cyborgworkshop.org

I added the first part of the pressure and safety monitoring section of WHIA. It’s still a work in progress, but the outline is there for what I’m working on now.

Christmas has come early! Fedex brought me a whole bunch of packages today stuffed full of pressure sensors, audio amplifiers, LED controllers and all sorts of  geek candy.  I’m playing with the freescale MPX5999D right now and the MPX4250.  I thought the 5999 was going to be my sensor of choice, but it looks lke the 4250 will be better.  Its more sensitive and it has a nice port connector so I dont have to mount the sensor in the plenum.  More to come!

I just got a whole bunch of emails from freescale, analog devices and marvell about samples that are shipping out! Closer and closer.

First off, thank you to everyone that has been commenting and leaving advice, it’s very appreciated!

OK, I have a few MPX5999D pressure sensors on the way from the fine folks at freescale, so I’m starting to code up the logic that WHIA will need to use them. I’ve decided that I will have one unit in the main duct, as planned, measuring pressure inside the plenum. The trick will be to make sure that the sensor is mounted in such a way that air is not being forced into the port but that it isn’t causing a vacuum as it rushes over it either. We’ll cross that bridge shortly.  Since I have multiple sensors on the way, I thought why not do a barometric pressure sensor too.  The sensors measure pressure in kPa and we usually talk about atmospheric pressure in inHg. Well, 1 inHG is equal to 3.3860 kPa or .49109778 psi. The barometer says that it’s 29.86 inHg at the moment, so I would expect a sensor reading of 101.1596 kPa or455mv (4.5mv per kPa) , and if I had a sensor I would happily test that.  The 5999D measures from 0 to 1000 kPa  so we’re WELL in spec, but here is where things get ugly.

What we call barometric pressure is actually a device created and employed during the crusades as an advanced interrogation technique. I’m being dramatic, yes, but when you don’t know a whole lot about the underlying theory, even a simple equation can look like water boarding.   Using the steps outlined here, I can take the pressure and temperature data from my probe and eventually come out with the barometric pressure.  Or I could buy an scp1000.  But for that matter, I could have just bought a programmable thermostat and some oscillating fans So when the sensors arrive, we’ll add a barometer.  A tweep also suggested a hot-wire anemometer might come in handy, so I’m adding that to the sensor pile so I can get velocity and pressure from inside the HVAC main plenum.

Outside of planning next steps, the system has been working remarkably well.  My basement temperature spiked a bit this evening and I was surprised to see the jump in my graphs, but then I found that the basement door had been left open. Mystery solved thanks to those medeling kids and their dog.  More to come.

The pressure transducers are on order from freescale so, while I wait for them, I’m starting to look at moving to the next generation of WHIA.

What I’ve learned.

• I’m a sensor whore and will always want to tack on more sensors.
• The arduino is amazingly resilient, but the USB connectivity can be unstable over long periods of time.
• Manual HVAC dampers aren’t exactly what one would call high quality.
• Outdoor temperature sensing can be a PIA

With that in mind, first we need to layout the goals for v2

• 10 temperature probes (DS18S20P 1-wire)
• 10 Humidity probes (Unknown, but 1-wire)
• 15 window servos (to control the blinds)
• 10 vent servos (Damper control)
• 5 HVAC pressure sensors (Main vent and branches)
• 2 Anemometers (Main Vent and outdoor)
• 1 Rain gauge (outdoor)
• LCD control panel (Feedback)
• 1 blinkenlight for each sensors (Geek Candy)

Without the blinkenlights, we’re talking about between and 39 and 53 IO pins depending on how I use 1-wire.  I’m planning on driving the LEDs with a MAX7219 8×8 LED driver. I should be able  to access 64 lights using 3 pins on the uC, so that’s a good match and allow me to blink a light every time a sensor or driver is accessed.  The LCD panel will just be a simple old i2c planel giving some generic status messages and essentially eye candy.  Now, the big question.  Multiple Arduinos connected via i2c or that fancy new Arduino Mega?… If I use the Mega, I can just create a Shield again and that will make my PCB a little simpler but multiple arduinos talking i2c is just cool!  I am dumping the protoboard this time, it will be all bread board and then sent off to the PCB guys for some fiberglass and copper.

FYI, i’m not abandoning WHIA 1, it’s going to keep getting updates and features as my test bed system so I’ll have a full write up of the pressure sensor integration as soon as they show up.