PIR sensor distance connection?

Oldbitcollector (Jeff)

I'm dealing with some "teenage" issues and need to know
when my front door (or porch) is in use. I'm thinking of
using a PIR sensor to a remote propeller board.

Question: What is the max distance I can put between
the connection of the PIR to the propeller? Can I put
20-30 wire connection (twisted pair) without losing
accurate signal?

May just have to unroll some cable and try this today.

OBC

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Paul Baker

I don't have any hard and fast figures, but since it is a level shift signal which has a low bandwidth, I would think the distance could be quite large. There are obvious factors which must be taken into account. First there is the associated voltage drop due to conductor losses, because the PIR sensor is outputing at 5V, using the resistor trick to limit the ESD diode current flow will caus a non-negligible current flowing through the wire leading to a voltage drop. To combat this, it may be best to perform a level shift to 3.3V at the PIR. Having the signal at 3.3V will cause minimal current to flow in the line during steady state conditions. Theres also the common ground issue to be aware of and how conductor losses can lead to a ground loop. Also there is the issue of interference on the line, this could be overcome by using a differential transmission protocol such as RS-485.

I would go ahead and wire it up as if there were no problems to be expected, but with a mindful eye on what could potentially go wrong and devising an effective way to diagnose any issues, and a solution for it in your back pocket if needed. A simple multimeter should be all you need, measure the voltage on the line, the current on the line and the current on the ground. EMI could be a little more difficult to devise a good test for. If you see issues in your measurments, then it's time to incorporate some solution to combat this.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Mike Huselton

Might an optical isolator help avoid ground loop, noise and current loss issues? Especially if the optoisolator is mounted next to the PIR.

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JMH - Electronics: Engineer - Programming: Professional

Post Edited (Quantum) : 9/25/2008 5:12:51 AM GMT