Using a Sensor to Influence Node Behavior

1. A sensor is added to the protoboard in the same way that we added the LEDs in previous exercises. The GPIO pins on the RF220 don’t care what’s attached, they just need to know how to interact.
2. Disconnect the power from the protoboard.
3. Find your kit’s tan pull-up resistor and photo cell. A photo cell senses light levels, and we’ll use it to create a stimulus for our node to react to.
4. Bend the leads of the pull-up resistor toward one another as seen in the picture, then place one resistor lead into the terminal block at the pin labeled GPIO 12 but don’t tighten the screw yet.
5. Place the other lead into the connector at GPIO 11. There is no polarity on a resistor, so it does not matter which lead you choose for which connector.
6. Tighten the screw for GPIO connector 12 only to lock the lead of the resistor in place.
7. Place one lead of the photo-cell into the block labeled GPIO 11 (don’t worry; it’s supposed to share the connector with the resistor) and the other lead in the adjacent slot labeled GND, the last slot in the connector. (The Photo cell has no polarity, so it doesn’t matter which lead you choose.) Tighten both screws and try a gentle tug on the components. They should not move.
8. Carefully bend the photo cell so that the face points up. Refer to the pictures; they really can be worth a thousand words.

Historical Note: That phrase was coined in 1911. When adjusted for inflation, a picture is now worth about 41 words. We expect that cell phones have flooded the photo market. Still, pictures are pretty handy.

9. Reconnect power to the SN171 protoboard.

The “HolidayLightShow” script is already monitoring for changes in light levels on the SN171 protoboard. (Sneaky, we know.) However, the script includes an “auto-calibration” capability and, since we have not “calibrated” the sensor yet, no values will be acted upon.

1. Place your finger over the photo-cell to represent “complete darkness”. This will give the script a max reading for calibration. Now, if you pull your hand away, the sensor will be reading the relative light/darkness.
2. Place your finger over the sensor for a second and then uncover it. The red and green LEDs will begin to flash once your finger casts a dark enough shadow over the photo cell.
   Look over at the SN132 SNAPstick. You will notice that the green and red LEDs are lit up as well. The SN171 protoboard has communicated the change in sensor status over the air to the neighboring device to extend the Christmas show.

NOTE: Sometimes holding your finger close to the sensor will trigger it twice. If the LEDs behave erratically, move your hand away and then close again to trigger the transition again. Remember, a sense and respond network is only as good as the sensors.

3. Use your finger to cover the sensor again, then uncover and cover it again. A Halloween light show (amber and red LEDs) will begin to flash. As before, the SN171 protoboard has wirelessly communicated the message to change the light pattern on the SN132 SNAPstick.
4. Finally, pushing the button on the SN171 protoboard turns the lights off, or you can execute the lightsOff() function using Portal.

Success! We now have a device that will sense when the sun goes down and begin a Christmas light show one night and a Halloween light show the next.

When the node boots it begins monitoring light levels. As soon as the sensor calibrates, the node's script sends a multicast message each time light levels drop below the defined threshold, and it instructs all nodes to change the light show by invoking the christmasBlink() or halloweenBlink() function.

The RF220 node on the SN171 protoboard is running the HolidayLightShow script, which defines christmasBlink() and halloweenBlink() as commands to GPIO connected LEDs. The SN132 SNAPstick is running the HolidayBlink script which defines them both as commands to the LEDs on the SNAPstick.

In this way, a number of different devices can respond to the same command in unique ways that are each suited to their application. This was a simple example, but broader applications can get quite complex.