Hello
wanted to know
I have circuit that act strange near RF of 400Mhz
when I use the circuit near a device that transmit on freq 400Mhz It start to act crazy
how difficult it will be to built to it a "RF guard " that will cancel the noise from this freq?

 

What is the device? What is the source of the 400MHz RF? What does "act crazy" mean?

Wrap it in tinfoil and connect it to a good ground.

 

If you can post more information about the device that is made to misbehave with the 400 MHz signal, and describer the nature of the misbehavior, somebody here might be able to make specific suggestions as to how to improve the RF immunity of your device.

 

I will explain
I have a pir sensor with a circuit that give me "1"\"0" in it's output -
so when the sensor detect a movement I'm getting "0".
I also I have a device that transmit 2-7 dBm in freq between 400-500Mhz
when the sensor is near it ~2-3 meter I get "false alarm" - the sensor showing "0" all the time - like he detect a movement.
when I close the RF - it works just fine.

I have try to use tinfoil and rap the sensor and also the circuit - but still getting "0" all the time.
this is the sensor I'm using
http://www.kube.ch/downloads/pdf/kube_pyroelectric_c172.pdf

the image is the circuit I have built

 

Hi David,

Looks like the long leads make a perfect antenna!

Just wrapping with tin foil is not good enough.

You should have no problem with the PCB in a metal enclosure and grounded to it.

Regards,

Nandu.

Hey, Nandu! Good to see you on AAC! Looks like quite a few "Zedders" are finding their way here!

 

how can I ground it?
if I take a metal box and close it with the PCB inside ,then connect the box to my wooden desk
will it do the job?

also you think the problem is in the PCB and not in the PIR sensor?

Thanks ,

 

You would ground the shield by connecting the shield to the (common) ground connection on your circuit.

For shielding the PIR detector itself, you might want to use brass or copper screening -it would let the IR through but stop the RF.

As v2unan pointed out, the long leads are quite likely a problem. If at all possible, put the PIR sensor on the circuit board.
Aluminum foil is usually a poor choice for shielding because it is difficult to make a good electrical connection to the aluminum through the oxide film on the surfaces.

Low value (hundreds of pf or lower) high quality capacitors to decouple various points in the circuit to the circuit's ground might solve the problem. I would start with the input circuit by putting 470 to 1000 pf capacitor across C1 and C3. A couple cm in placement and lead length can make the difference between the decoupling working or not.

Those are some ideas that might take you in the right direction.

 

just to see I understand you correctly :
1. I'm going to use less then 1 cm cable between the PIR and the circuit .
2. I tool a metal box and connect it to the GND of the circuit (Then circuit is in it ) and the PIR sensor is outside of the box.
3. I will take 470 pf (this is what I have here right now) and connect it in parallel to C1 and C3 , right? or in line?
4. also I going to take a very thin copper something like paper width and put it on the sensor ,right?

 

When DickCappels says "screening" I think he means mesh rather than foil. Copper/brass foil is not going to let the IR through

 

did all of it , didn't help
any more ideas?

 

Maybe its time to do some troubleshooting on the circuit while operating. Using whatever test equipment you have on hand, try find the earliest part in the circuit that is adversely affected by the RF. That would be where to start applying additional countermeasures.

 

Check the cabling to the PIR sensor, is that screened? is the screen gounded? You could consider putting a choke on the cable close to the PIR.

 

you think the problem is the PIR sensor ?
I will try to cover as much as I can the pins of it