22 April, 2012

Simple IKEA transmit indicator

IKEA Oleby automatic wardrobe light with passive
infrared sensor upper right and four LEDs in the center.
Passive Infrared (PIR) detectors have the bad habit of being sensitive to radio frequency (RF) radiation. This can be a nuisance for neighbors of radio amateurs. But we can use this property to our advantage as this makes them suitable for transmit indicators next to a feedline.

The IKEA motion activated LED sensor light, which one can get for a few dollars or euros, turns out to be quite simple to hack for this purpose. You can only buy them in pairs so the other one can be put to use in the darkest wardrobe you have.

Like in most such circuits, the brain inside is a BISS0001 Micro Power PIR Motion Detector IC. The circuit has two time constants:
  • Output pulse width control, Tx, which is controlled by R13=10k and C7=100n in the IKEA design (R10, C6 in the datasheet). In the IKEA circuit Tx = 24576 * 10k * 100n = 25 sec. 
  • Trigger inhibit control, Ti, given by R12=470k, C6=100n (R9, C7 in the datasheet), Ti = 24 * 470k * 100n = 1.2 sec. 

Remove three screws to access the circuit board.
See lower right corner where C6 and C7 are lifted
Both of the time constants need to be made smaller than the shortest unit in a morse code symbol in order for the indicator to be agile enough. As the dot is 1.2/WPM seconds long, this gives e.g. 50 ms for 24 words per minute (WPM) morse code.

I did the simplest thing and just lifted C6 and C7 as shown in the image of the circuit board (on the right-hand edge of the circuit board, next to the integrated circuit). That was also how I was able to measure the values of these unlabeled capacitors to 100 nF. The capacitors are easy to find as the circuit board is labeled with C6, C7 etc.

That gave me an RF indicator which when placed next to my ladderline will react to RF at levels down to well below 5 Watts on most of the bands between 3.5 and 50 MHz. It is a good idea to tape over the PIR indicator window so that it stops reacting to movement.

Was it fun to make? Yes. Is it useful? Mmm, perhaps.
  • Link to BISS0001 datasheet
  • Here's another modification which ensures that the IKEA light only turns on when dark. The BISS0001 has a dedicated input for a light-dependent resistor so it is very easy to add.