of objects without physically touching them. You have most likely seen them at work, automatically turning on faucets, opening doors, and acting as a touch-less switch.

There are several types of proximity sensors to choose from such as capacitive, inductive, magnetic, or photoelectric. Each has their use pros and cons along with specific uses cases. The MetaDetector board uses a photoelectric proximity sensor so we will focus on that type of proximity sensor in this post.

How They Work

Photoelectric proximity sensors combine a light emitter with a photodiode. When residing on the same die, the light emitter emits infrared light (can be other wavelengths but usually infrared) which is then reflected off of the target object and detected by the photodiode.
princ

Amplifiers boost the photoelectric currents and analog to digital converters transform the signal into digital values. As the sensor moves closer to an object, more of the emitted light is reflected back to the photodiode resulting in higher current. Conversely, an object far away reflects little to no light back to the photodiode resulting in no photocurrent.

TSL2671 block diagram

TSL2671 block diagram

The emitter and receiver can be also separated so that light is being emitted directly to the receiver. This configuration has higher accuracy and range than the reflective setup though it does require two install points rather than one and one must purchase both receiver and emitter.

MetaDetector Proximity Sensor

The MetaDetector board comes with an AMS TSL2671 proximity sensor. This model compensates for ambient light allowing it to function in bright sunlight and dark rooms, is a mere 2mmx2mm, and consumes a tiny 2.5µA of current in sleep mode. Some relevant operational parameters are in the below table:

Parameter TSL2671
Temperature Range -30C – 70C
Supply Voltage (VDD) 2.6V – 3.6V
LED Driver Current 12.5mA, 25mA, 50mA, 100mA
Recommended Distance <15cm
Active Current Consumption 3mA

Applications

Given that the MetaDetector proximity sensor is a low power model, it is most suitable for applications such as an optical switch (automated faucets), printer paper alignment, mobile phones (automatically enable speakerphone), and security (place on valuables). Higher powered proximity sensors can sense further distances enabling them to act at distance monitors, useful for things like alerting drivers of on coming obstacles. If you separate the emitter and receiver, you can then detect if an object has passed through the beam which could be a person walking through a door.