SenseLamp is a lamp shade that can be remotely controlled and gathers temperature, humidity, light-levels and motion data

SenseLamp: a ultra-low cost, WiFi enabled sensor platform

March 31, 2013 – 2:46 am

The SenseLamp is an ultra-low cost (less than 30GBP/45US$), WiFi enabled sensor platform that is easy to build, easy to deploy and fully open source. A SenseLamp is a lamp shade that can be remotely controlled and gathers temperature, humidity, light-levels and motion data. It runs Linux and can do some on-board processing before passing the data on to e.g. busfarhn or COSM.

jump to: the story, How it's made, Do it yourself.

The story

One can write a story from two angles: a historical account or coming from the motivation. Since my motivation is something like "it sounded like a fun thing to do", I'll give the historical account.
The purpose of all this is to put a temperature, humidity, light-level and motion sensor in every room. And while we're at it, throw in a relay to control a light-bulb. This combination allows some nice home-automation to be implemented. Even further: it provides a test-bed for activity recognition and helps to implement the idea of a quantified self. But in the end, it really just sounded like a fun thing to do.

Mark 0

SenseLamp v0.0

After I had finished a first version of busfarhn and implemented the central heating control, I wanted to take things to the next level and started working on the SenseLamp. It didn't take too long to whip up a first prototype: SenseLamp v0.0. It consisted of a TL wr703n, TI Launchpad with an MSP430G2553, a relay module, HC-SR501 PIR sensor and a DHT11 temperature/humidity sensor. All neatly wired up with jumper cables and attached to a lunchbox to which I went to town on with a Dremel. It wasn't pretty, but got the job done – and it was running for 2 months!

I learned three things from this first prototype:

The Linux CDC ACM drivers are buggy: it seems to be a well known problem that the Launchpads serial interface doesn't work so well in Linux. The reason for this is a bug in the CDC ACM kernel module (see here, here and here). Despite the patches provided, I was unable to fix the issue on the WR703N resulting a bit of instability. A cronjob to reboot the router every 6 hours served as a usable workaround, but certainly does not fix the issue.
HTTP only goes so far: using a simple HTTP based interface implemented via CGI worked beautifully for the central heating control as the interface was unidirectional. The SenseLamp however, needs to be able to send sensor data as well. So a different protocol/interface is needed.
It will have to look better than this: While the dremeled lunchbox worked fine as a first prototype, I wouldn't want plastic lumps hanging throughout my flat. So if I was to deploy this on a bigger scale (count > 1), it would have to look better than this.

Mark I

SenseLamp v0.1

The next iteration was designed to do away with the flaws of the prototype. To get rid of all those jumper cables and get things neat and tidy, I designed a PCB and sent if of to SeeedStudio Fusion. As much as I favor local production and business – SeeedStudio was simply 10 times cheaper than anything I found in the UK – even with similar lead time. It only took about 2.5 weeks until I got the PCB in the mail, with the cheapest delivery option that is.

The goal for the SenseLamp PCB is to serve as a "shield&quot: for the WR703N router. The shield is powered by the router and directly talks to the routers AR7240 CPU via the serial port exposed via two test-pads (yep, fiddly soldering included). To make the power supply work, the board sports a 3.3V LDO. All sensor protocols required (the single wire protocol of the DHT11/DHT22), GPIO stuff for the PIR sensor and ADC stuff for the LDR are handled by the MSP430G2553 which is also on the PCB. A neat package all in all.

At the same I started playing around with different frame designs. The lamp shade side of things would have to support the boards, mains to 5V power supply (which handily comes with the router), some space for the light bulb and a spot to place the PIR and temperature sensor away from the light-bulb. As I happen to be interested in personal fabrication (sigh) I laser-cut a triangular frame that unfortunately is not really portable across different machines and can certainly not be ordered from services like Ponoko. But then, any other form of shade/lamp holder would do.

I ended up deploying that version in the kitchen and ran it there for a while. It successfully did away with the communication issues that plagued version 0. Also it looked quite a bit better. But it still wasn't quite there yet:

forgot the PIR sensor: the lesson learned here is never design a PCB in a hurry. For some reason I forgot to add the the appropriate pinheaders for the PIR sensor on the PCB. So I had to compensate for that while wiring up the whole thing – it worked, but it was messy. Because like this the PIR sensor can not simply sit on the board, it has to be placed somewhere on the frame. Yet another thing to consider.
assembly was still too difficult: which is in part due to the PIR screw-up. Until this version I assumed I could simply tie it all together using some good old zipties. But as it turns out the holes in the relay board, as well as the router board are close to M2 and that's too small for those standard 50mm zipties. So, I'd have to put in screws. Combine that with my tendency to pack everything too tight (on the frame) and you end up with something that's very hard to put together.
DHT11 sensors are too inaccurate: according to their datasheet, DHT11 sensors have a tolerance of +- 2 Celsius and 1% humidity. That's quite a lot when one wants to monitor temperatures in the range of 20 +- 3 degrees. So I needed better sensors: the pin-compatible DHT22 to the rescue.

Mark II

SenseLamp v0.2

That's the version that I ended up building four times and putting one up in every room. It uses a new revision of the SenseLamp PCB (taking the PIR sensor into account), has a way cleaner cable structure and sports DHT22 sensors. Let's see how it's made in the next section.