Is a Fresnel Lens necessary?

Jaggermax

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I purchased some raw RE200B PIR sensors and have designed my own detection circuit around them.  The problem is that I did not purchase any fresnel lenses and they seem very hard to come by.  I really dont want to blow $9 on a motion light from Wallyworld just to steal the lens.  

I went to Dollarworld today and found some cheap 1 1/2" plastic magnifying glasses.  The focal length is about 1 1/2".  Do any of you know if I can use this as an alternative?  I know that the fresnel lens is simply a low profile magnifier so I figured the convex (or is it concave?) lens would work since I had enough space in my box.  Any thoughts guys?  
 

Tinhorn

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uh-oh, I don't think a single magnification lens will work.

The PIR's fresnel lens is actually a series of lenses (as I'm sure you noticed)  

When you walk by, one lens focus's your IR heat on the intenal sensor, then, as you leave that len's focal point, there is a pause in the IR heat detection until a 2nd lens focus's your IR heat on the sensor.

so

the Sensor electronics sees a sort of morse code and knows you are moving by the series of Dots (or dashes)   It counts these pulses of IR heat,  get it?

Some Home Burglar Alarm PIRs allow selecting between 1,2, or 3 pulses to prevent false alarms.  This is the pulses it's talking about.

it was 2AM when you posted, quit thinking about the cams and get some sleep!!!  ha ha

Tinhorn


(Edited by Tinhorn at 12:24 pm on Oct. 20, 2001)
 

Archilochus

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Hi Jaggermax,
I believe Tinhorn is right on the money there.  Give it a try though, who knows?
>>>>
If you want to buy some really nice Fresnel lenses (or get some samples) try:
http://www.fresneltech.com

They now have most of their lens descriptions available on-line.  They also have some basic descriptions of Fresnel lens theory in PIR sensors.
Minimum order was $30 US$ - The lenses run about 35 cents each - maybe a bit more for complex ones.

Archilochus
 

Jaggermax

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Thanks for the info guys.  You guessed it Tinhorn, I was up late trying to finish another cam so I could put it on the woods on this morning.  I ran out of fresnel lenses and did not feel like making a run to Walmart.  It's great to hear fresneltech now sells direct Arch.  .  I felt guilty about asking for free samples.  I tried the magnifier lense and it worked out to about 10 feet, but that was it.  I stopped on the way back from the woods this afternoon and picked up another Mainstay.  Thanks for the info guys.  
 

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