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[Solved] HC-SR501 PIR onboard LDR

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(@pugwash)
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Joined: 5 years ago
Posts: 923
Topic starter  

I intend to solder an LDR onto HC-SR501 breakout board.

Has anyone done this and if yes, what size LDR did you find optimal. I have got a selection of LDRs lying around from 0.5M to 10M. Internet was not very helpful on this matter, as I couldn't find a manufacturer's datasheet.

A ballpark figure would be good, as I don't want to solder and de-solder a pot to the board just to find the correct threshold.


   
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Mandy
(@amanda)
Member
Joined: 5 years ago
Posts: 74
 

The LDR pins is connected to ground and is in series with an resistor of 1MΩ connected to the 3.3V supply.  The LDR and the resistor form a voltage divider :

  • if the LDR has a resistance of higher than 250K, the controller will turn on
  • if the LDR has a resistance lower than 250K, the controller will turn off

So the LDR should read 250K at the switching light level so the best way is to measure it on a multimeter.

The 600 series had rubber skin. We spotted them easy, but these are new. They look human... sweat, bad breath, everything. Very hard to spot.


   
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(@pugwash)
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Joined: 5 years ago
Posts: 923
Topic starter  

Thanks for your reply.

I have now found and attached a useful datasheet for the CMOS chip on the PIR breakout board.


   
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(@pugwash)
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Joined: 5 years ago
Posts: 923
Topic starter  

It took a while, but I finally figured out where the 250000Ω fits into the equation!


   
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