On 2/24/2010 2:38 PM Jim Yanik spake thus:
William's right. The whole unit is a motion detector; the PIR is not. All it does is sense infrared, as its name implies. It's the circuitry downstream of it that analyzes the deltas and comes up with "Aha! thermal source moving across my field of view!".
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That wasn't what we were last discussing, but rather the issue of active versus passive sensing.
"William Sommerwerck" wrote in news:hm4j4o$hgf$ snipped-for-privacy@news.eternal-september.org:
not relevant to the thread subject.
-- Jim Yanik jyanik at localnet dot com
It might not be (though I feel it is), but it came up.
"William Sommerwerck" wrote in news:hm5ue9$ls8$ snipped-for-privacy@news.eternal-september.org:
note how the other guy moved the goalposts by asking about "active - thermal- IR sensors",deliberatly excluding other IR wavelengths that ARE used in active IR sensing.
-- Jim Yanik jyanik at localnet dot com
The other guy? You mean Phil Hobbs. He is an optical engineer.
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Some of the hard wired break beam sensors fed a square wave from the transmitter to the detector to synchronize them. This prevented ambient light from tripping them, and also allowed multiple units to be used where reflections wouldn't cause problems due to differences in the time period. They were great for lighting controls and for security systems.
They also came with multiple sets of lenses to allow you to select the beam width, shape and distance. You could shoot one down a long hallway to turn on the lights whenever anyone stepped into the hallway. The rest of the time a few night lights gave just enough light to see if the other lights failed.
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Circuit-wise, it works like this: a light sensor produces a voltage for a given pattern of light. That voltage is read by a diferentiator op-amp. The differentiator measures the rate of change of the input voltage. The output of the differentiator drives a comparator, which drives a relay to turn on the light.
So, if the light pattern changes, the voltage changes. The op-amp, which is acting as the brain here, is designed to detect changes in that sensed voltage with respect to time. In other words, if the op- amp sees 4V at one time and then 6V one second later, that may be what is called a "significant change", which is sufficient to activate the light. However, if the voltage changes from 4V to 6V over the course of 8 hours, then that rate of change is much slower, and is not likely to cause the system to switch on.
o
If you are interested in the circuitry of these devices, you can buy the sensors for less than $2.00. The remaining circuitry might cost you $5.00.
I learned how they work over 25 years ago, but thanks for the link.
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