This Instructable will show you how to make giant, super cool, glowing balloons that react to surrounding air quality. Inside each balloon is a tri-colored LED. This LED reacts to data from an air quality sensor, turning green, yellow or red based on low, average, and high values.

Even without any fancy circuitry, these look pretty awesome at night. The project could be improved by adding a transistor to make the balloon illumination brighter, and by more carefully calibrating the air quality sensors to temperature and humidity.

This is a project by Stacey Kuznetsov, Jian Cheung, George Davis and Eric Paulos at Carnegie Mellon University. Photos were taken by Chloe Fan and Iris Howley.

Please feel free to contact Stacey Kuznetsov (stace@cmu.edu) if you have any comments or questions.

Step 1: Gather Materials

Here are the materials you'll need to make a single balloon:
- Air Quality Sensor from Figaro (I recommend the VOC or the diesel/exhaust sensor, $10/$20 each)
-PICAXE micro-controller - 8 pin ($2.95)
- DIP-socket for the PIC ($1.50)
-Rechargeable lipo battery ($8.95)
-Tri-colored LED ($7.95)
-Either a 10Kohm resistor for sensing VOC or exhaust, or a 100Kohm resistor for sensing diesel
- Weather Balloon ($3.95)
- Access to Helium (you can probably go to a party store, or rent a tank from a welding supply shop)
- rubber band or string to tie the balloon once inflated
- (optional) connector for battery

You will also need to use
- Picaxe starter kit and cable, or some other way to program the pic
- wire
- solder/soldering iron
- electric tape
- head shrink /head gun
- pliers/wire strippers
- power-supply or some power sources around 5V to preheat the sensor

Step 2: Preheat Sensors

Data sheet for both sensors suggests 48 hours of preheat to make the elements most stable. I've run them for about 24 hours and it seemed to work fine.

All you have to do is hook up the Power and Ground on the sensor to a power supply (at 4.5Volts) or battery (3.7-4.5 volts should work). If using a power supply, make sure it's not current- limited because multiple sensors will draw a lot of current.

You can do this on a breadboard or using alligator clips and leave it on over night.

Step 3: Program the PICAXE

First, you need to install the PICAXE IDE and drivers for your computer from http://www.rev-ed.co.uk/picaxe/software.htm

Second you have to assemble your PICAXE programming board or whatever you choose to use to program the pic. Refer to http://www.rev-ed.co.uk/ for details on starting out with the PIC.

The code is very simple. It samples the sensor output, and if it's below, within or above certain thresholds, it turns the LED green, yellow or red. Each sensor might have slightly different thresholds. Here are the values I used, based on collecting data around my city:

main:
'w5 = 270 'DIESEL GREEN'
'w6 = 295 'DIESEL RED'

'w5 = 448 'VOC GREEN'
'w6 = 470 'VOC RED'

w5 = 360 'EXHAUST GREEN'
w6 = 380 'EXHAUST RED'

goto runsensor

runsensor:
readadc10 4, w3 'SENSOR VALUE'
if w3 < w5 then 'GREEN'
high 1
low 2
endif

if w3 >= w5 and w3 < w6 then 'YELLOW'
high 1
high 2
endif

if w3 >= w6 then 'RED'
low 1
high 2
endif
pause 500
goto runsensor

An easy way to guess what the green, yellow and red values might be after you already wired up the circuit is by using a voltmeter. You can see what voltage the sensor is giving off at different locations and translate that into analog values:

1023 * sensor voltage / your battery voltage

Step 4: Solder the Circuit (VOC Sensor)

This is the trickiest part of the project. All connections must be very solid or they will fracture from the movements of the balloon. I recommend lot's of heat shrink or electric tape around each joint.

Solder the pic socket (not the pic itself to allow easier programming). Leave about 6 inch wires between the LED and the picaxe. Solder the picaxe, sensor and battery close together.

Sensor Pins
1 - ground (this could be the battery ground or the picaxe ground)
2 - picaxe pin 4, with a 10K resistor across ground
3 & 4 - VCC (this could be the picaxe VCC or the battery VCC)

LED Pins
Green - picaxe 1
Red - picaxe 2

Step 5: Solder the Circuit (diesel or Exhaust Sensor)

This is the trickiest part of the project. All connections must be very solid or they will fracture from the movements of the balloon. I recommend lot's of heat shrink or electric tape around each joint.

Solder the pic socket (not the pic itself to allow easier programming). Leave about 6 inch wires between the LED and the picaxe. Solder the picaxe, sensor and battery close together.

Sensor Pins
1 - picaxe pin 4 with a 100Kohm resistor across ground (for diesel)
OR
3 - picaxe pin 4 with a 10Kohm resistor across ground (for exhaust)
4 - ground (this could be the battery ground or the picaxe ground)
3 - VCC (this could be the picaxe VCC or the battery VCC)

LED Pins
Green - picaxe 1
Red - picaxe 2

Step 6: Lot's of Tape

Put electric tape all around the sensor and piaxe, without blocking the sensor element from getting air. Tape is a DIY container for this circuit (this way you don't need any other enclosure).
Put some tape around the LED too- it could get too hot or too sharp and make the balloon pop.

Step 7: Insert Into Balloon

Insert the LED into the balloon, but keep the picaxe, sensor, and batter outside.

Most likely, your sensor will float at this point and the LED will turn red no matter what threshholds you set in step 1. Give it some time, it'll settle back to normal in 20? minutes.

Step 8: Inflate the Balloon!!

This step is awesome. You can now inflate the balloon! We used rubber bands to tie them off since tying the actual balloon might break the already-fragile wiring. If you're planning to set up a balloon installation, I recommend inflating the balloons on-site. They will not fit into a car :)