using PIR sensor without lens?

[ditch08]

Good day to all,

I need some expert advice on this. Right now, i am making a motion detection circuit with the use of PIR sensor lhi 968. Well, the circuit is working.. as a heat detector, and not that efficient also. To activate, i need to put the tip of a soldering iron near the sensor, about 1cm. I followed the circuit design in this link

My problem is can i use it without a lens (fresnel lens) ?
please enlighten me about this, it's my first time with PIR sensor

 

[tpetar]

Use lens and check circuit.

Posted link do not work.


See this link:

https://www.ladyada.net/learn/sensors/pir.html



Best regards,
Peter

 

[ditch08]

sorry about the link, here it is: https://www.google.com.ph/imgres?imgurl=http://www.brightsparks.org.nz/userfiles/forumcontent/8f46fb30-4539-444a-9b9c-9fb983c5956a/image/PIR%2520D203B.jpg&imgrefurl=http://www.brightsparks.org.nz/the-forum/beginners-forum/brain-box/&h=399&w=717&sz=28&tbnid=s3ll49O9wVvFfM:&tbnh=76&tbnw=137&zoom=1&usg=__TcjhyQNnXPF3XwEX79l83qIxG58=&docid=OyUXH9cStnNMWM&sa=X&ei=23YMUsrAKMj-rAffyIHAAg&ved=0CEYQ9QEwBQ&dur=85

**broken link removed**

well, that's my problem. I don't have a fresnel lens. Maybe i will try to buy one. I guess i can't confirm if this circuit is working unless i have lens. Thanks for the reply

 

[jiripolivka]

Instead of a lens you an use a grid. Use a dielectric like Teflon to make a grid.
Lens or grid are used to modulate the input temperature due to an object moving across the field of view. Only these changes are detected and amplified in the sensor circuit.
A PIR sensor can detect the presence of an object but its response is unstable due to 1/f noise. By the lens, object to sensor IR "light" is modulated by a low frequency and this is detected.

 

[ditch08]

thanks for the reply

Grid? how does that work, made from teflon? hmmm i'll try to search that in the net. thanks

 

[jiripolivka]

If you have a chance to see the Fresnel lens, you will see thinner and thicker features on the strip or ball. If an object moves across the field of view, its temperature is alternately passing to the PIR sensor or not (diagrams in the Specifications). This enables the sensor to detect moving warm objects. The same thing you can make with a grid, thin strips cut from Teflon with similar gaps of air. Using Teflon (or another dielectric) modulates the response like Fresnel lens does. If you use wire grid, metal strips may radiate the high ambient temperature and overload the sensor.