I was recently tasked with determining an effective method of detecting the removal of a small product from an enclosed space. This was an excellent opportunity to break out the breadboard and multi-meter to construct three quick sensor prototypes and explore their potential benefits and drawbacks. Here is a summary of the sensors that I explored and my impressions of them.
My test environment was a simple Arduino Uno with an attached breadboard, connected to a PC via USB.
Sensor #1 – Velostat
The first sensor is the most unique! Velostat is actually a conductive packaging material that many electronic components ship in. Inspired by the tutorial on adafruit, I was able to create a pressure sensor with two pieces of acrylic and some conductive thread. I also tested some copper tape as an alternative material. The Velostat sensor can (sort-of) report a relative difference in weight as items are added or removed. It behaves similarly to a variable resistor.
Pros: Cheap, Low Profile, Configurable (in that you can cut sheets to any desired size)
Cons: Inaccurate, Homebrew Quality
Sensor #2 – Passive Infrared (PIR) / Ambient Light
The VCNL4000 is a small (dime-sized) sensor which is able to report on ambient light and proximity (up to about 3 inches) at the same time. An array of these sensors could detect movement from one location to another. Because it has a self-contained IR emitter and receiver, it makes it easy to integrate and unaffected by different lighting conditions.
Pros: Small, Fast, dual-sensor, functions independently of ambient light
Cons: Difficult to integrate multiple sensors. This particular sensor does not measure well at great distances
Sensor #3 – Light (Shadow) Sensor
The TSL235R is a photocell similar to the Passive Infrared Sensor, but operates 100% off of ambient light. They could also be placed as an array to detect when a hand passes near them. This has a danger of relying on ambient light for detection. If it was installed in a dark corner it may not work at all. A supplemental light could be integrated into the installation as a work-around.
Pros: Cheap, Fast, Simple
Cons: Relies on ambient light which can vary depending on where it is located or the time of day