It is always interesting to see "common" electronic components used in fun, novel ways. Often, these methods have been well-known for quite some time and may even be the premise behind dedicated, more expensive sensors. However, the methods have a tendency to become vogue every few years. The decision to use these "alternative" components can be boiled down to a few key factors: cost, space, novelty, and usually the most important, availability (it's a nice alternative when you don't have the dedicated sensor sitting on your desk). I'm going to discuss two examples -- using LEDs as touch sensors and diodes as temperature sensors. Can you think of any others?
As a first example, let's look at one of the most visible (and often annoying) components that everyone is familiar with -- the light-emitting diode, or LED. To get a handle on what I'm referring to, watch this video of an LED array being used as a touch-sensitive panel.
This project comes from New York University, and as you can see, they're using the LEDs to emit light while simultaneously using them as touch sensors (actually, reflective light sensors). You see, in the "normal" operating mode (forward-biased), LEDs consume power by emitting light. However, LEDs can be hooked up backwards (reverse-biased), where they will exhibit light-dependent characteristics similar to photodiodes. This means that the LED can be used to both transmit and detect light simultaneously! Of course, the LED can't actually be sensing and transmitting light simultaneously -- the effects are mutually exclusive since they use different operating points. However, if you're toggling the LED between modes fast enough, it will have the appearance of simultaneous emission and detection.
Now, if a single LED is hooked to a microcontroller as shown below, it can be used to both transmit (emit light) and receive (sense light) using just two I/O lines (by using the RC characteristics of the microcontroller I/O lines, no analog-to-digital converter is required). This means that virtually any LED can serve as a bi-directional communications port! This was explored in a paper entitled "Very Low-Cost Sensing and Communication Using Bidirectional LEDs" from Mitsubishi Electric Research Lab.
Of course, this capability has also been known for quite some time, as illustrated by a patent entitled "Proximity sensor with a light-emitting diode which simultaneously emits and detects light" (Patent #4564756) from 1984.
My second example of a "clever" sensing device using a cheap component comes from µblog. Basically, Nick couldn't find a thermistor (a cheapish part that changes resistance with temperature), so he used a run-of-the-mill diode instead. Basically, the reverse bias leakage current of a diode varies with temperature, as shown in the graph below.
Thus, the diode can be used as a temperature-dependent resistor (or essentially a thermistor) when held at at a constant reverse-bias voltage. Nick used a custom analog sensing and control circuit (to control a CPU fan), but consider a circuit similar to the one above (with "reverse bias" and "discharge"). You could use the same circuit (again, knowing the RC characteristics of the microcontroller I/O pins) with the regular diode in the place of the LED to sense temperature.
Again, this technique isn't exactly "new." The same concept is empolyed in many higher-end microcontrollers with internal temperature sensors.
So, I've listed two examples of clever sensing techniques using cheap, common components. Can you think of any others?
NOTE: This post originally appeared on Hizook.com, which has been transformed into a robotics-centric website. To preserve all of my original posts, I've transferred the content to my new homepage TravisDeyle.com. I also encourage you to scope out the new Hizook!