Monitoring Radon with Arduino

I live in the Midwest, in Chicago suburbs. When I bought my house several years ago I ordered Radon gas check and the levels were in normal range. From what I read about Radon, it's levels can change during the year and season. I always wanted to check its level real-time and find the factors causing its fluctuation if it is possible.

I bought a Safety Siren Pro Series 3 Radon Gas detector from Amazon (Model No: HS71512) and did some initial measurements. On my living floors the radon levels were OK, but I was mostly interested in basement, which is unfinished non-liveable "crawl space", used mostly for storage. Would be great to have the Radon measurement process automated, without the need to crawl into basement every time to check the readings.

I found excellent article written by Chris Nafis which exactly targeted my task. Recently I've got a spare Arduino Kit from SparkFun Electronics and decided to use it for the Radon detection automation. You can get a lots of basics and instructions from Chris's radon work, I will explain here only what I did differently.

First of all, the Safety Siren Radon Gas detector has to be modified, which will void your warranty. Instead of having a set of cables directly attached to a Safety Siren detector board, I installed a 25-pin DB-25 connector on top of Radon Detector, and cut a LPT-port cable in half from my old PC. I soldered the cable wires to my Arduino and put it along with Ethernet Shield into a small plastic box. See the picture below of what I have today running in my basement.

 

Below is the picture of LPT cable ends with soldered wire pins I used for experimenting. Later I removed all the pins and soldered cable wires directly to the Arduino board, so the whole construction would fit into the box.

An interesting challenge was the detection of Long / Short setting of Safety Siren detector (1 week or 1 month of approximate radon data). I noticed that after reset the detector automatically turns on into Long setting, but I wanted to rely on the actual settings and not assume anything. Chris has an Arduino program for older Model HS80002 listed on his website, but I had a newer HS71512, and Long / Short detection logic were missing in the program. First I thought I can read the Long / Short LED, but unfortunately soldering into it was not an easy task without disassembling the Safety Siren board, so I decided to take a programming approach.

I noticed that the Long / Short settings has some timing differences. See two screenshots below, each screenshot has two channels displayed. On a first screenshot the DIGIT_4 strobe (lower channel on screenshot) goes 0 almost immediately with LTL signal (upper channel) when we are in Short setting. In Long setting, this two signals has a timing difference, so the second screenshot illustrates it.

 So to handle this timing difference, I used interrupts functionality of Arduino. The Arduino Uno has int0 on pin2, which I attached to LTL from Safety Siren detector.

attachInterrupt(0, detectShortHandler, RISING);

The interrupt service routine is very simple - if DIGIT_4 is 0 - we are in Short mode, if opposite - we are in Long.

void detectShortHandler()
{
  delayMicroseconds(100);
  if (digitalRead(DIGIT_4) == LOW)
    state = ST_SHORT;
  else
    state = ST_LONG;
}

Please note a small delay I had to add before processing the DIGIT_4 strobe value. The DIGIT_4 goes to 0 "almost" immediately, see a screenshot below, hence the pause.

Finally, I had the radon values detected, processed and periodically (once per ~30 minutes) published to Xively free service (former Pachube). Below is a graph of the Radon level in my crawl space after several days of monitoring.

Note that I have only "long" setting presented. After several weeks of collecting data, my Safety Siren detector failed to switch to Short. I think it is a detector issue (probably internal firmware fail), because it still switch into Long and stay there even after I disconnected Arduino and did a full reset of Safety Siren detector. Well, I cannot exchange detector because I opened it and I will not buy another one (~$100 is too steep for a small project), so I'll stuck with Long setting for now.
Update: after about 4 months both the "long" and "short" settings started to operate normally. I think the reason might be a power supply, this is the only thing which I changed with my current setup during that time.

For those who would like to repeat this project - I published my Arduino program on GitHub, feel free to use it and get back to me if any questions.