[Max]

Originally Posted by Aurificus

PULSING IR LED DETECTORS - Theory & Practice

Background : IR remote sensing

Common uses currently include: Satellite weather imaging, crops and land clearing studies, monitoring river, dams & lake levels. etc, etc.

Movement sensors to operate security, lights, alarms, cameras,etc.

For less than $50 you can purchase a quite accurate, IR, remote sensing, digital pyrometer (thermometer), (pay the extra and get the one with the laser pointer).

These detection systems rely on receiving “black body” IR radiation emitted by objects.

ATTENTION: A pulsing LED detector DOES NOT use this method.

What we are attempting to detect is far more subtle and is the type of effect that is considered inconsequential noise in standard EE practice.
In fact, standard theories & practices are designed to dampen, cover-up, ignore or over-power these effects.

The LED emits energy as IR radiation.
When it contacts matter this energy can be transmitted, absorbed or reflected.
Standard theory assumes that the matter involved has a constant temperature and therefore the rate of transmission, absorption or reflection is constant.
In a real world, (the one I’m in, maybe not yours!!) matter is constantly changing
its temperature. The rate of change depends on the introduction or removal of an energy source (radiation, let’s call it ‘heat’) and the composition and physical properties of the object in question and the medium it is in.

i.e. What size is the object?, what is it made of?, what it is buried in?, how deep is it?, is heat from the sun warming it?, is it cooling down?, etc, etc, etc, etc,…….

The zone around a buried object will therfore have a thermal energy gradient, except for brief periods where equilibrium might exist. Depending on conditions the gradient could extend to the air space above a buried object ( phenomenal!)

So what?!!! The emission from an IR LED is tiny and will have a negligible effect on any of “that BS” at any sort of distance!!!!!

True…..but what if a remote energy level change has an effect on the LEDs?

An LED will have a “rise time” from the application of power to its full IR emission level. Energy is required to excite (don’t say heat) a diode each time it is triggered.
The amount of energy needed will depend on how much energy the LED has dissipated whilst switched off. If the pulse is constant the LED’s start-up power requirement will change according to how much energy it is transferring to the environment while it is emitting.

Monitoring these undoubtedly very small voltage and/or current fluctuations might best performed with a sensitive ‘amplitude’ detector or similar that is separated from the power circuit to minimise any effect on it.

In conclusion, the theory is this: The pulsed beam is a ‘detection probe’ or a ‘transmitting antenna’, the zone around the target absorbs or rejects ‘additional’ energy at a greater rate than ‘ambient’ and the response is measured as power fluctuations at the transmitter not in reflected signals from the target.

No magic, No mumbo Jumbo. Just science and physics and not letting “the big stuff over-power the small stuff”. May I have a Patent, please? Plenty granted for a lot less than that!! I’ll share it with Esteban, we’ll be rich and retire and go treasure hunting.

P.S. For practical use, the variables involving real targets are so numerous that results might be “Very Hit & Miss”. When it works, it works, when it doesn’t……..try again, under different conditions……try again…… (more system development and/or control of the variables is required)

P.P.S The concept, however, has a lot of value. This type of sensing of seemingly insignificant, but quite measurable “side effects” can be applied to many different real world problems.

P.P.P.S. My deep RESPECT to Esteban, who politely and patiently shares elements of his years of work on Remote Detection equipment. The good, the bad & the otherwise. Without his input this thread would be little more than sceptics teasing novices & stroking their own and each others……prejudices.

Cheers, Aurificus

Hi,
Congratulations! For the big pile of BS.

I think it doesn't work, and will never work in the real world, maybe yours is different.

Have any idea of what percentage of IR radiation arrives to a deep buried target ???

ZERO. The soil will dump anything your ridiculos 10mW from the led!

The LED you'll not read any measurable variation.

Follow this thought: suppose you have a 23mm diameter/4.15cm^2 area coin buried at half a meter underground... and you're 10meters far from target on the horizontal plane...

OK the half-meter means 50cm of soil above the coin...

The soil will swamp any reading... or you're talking here that a target like a coin cause if the thermal gradient is what you're looking for you must know that heath transfers are related to the MASS of objects ...so considering a deep buried small target (50cm and a coin) you're trying to let people here think that the coin retains or delivers heath to/from the soil in a way that's detectable by a simple IR LED + amplifier ! Isn't it ?

Now... the heath radiation happens from/to coin isotropically, so in all direction from/to coin vs soil , right ?

So... a coin placed at 50cm will radiate or receive heath equally from a volume of 1m^3 soil if we consider soil temperature about flat constant in the 1st meter (a good approximation in most cases during the day).

1m^3=10,000,000 cm^3

Now look at the coin that is e.g. a 23mm diameter one, 1mm thick: you'll get a volume of around 4.15 cm^3 means something 4*10^-7 order to be clear... so the coin's volume is about 4/10,000,000 the volume of soil we are talking about...

About mass... soil mass for 1m^3 vary due to soil composition/density but in organic soil (farm soil in most places) is about roughly 1200kg/m^3 so 1200Kg mass for 1m^3 and for coin, say it's silver... with density of 10490Kg/m^3 , so 10490*0.000000415= 0.00435Kg or 4.35grams

Now look at energy transfer (heath) in ideal conditions between these 2 mass... in the case the silver coin is hotter than soil around it... with a temperature difference of 20°C = 20°K....with wide approximations it's something like this:

The energy release from silver coin is due to Qc= volume_of_coin*volumetric_heat_capacity_for_silver *delta_T(°K)...

so Qc= 4.15*2.44*20= 202.52 joules

... then we suppose the soil that's 20°K lower in temperature will get that heath by conduction mostly (an approximation close to reality) and so the temperature in soil will increase , but what's that increase in °K considering all energy will be transferred with no losses and no dispersions ?

here the energy Qs=volume_of_soil*volumetric_heat_capacity_soil(si lica,water)*delta_T_soil(°K)

Qs=Qc (perfect transfer)

delta_T_soil(°K) = Qs/(volume_of_soil_volumetric_heat_capacity_soil(sili ca,water)) = 202.52/(10,000,000*2.9)= 6.98*10^-6 °K

In other words... if your soil is humid, organic and hi content silica... say it's at 30°C and the 23mm silver coin buried at 50cm reach 50°C, so with gradient of +20°C at coin, and we approximate full heat transfer between coin and soil, you'll get just about 7u°K or 7*10^6 °C variation in the considered 1m^3 soil.

GUYS !? We consider having 20°C gradient at coin soil interface and end up with 7 MICRO CELSIUS variation at soil volume... so at surface.

Can you detect 7*10^-6 °C variations with a LED diode ???

From 10meters away ???

Consider also S/N... 7uCelsius variations will be swamped out by any e.g. wind movement... that will cause the soil to cool down at really faster rate than 7uCelsius!

Consider the evaporation of trapped water in soil... will that cool the surface or not ???

It's a themodynamic-hell for you PYROMETER OR LED... OR WHATEVER!

Is that the new LRL-science-fiction!

I'm sure you didn't realize any of these fairy-tales devices... otherwise YOU MUST KNOW that's impossible detecting something this way.

On the air... of course, things are quite different... but soil it's a well known dumping filler... that's cause e.g. an atomic bomb shelter is usually underground... (not only... there are other reasons too ) the heath is dispersed by soil/water volume... will not work at ground zero just under the bomb maybe... but at few distance the soil will shield from excess heath... not like painting white your room and wear your preferite sunglasses!

Kind regards,
Max