Olin, thanks for the PIR explanation! A bit more words for my question. I'm working on an unusual problem, that is discovering airflow exiting from small rock cracks on a karst mountain. Caves entrances are often only small fissures opened in the rock surface and often hidden by soil debris but maybe evidenced due to the internal-external air circulation and temperature difference in some seasons (typically Summer and Winter). In Summer cave air is generally more cold than external and these "blowholes" can be detected due to the cold air flux. My actual objective is to survey for these in a somewhat large area mountain flank (rough, 0.5 square miles) and mapping them with a GPS. I have 2 surveying methods: 1) observing the area from distance (nearest is 600-800 feet) 2) recognizing directly on the terrain In case (1) I was considering a Thermal IR Camera but price is still excessive for me even for low-end models. IR Thermometers have a reasonable cost now but from that distance (and without a view-finder and a very spot measue area) the results seems unreliable. Case (2) approach is the reason for which I have asked for the PIR sensor: walking with the PIR looking in ahead, and wait it triggers (a led or a sound) when sensing a temperature difference. I hope to have explained the context... regards, Marco ----- Original Message ----- From: "Olin Lathrop" To: "Microcontroller discussion list - Public." Sent: Wednesday, April 01, 2009 2:35 PM Subject: Re: Re:[EE] Temperature detection with a PIR sensor > Jinx wrote: >> Think it might be. I tested some PIR circuits a few years ago and >> found that cold outside air blowing through the open door into a >> warm room was quite apparent on the 'scope and would have >> triggered the following logic/micro. > > Are you sure you were really measuring the air and not hard objects that > it > cooled down? > > > ******************************************************************** > Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products > (978) 742-9014. Gold level PIC consultants since 2000. > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist