Air quality monitor and logger

Jeremy Harris

Senior Member
As mentioned in a thread on the active forum, I've been working on building an air quality monitor that measures and records CO2 concentration, temperature and humidity. The idea is to use it to monitor home ventilation system effectiveness. The hard part was finding a suitable (and affordable) CO2 sensor. There is one hot wire type sensor available, the MG811, but it isn't either accurate or sensitive enough for this task. It also needs a fair bit of power and signal processing to try and get the very tiny output voltage up to a level usable by a Picaxe, plus it would be difficult to calibrate.

Luckily, I found a source of surplus Telaire 6004 Non-Dispersive Infra Red sensor modules (see Telaire CDS-1 data sheet attached). These are accurate, self-calibrating, and best of all they can output serial data that can easily be read by any Picaxe. They were being sold off for around 1/10th of the normal price, so I bought a few to play with. Unfortunately, the postage and duty/VAT cost more than the sensors, but at around £17 each inc everything they still seem to be reasonable value, as the closest sensors of this type I could find cost around £80 each.

Telaire were helpful and emailed me the serial data protocol, so after a few minutes with a breadboard I'd managed to get accurate CO2 data in parts per million, exactly what I was after. I used an HH10D humidity sensor module to measure relative humidity and a DS18B20 temperature sensor for room temperature. To store the data I used an OpenLog µSD card unit from SparkFun (a very versatile way of interfacing a µSD card to a Picaxe). As I wanted a reasonably responsive display, I opted to use a cheap LCD (from Ebay) connected in 4 bit parallel mode. The whole unit, including a 4 x AA battery box, just fits into a small Hammond alloy case from Maplin.

Attached are some photos of the finished unit, together with the code and the circuit layout. So far it seems to be working well. It can run for a few tens of hours on 4 alkaline AA cells, but is best run from a 6V power supply. I used an old mobile phone PSU, and added a diode so that the unit keeps working on battery power if the power supply is unplugged. The power supply absolute maximum voltage is limited to 6.5V, by the use of the MCP1700 low drop out voltage regulator. This was chosen for two reasons; it allowed the use of a 6V battery pack and it delivers over 200mA peak current, needed as the NDIR sensor draws a short pulse of around 130mA or so every 2 seconds (although it only draws around 2mA in between pulses).

I'll attach more info in the next post.

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