Monday 7 October 2013

At last! low cost wireless sensing for couch potatoes.

Update 9/10/2013

Texas Instruments SensorTag 

Wireless sensing or The Internet of Things has been around for a while but at last we have a multisensor  device for £25 . People have designed many wearable health and sports monitoring  applications, initial research by me at Microsoft in 2001, so  below I cover some new useful designs for busy couch potatoes, including The Door Monitor, Watch My Cooker, Cat Burglar Alarm and finally Am I Alive? It looks like these can be practically built and tested and used by people with Smartphones. 

 Nov 2012 Texas Instruments  announced  a multi sensor wireless device  (6) for  £25, inc shipping and tax with all the hardware sensors included. This looks like a loss leader price for development purposes  but will be delighted if can stay at this price. I have seen many sales promotions for this but not sure how many shipped as not many third party apps on the web.  I am due to get  received one on  7/10/2013.

  This devices makes obsolete many other wireless sensor hardware designs. I will be very pleased to start creative programming on  it  without worrying about my hardware design issues such as eyesight,  wires falling off, batteries, antenna design  etc.  
Texas have done a great job with this design, A SOC with built in Bluetooth and  8051 8 bit microcontroller for low power, and also the heavy duty signal processing  is done on the smartphone device. The CC2541 SOC is around $3.08  in 1000 off quantities, i.e. the Bluetooth and microcontroller but no sensors. At this price, a design can be made disposable.

CC2541  6 x 6 mm

From  the Texas Data sheet, click to enlarge.

It can be used with low energy Bluetooth e.g the  iPhone.

Here is the spec. 


  • Humidity Sensor
  • Pressure Sensor
  • Accelerometer, 3 Channel, up to 14 bit resolution 
  • Gyroscope
  • Magnetometer
  • 2 contact switches 
  • Bluetooth low energy, range 50 metres. 
  • Radio Signal Strength transmitted so useful for location services.
  • CR2032 coin cell providing months or years of battery life. 
  • There seems to be  no Time Of Day clock but this is provide by the host computer. 
  • 32KHz sleep timer
  • Missing is a low cost light sensor but could reverse bias a LED to measure light level.
  • GPS not need as you need to be just a few metres (50m)  of your smartphone to read the wireless data.
   size 71.2x36x15.5 mm   57x25x1.5 mm (PCB)  

You will find most  the above sensors in smartphones but remote sensing has many more applications. 

All I will add at first  to the hardware for my designs  is a neo magnet for attachment to walls etc. Modern homes have steel pillars in unexpected places e.g.  hidden behind the plaster. 

I also require a waterproof version and the PCB and a larger battery will fit inside a  35mm film cannister  ( internal  64mm in height and 38mm diameter). Buy from eBay.

Here is a teardown.


Texas and others cover many of the usual wireless sensing designs including sports applications, games controllers, wristwatches  and  the perennial lost key detector. Below are some new ideas  for couch potatoes. 

 The  Door Monitor

After I have learned how to program the SensorTag, the first object I will attach it to is my front door, with messages sent to iPad. 

 Yes a camera can be used but also useful to have simple text alerts sent to your smartphone, tablet, etc. 
  • The thermopile will detect if a  person stood at front door.

Field of view from Texas Data sheet
  • Accelerometer/gyro  will send message re door open or closing. Accelerometer can detect vibration off a door knocker, useful if hard of hearing. Gyro can detect opening and closing of door. 
  • Will the magnetometer be sensitive enough to detect the the steel from a visitor's parked car 6 feet from my front door?
  •  The humidity/pressure  sensor gives current alerts re rain, so we don't let our washing get soaked on the line. 

Watch my Cooker - Kitchen Safety and Dementia.

 The Sensortag.  has a thermopile for measuring heat at a distance,(-40C to 125C)  ALSO humidity, steam  so can point at gas cooker hob and message sent to ipad/iphone if pots boil over. Useful for forgetful cooks, Alzheimer's and dementia. 

None contact temperature sensing using thermopile, 68C. 

Adding a Sensortag to a fridge door provides the imagination with many uses, such as diet control and care of people who live on their own. (if you don't open a fridge for a while, are you well?).

Another Pet Project - Cat Burglar Alarm 

I might write something simple to test it with like cat burglar alarm, 50m range to ipad. Using the wireless accelerometer , can use direction to count N cats IN and OUT on cat flap, and if N+1 IN counts,  it implies it  the neighbour's big cat!  Saves me getting up at 5am to sort out cat fights and who steals the food at 3am. (My cats sleep  upstairs). 

It wasn't me!

And finally, Am I Alive? Wireless accelerometers useful for fall detection on people. They really need to be small enough to add to devices like spectacles (thank you AM for this idea) so people wear them all the time. An accelerometer can also be worn on the chest for respiration monitoring, so the SensorTag may be applicable.


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