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03.06.2015 New ultrasonic sensor and some data
06/03/2015 at 21:00 • 0 commentsIt's becoming obvious that ultrasonic sensors are not the best way. I have failed to make another one work with a different model of a chair, which means they will not work everywhere. Apart from that, they are very bulky, coupled with 3AA(A) batteries will have to create a reasonably large sensor. Time to think of alternatives.
I have just received the new US-100 ultrasonic sensors from ebay. As they operate from 3V, a 3.3V supply can be common to the microcontroller and WiFi. Current consumption is also lower, I am measuring 1.8mA vs 7.5mA with the older sensor, 4 times less, this should improve overall power consumption.
I have set up an emoncms panel to watch over the data. Unfortunately it takes a precise level of zoom for the first graph to look this great: the visualizer decimates the data and then interpolates the remaining points linearly, which usually causes it to draw diagonal lines between some sitting and non sitting event. However, here is a section that looks great:
On Saturday, 30th may I left something on the chair by accident, which caused it to record a lot more hours. Human and non human distinction would be great for new types of sensors.
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25.05.2015 First experiments with chair version
05/26/2015 at 23:05 • 0 commentsThe chair version will work with any user and should be as low cost as possible so that it will be installed on a high scale on any possible chair. Alternatively, if the chairs are dedicated per person they may provide insight on time spend on each chair, like at work, at home or in the car.
I started doing some experiments with an ultrasonic distance sensor. As it turns out, it is not the ideal thing to use: this particular version requires 5V and quie a lot of power, but it will do for the time being, as proof of concept.
The circuit is build around an XMEGA32E5 one some header board, with an attached ESP8266 and your tipical HC-SR04 ultrasonic ranging module, nothing special. It is not optimised for power consumption in any way so it will burn a charge of batteries in 3-4 days. For now, it simply uploads the data to emoncms on my website, similar to Led logger V3.
As it turns out, putting the sensor on the top of the chair is not a good idea: some fabrics seem to be pretty bad at reflecting ultrasounds and crouching does not detect the person.
Next up, i mount this sensor between the seat and back of the chair. It is a delicate position, as a bit higher or lower will cause the sensor to pick up the cushions as obstacles. However this seems to work a lot better, usable with all the types of pants that i have. (well a long wool sweater will still break the thing).
Conclusion on ultrasonic: might work, delicate to place, large. The high voltage and high power consumption may be managed to get decent battery life.