Tag Archives: sensor

Arduino and Sensors

I’ve been busy putting the finishing touches on a class I am teaching at NYC Resistor called Arduino and Sensors. The purpose of this class is to teach people how to use common sensors with Arduino so they can build awesome interactive projects. The class features the Adafruit Sensor Pack 900, as this pack contains a nice selection of common sensors. I’ve written some sample code for each of the sensors in the pack. We will discuss both digital signal and analog signal sensors.

Sensors!

Digital signal sensors are the simplest to use. They simply return a 1 or a 0 based on the reading of the sensor (just like a switch, it’s on or off). Therefore, reading the state of one of these sensors is as simple as hooking the output of the sensor to a digital pin on the Arduino¬†(pins 2-13 on the Uno) and calling digitalRead() on that pin. Here is a simple example – an IR sensor:

IMG_6848

Analog signal sensors are more complex. These sensors return a voltage on an analog pin somewhere from 0 volts to the max voltage of the microcontroller (with the Uno, it’s 5 volts). In order to read an analog sensor value, the sensor output needs to be connected to an analog pin on the Arduino (pins A0-A5 on the Uno). In the code, calling analogRead() on the analog pin will give you the sensor reading. The Arduino automatically converts the voltage on the analog pin to an integer between 0 (no power) and 1023 (full power). Generally, the reading can be mapped back to some meaningful value. For example, here is a simple analog sensor – a temperature sensor:

IMG_6832

According to the datasheet, this sensor returns 0 volts at -50 degrees Celsius and 1.75 volts at 125 degrees Celsius. It has a scale of 10 millivolts per degree Celsius. To get the raw voltage reading, we take our reading value, divide it by 1024 (to get the percentage of the full voltage) and then multiply that by 5 (since the microcontroller is supplying 5 volts). To scale the voltage to the range, we can simply multiply the voltage by 100 (according to our scale factor, 1/100 volt is 1 degree Celsius) and then subtract 50 (since zero volts is -50 degrees Celsius).

Of course, many sensors are more complex than just reading a simple pin. We’ll discuss a number of different scenarios and how to handle them. Did you ever wonder what the AREF pin is for? It’s the analog voltage reference pin and we will be discussing how to use it. We’re also going to use potentiometers to tune the sensitivity of some of the sensors.

The class is already almost sold out! If all goes well, I will hopefully teach it again soon!

Motion-Sensitive Paper Lantern

Back in February, I made a motion-sensitive lantern for the Lunar New Year. The idea was simple: have a lantern that appeared to be mostly plain but would reveal a design when a person moved closer to it.

Lantern from far away

The easiest form to use was a round paper lantern. I started with a 14″ white paper lantern and some markers. I then attempted to draw some snakes on it, as 2013 is the year of the snake. As I am not an artist, let’s pretend that these squiggles look like snakes.

Lantern up close

The next part was to add some lighting. The lighting needed to change color. I decided to go with LED strips from Adafruit, as I could wrap them in the center of the lantern and have fairly uniform lighting. I had some left over pieces from a previous project and this seemed like the perfect occasion to use them.

Next, the lighting needed to respond to motion. There were a couple of sensors that would have allowed me to detect motion, but I decided on a passive infrared (PIR) sensor. Interestingly, these sensors work by detecting rapid changes in infrared radiation (including those given off by body heat).

PIR Sensor

The code for this was very straightforward. The PIR sensor sends a high signal on its output pin whenever motion is detected. Therefore, it’s as simple as polling the output pin with digitalRead() and transitioning the lights based on changes in the output pin state.

Finally, I had to find a lightweight power source. I found an Energizer power pack, which was the perfect power supply for Teensy. It even came with a mini USB adapter, which meant that I could plug it directly into the Teensy without having to solder anything. Here is the final internal assembly of the lamp!

The final assembly

And here is the lamp, fully assembled and running!