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[on:]

I've recently got SparkFun PIR sensor (https://www.sparkfun.com/products/8630) and hooked it up based on bildr instructions (http://bildr.org/2011/06/pir_arduino/).

The sensor wasn't reacting at all at first. Then I swapped power and alarm pins and is started detecting motion. However, now the sensor is perpetually in LOW state. I connected a pull up resistor but nothing changed (maybe I'm doing it wrong).

What do you think could be causing this?

[Reply #1 on:]

Can you post a photo, so we can see how it is connected.
Can you post the sketch, or is it still the same as on bildr ?
Do you have a multimeter, to measure the signal from the PIR sensor ?

Is it a 12V PIR sensor or 5V ? The Sparkfun PIR sensor is 12V.
If you have connected the 12V wrong, the pir sensor or a pin on the Arduino could be damaged.

[Reply #2 on:]

I've attached the photo. I'm using the sketch here: http://itp.nyu.edu/physcomp/sensors/Reports/PIRMotionSensor

The analogRead on the alarm pin is showing 1023 without any change. The PIR sensor is connected to 5V. I tried replacing the short brown connection to the VIN pin but that didn't help.

Am I missing something really obvious? 

[Reply #3 on:]

Without a reaction on the alarm pin, how do you know the sensor is detecting motion? Is there an LED showing the motion detection on the sensor PCB?

[Reply #4 on:]

Yes, the LED should light up when the motion is detected. I'm also tracking the readings on the alarm pin with analogRead.

[Reply #5 on:]

double check your connections from your arduino to the breadboard. looks like you might have 5v connected to the GND rail on the breadboard. And I don't see where you have anything connected to ground on the arduino. hard to tell from that angle though

[Reply #6 on:]

Yes, the LED should light up when the motion is detected. I'm also tracking the readings on the alarm pin with analogRead.

You didn't understand my question. In your first post you stated:

Then I swapped power and alarm pins and is started detecting motion.

So how have you examined that it started detecting motion? The LED we see on the photo cannot detect motion if the Arduino doesn't get an according signal. You told us that the Arduino doesn't get a signal when there is motion. So I ask again: How do you know that the sensor is really detecting motion when you don't have a signal on the Arduino?