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PIR wiring

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gonzo350

New Member
I'm working the the motion detector out of a basic off theshelf outdoor light. I need to remote mount the PIR, so I desoldered it from the board and attached 10" leads. The PIR now does not trigger the light. I have checked my connection numerous times, and when I resolder the sensor to the board...all is well again. Anyone have any idea why taking it off the board and attaching leads causes it to fail? I've been through about 4 boards now...this is getting expensive. Merry Christmas and thanks for the help!
 

Johnson777717

New Member
It sounds like the additional lead length is adding resistance to the PIR circuitry. The circuit was designed to provide the PIR with the proper amount of power. When the 10" leads are added, you've added more resistance to the circuit, which means you have less amperage going to the PIR. What you may have to do is alter the resistors that are before the PIR to allow for the additional resistance caused by the 10" lead length. The more wire you have, the more resistance you've allowed in the circuit.

If you have access to a multimeter, you can find the resistance that is caused by the 10" lead length. Then you can adjust the circuit resistors to allow for this additional resistance.

Do you know what guage wire you're using for the 10" leads? If the wire is thick, you've added much more resistance than if you used thinner wire. You can try using a thinner wire for your leads. Be careful though, because we don't know the amperage going through the wire. High amperage through thin wire can cause overheating of the wire, or surrounding components. You may need to find how many amps are running through the wire, then chose the best wire guage for that amperage. You can check the amperage with the multimeter.

Would you possibly have the ability to post the schematic of the circuit so that we can help analyze?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I would suggest the reason is noise, don't PIR's have massive gain in order to detect small movements at long distances?. The output from the sensor itself is minute, if you extend the leads to the sensor the noise picked up on the wires is going to be larger than the signal. You really need to keep the sensor and amplifier as close as possible - there doesn't really seem much reason to want to seperate them.
 

Artie Ziff

New Member
Nigel Goodwin said:
I would suggest the reason is noise, don't PIR's have massive gain in order to detect small movements at long distances?. The output from the sensor itself is minute, if you extend the leads to the sensor the noise picked up on the wires is going to be larger than the signal. You really need to keep the sensor and amplifier as close as possible - there doesn't really seem much reason to want to seperate them.

Hi Nigel is correct,
The output from a PIR sensor is a very small signal and everything is
tuned in the circuit to give the best performance adding 10" leads will not
work.
The leads will pickup noise from around the enviroment.
You should also avoid putting your fingers o the sensor window.


Also you say you took it off the board. Does this mean that it is no longer
in it's box with the fresnel lens?
If this is the case then you will also have problems. There is a lot of design
work that goes into the lens and case for one of those things the sensor is
set back a certain distance and the lens creates the patterns for the beams.
The performance will reduce if it is not kept in the box.
**broken link removed**
is an app note that sort of gives some insight into a PIR.
Here is a product spec for a typical PIR **broken link removed** used for light sensors
 

gonzo350

New Member
I too agree that the problem is noise. My reson for removing the sensor is mechanical....I only have enough room for the sensor itself....not the board as well.....thus the reason I need the remote mount capability. I have checked to see if removing the fresnel lens renders things inop....but it does not....the sensor continues to opperate. So my question now is....what can I do to reduce the noise created by the length of wire (22awg)?
 

Artie Ziff

New Member
It's all down to experimentation at this point,

you could try shielding ALL the wires to the sensor.
(I would try shielded audio cable)
If that doesn't help,

try to locate the resistor and capacitor across the pins of the sensor as per App note. Try them at the pir end (solder them with very short leads close to the sensor. (with the shielded cable)

try to locate the amplifier on your board and relocate that circuit (including caps resistors etc) to the sensor end.
This will bring the amplified signal to 'your box' instead of the really small signal.
or build your own circuit....

I didn't say that the circuit would stop working without a fresnel lens just that performance would not be the same. Do you have an alternative lens?

What exactly are you making with the sensor out of the outdoor light maybe someone else has done this before and can share their experiences if they know more details?
 

linhthai75

New Member
Motion Detector

Looking for a motion detector schematic. Does anyone have a link I can follow ? If not which magazine I should look for such a thing.

Thanks ahead
 
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