PIR Motion Sensor get RF interference

Hardware
1

hi,

i had a Motion Sensor in parallel with smartmaker breakout, both sharing same usb power source.
the issue is the PIR will self trigger when the imp is trying to send data. to proof that it is cause by RF interference. i setup another PIR with difference power source and place next to the imp card and it did turn on by itself with exactly same timing.

i had been read through this article http://forums.electricimp.com/discussion/1607/eliminating-interference-with-external-module and adding 10pf to + - but it didn’t work. so i just try my luck by attach more capacitors… 1000uf, 0.1uf, 0.01uf, 0.001uf and the interference is still there.

anyone have solution for this?

my breakout
http://smartmaker.com/index.php/electric-imp/break-shield-impee-for-electric-imp.html

my motion sensor
http://www.raspberrypi-spy.co.uk/2013/01/cheap-pir-sensors-and-the-raspberry-pi-part-1/

2

I don’t know if this will work for your setup, but I experienced occasional interference when reading thermocouple data over SPI if the Imp was transmitting. To get rid of it, I do a server.disconnect(), then read all my SPI data and do whatever, and then reconnect by sending the data to the agent and then to Xively. I haven’t seen any highly deviant values in my Xively logs after doing that.

Anything you do that would need to go out to the imp servers will cause the device to reconnect, even server.log

Can you set things up to ignore the sensor when you transmit?

3

The RF issue may not be power coupled, but coupled elsewhere in the circuit - low power PIRs rely on some very small bias currents.

You may be able to improve this with small value pF caps (eg 10pF) on the op-amp inputs, or simply by shielding the PIR from the imp with a grounded conductive surface, eg a bit of copper or aluminium.

4

Here are my finding.

I had tried shielding the PIR with aluminium, putting the capacitors (see my attachment)…and all not working. the PIR is place back to back with ImpCard in a project box, I suspect the RF signal is too strong, but if I move the impCard 2cm away from PIR, the interference gone.

and I found something interesting. If the signal is weak (RSSI 80-90 dbi), I get tons of RF interference. so I put a wifi repeater in between my router and ImpCard the interference gone (RSSI 60-70dbi).

also if I move my ImpCard 1.5cm away from my aluminium window, the signal improves from 80dbi to 70dbi.

I hopes anyone who want to permanent place their impCard should check for best RSSI signal, a few cm difference can makes a huge improvements.

5

When you are at a low signal strength, the imp will be dropping down to 802.11b (CCK) modulation, and it transmits at a higher power when in this mode.

Likely your PIR is being affected by CCK but not the OFDM modulation of 11g/11n.