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Strange Oscillations

Tuesday, July 5, 2011 by darco

I stuck one of my soil moisture sensors in a potted plant last Friday before I left for a weekend trip. It was working fantastically. I watched as the soil moisture reading increased as I added water to the plant—working exactly as expected. However, when I returned Monday evening, I noticed that the values were all over the place. I took some samples on 100ms intervals and plotted the results:


Oh my. It's oscillating with an amplitude of around 10% of the capacitance reading. Not only that, my readings are an order of magnitude larger than what I was getting on Friday—when if anything they should be smaller.

I'm wondering if the 60Hz mains frequency is somehow affecting my readings, which would imply that what I'm looking at is the alias of 60Hz sampled at whatever interval I'm polling sensor. The power company regularly adjusts the frequency by ±1Hz or so, which could explain why I didn't notice the ripple earlier (Slight changes in the mains frequency would yield large changes in the alias frequency). This doesn't explain the large increase in the reading, however.

I also noticed some interesting behavior back on Friday. Whenever I grasped the stem of the plant, it caused the moisture level to increase. This makes sense, now that I think about it, but the implications are that each soil moisture sensor may have to be individually calibrated for differences in soil type, amount of soil in potted plants, etc. Not ideal, but not a show-stopper either.

Obviously, still some kinks to work out.

I hope to have the wire protocol solidified this week, at which point I'll actually deploy a few of these things in the yard to see what kind of readings I get. Should be interesting.

Final Soil Moisture Sensor Design

Tuesday, June 28, 2011 by darco

I think this is the final re-design of the soil moisture sensor that I will be sending off to Gold Phoenix for panelization and a small production run.



Just seven parts: two capacitors, three resistors, one ATTiny25 and a circuit board. I switched over to largish surface-mount parts and an edge connector instead of a standard 3x2 header. This board will be able to temperature-compensate using the on-board temperature sensor on the ATTiny25. I've also increased the length of the sensor to nine inches. Here is the schematic:


By my calculations I should get back around twenty to thirty boards. Should be more than enough to cover the front and back lawns and the garden. Now to just work out how to wire them all up...

Soil Moisture Sensor Update

Friday, June 24, 2011 by darco

I've heard a lot of feedback on my soil moisture sensor project, and I wanted to update everyone on how it is going.

Last weekend I did a lot of work on these devices and I've made some significant progress. I now have the firmware to a point where I can remotely communicate with the sensor, perform readings, etc. I've also made a few observations after working with the most recent board layout.

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6LoWPAN Cat Feeder

Saturday, March 12, 2011 by darco

2011-03-06_17-30-16_2231I just wanted to point out that while I'm lagging quite far behind in my own 6LoWPAN-based home-automation system, it seems that yzf600's project is moving ahead at full steam. He just recently created "World's First 6LoWPAN Cat Feeder", based on a wireless controller board of his design.

Now I want one. I guess I had better get back to working on this stuff.

Updates and Musings

Sunday, February 27, 2011 by darco

Late last year I decided that I need to go ahead and get my workshop finished before I continued my work on my home automation network. Since that time I never seemed to find the proper time to get back to working on it. The longer I wait, the more difficult it will be to get back up to speed. I've got a lot of parts just waiting to be soldered up in the garage, but due to the recent construction work on the house, the garage (and, thus, my workshop area) is a total mess. Grr...

I'm looking forward to getting back to work on the home automation front. From what I can tell, there is still no CC2350 or ATmega128RFA support natively in Contiki, so it sounds like I may have to do the heavy-lifting myself after all.

Smart Switch

Monday, February 7, 2011 by darco

I've been working on the design of my Smart Switch almost a year now, and the other day I finally received my first set of circuit boards for making my first prototype. I'm not finished populating them, but I figured I would go ahead and show it off anyway.


The board in the upper right is the power and communications module. The board in the lower right is the load module, which in this case is a simple relay.

The board on the right is the touch sensor board. This board will eventually have a small piece of white-backed frosted glass glued to the white outline on the front of the board. This is what the user will touch to control the switch.

The board in the middle is the main board. It contains the microcontroller which controls almost all of the functions of the switch.

SmartSwitchPics/DSC_8266 SmartSwitchPics/DSC_8269

All of the other circuit boards plug into the main board like this:


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Soil Moisture Sensor Update

Saturday, January 8, 2011 by darco

I ended up building the prototype soil moisture sensor boards I mentioned in a previous post, and... it works! Somewhat.

This is what it ended up looking like:


The sensor section of the board I coated with black Plasti-Dip to protect it from moisture, and the results turned out rather well. In later versions I'll probably be potting all of the circuity in either Plasti-Dip or epoxy.

It turns out my method for trying to get two zones to work properly on the limited pin count of the ATTiny13a was not effective—instead of getting independent values for each depth, it acted like a single large sensor. In light of this observation, I've decided to jettison the cleverness and just focus on a single-zone capacitive moisture sensor design—at least for now. I also removed the ground plane as well, as it doesn't seem to be helping the readings. Here it is the new single-zone version:




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Smart Switch

Thursday, January 6, 2011 by darco

I just wanted to share a few pictures of project I'm working on. This is the next logical step from the appliance module I mentioned a while back: a smart light switch. The design is modular, which allows for all sorts of configurations, but the first prototype will be be a simple on/off switch with the ability to be remotely controlled.

There are four boards: a main board, a relay board, a power board(which also handles remote communications), and the touch board. Eventually I'll be making a dimming version of the relay board, and I may possibly implement a power-over-ethernet version of the power board.

The user interface is a Decora-sized 1-dimensional capacitive touch plate with a frosted glass surface, with slight indentations on the upper and lower portions. Pressing once on the top will close the circuit, pressing once on the bottom will open the circuit. If the switch is equipped with a dimmer module instead of a simple relay module, touching the top and dragging your finger down will dim, and the reverse will brighten. A piezo-electric element on the main board will provide some tactile feedback as you touch the glass, making a satisfying click when you press it. Right-angle Status LEDs shine light into the top and bottom of the glass, which is diffused by the frosted surface.

Cool, huh?

I've been sitting on these designs for a while now, so I figure it is about time I pull the trigger and order a few boards to populate.


power-board main-board touch-board

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