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PIR Motion Sensor Tutorial

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Step 6: Changing pulse time and timeout length

Picture of Changing pulse time and timeout length
pirtitx.gif
There are two 'timeouts' associated with the PIR sensor. One is the "Tx" timeout: how long the LED is lit after it detects movement. The second is the "Ti" timeout which is how long the LED is guaranteed to be off when there is no movement. These are not easily changed but if you're handy with a soldering iton it is within reason.

First, lets take a look at the BISS datasheet again (see image below)

Determining R10 and R9 isnt too tough. Unfortunately this PIR sensor below is mislabeled (it looks like they swapped R9 R17). You can trace the pins by looking at the BISS001 datasheet and figuring out what pins they are - R10 connects to pin 3 and R9 connects to pin 7. the capacitors are a little tougher to determine, but you can 'reverse engineer' them from timing the sensor and solving!

For the sensor in the Adafruit shop:

Tx is = 24576 * R10 * C6 = ~1.2 seconds
R10 = 4.7K and C6 = 10nF

Likewise,

Ti = 24 * R9 * C7 = ~1.2 seconds
R9 = 470K and C7 = 0.1uF

You can change the timing by swapping different resistors or capacitors. For a nice tutorial on this, see Keith's PIR hacking page
 
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