So I left the last article with the following:
Well apart from the hardware I need, so issues need to be addressed which may or may not require extra hardware – as I’ve just thought of them.
- DateTime equivalent object for when I register a pulse
- Work out how long these will last on battery
- Can I set an interrupt to go off when a digital value hits a threshold? Or does this require analogue input? If I can it would massively save on battery as no polling! But, it may require fiddly per-house calibration, which the brute force method ignores
- Laser/3d Printed box and some form of mounting which will let me attach to anything. Probably going to be velcro
So I’ll go through what has been done since then via this criterea.
The DateTime object style thing was achieved through an RTC module (http://www.sparkfun.com/products/99) which communicated to the ‘duino using the I2C bus and takes 5V. A microSD card shield was also added to the hardware for saving events into a simple text file (http://www.sparkfun.com/products/9802).
Rather than use my hastily build photosensor, I used a load of RFXcom sensors as they are well built and have a housing designed for sticking to the meter (Which is by far the biggest engineering challenge of this project). A board layout for interfacing with the sensor units was created and the gEDA/gSchem schematic file can be found on the git hub project page.
Well, apart from the stuff which interfaces with the RTC module and the microSD card, not much has changed code wise. The way the RFXcom modules work was backwards to my prototype, so I measured the time it takes to discharge a capacitor rather than charge. An LED on a meter normally flashes for .1s, so the timout is set to 0.05s.
Using a multimeter showed the whole sensor drew 92mA. It isn’t ideal, but with a 6 AA battery pack which packs ~3000mAH which lasts a bout a day ( ) However, that was with a ridiculously high powered infrared LED on the RFXcom board. Using just the photosensor (rather than the reflective) the power output was 42mA and that was with the SD card always powered. There is a lot of scope for battery life improvement on this project.
The source can be found on my github page at https://github.com/carl-ellis/Arduino-Gas-Sensor .