Sensors are the front line components in a wireless sensor network. They move a response from the physical realm into the electrical realm. Each sensor responds to some environmental events and generates a voltage or a digital signal or something that can be measured. There’s a great list of sensors on Wikipedia (hours o’ fun). I am constantly surprised by the seemingly endless list of sensors.
To list some things that can be sensed:
- Gas, Smoke
- Tilt, Acceleration
- Air pressure
Most sensors are what are defined as Passive. Passive sensors react to some environmental stimulus and put out a signal. Active sensors inject something into the environment and then respond to the effects created by the active aspect. Radar is a classic example of an Active Sensor.
Because we’re talking about “wireless” sensors, it’s hard to talk about wiring up the sensors without talking about the radios that make them wireless. We’ll focus on XBee radios but there are others that I’ll describe on the Radios post. On many sites on the internet sensors are shown as hooked up to a microprocessor (which then connects to a radio). I have found that for the sensors I have built so far, none have needed to use a microprocessor. Instead, I’ve connected the sensors directly to an analog input of an XBee. It seems you would need a microprocessor when you need to apply some kind of up front logic to a sensor’s readings.
When trying to figure out how to wire up sensors, it helps to have a basic knowledge of electronics. The most prevalent setup in sensor networks seems to be what is called the voltage divider (see the diagram below). There’s a lot of theory about voltage dividers but the basic idea is that you have a voltage at the top (Yin), you have 2 resisters connected inline (R1, R2) to a ground, and you have a voltage out (Vout) where the 2 resisters are joined. The output voltage (Vout) is some value relative to the ratio of R1 and R2.
As an example, you can imagine that R1 is a sensor that varies its resistance with temperature. If Vin is 5VDC then the Vout is going to vary dependent on the variable resistance of R1. Vout would be connected to an input on an XBee (to send wirelessly over a radio) or connected to a microprocessor (like an Arduino) for special handling.
There’s a cool page that shows lots of different circuit layouts for various sensors on the Wiring site (Wiring is kind of like Arduino). On this page you can select circuit types from a popup and some of the selections with display circuits.
Another great resource to get started is on Adafruit Industries’ Tutorial on Sensors which gives an overview of various sensors and how to wire them up.
- Tweet-a-Watt: The Tweet-a-Watt, originally created by Limor Fried (aka: Adafruit) is a modified Kill A Watt with an embedded XBee radio.
- Temperature Sensor: It is relatively easy to hook up a temperature sensor to an XBee radio.
- Gas Sensor: Hooking up a gas sensor to an XBee radio.
- FSR Sensor (Force Sensitive Resistor): Very easy to hook up even if a bit challenging to hook up physically. The FSR responds to pressure applied to its disk and simply changes the resistance based on the amount of pressure.
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