Two ultrasonic articles

[J_Player]

Originally Posted by Fred

i think we are loosing contact with reality...your example above were not aplicable here,and i would not call this last one as a "distant" effect.
In relation to IR wavelenght , a few meters is gigantic.
Here is one other example,how a tranmiter can be afected by external conditions: monitor the power output of a transmiter, then change the medium where the antena is or make it close to a metalic object.The absorbed power (or SWR) will change. But that doesn´t work at long distance in relation to wavelenght.

My example above comparing a modified PI detector with Esteban's IR detector is very applicable. I am talking about the exact distance that Esteban described for IR LED experiments. According to Esteban, the distance is not very large, used for pinpointing only. This means reality for Esteban is the distance of PI metal detector range or slightly better. If we were to change to larger ranges, then this conversation would no longer be applicable, because Esteban did not find reality in detection with IR at long ranges of 10 meters for example, and was not limited to detection ranges of maybe 5 cm, for example. Esteban never said anything about measuring the SWR or changing the transmitting medium. What he said is the amplitude of a 400 Hz square wave transmit power signal is changed when the IR LED is pointed at a long time buried metal object. He said the IR LED does not have very long range. This is very much reality, exactly as Esteban described.

The comparison of a modified PI coil at the same reality distances as the IR LED that Esteban described will also show that the power traveling to the search coil will change when the coil is moved in the detectable range of the coil. (This PI search coil is at the same distance, or reasonably similar distance as the IR LED Esteban tested if you have a sensitive PI search coil).

Originally Posted by Fred

Again,if you don´t have a receiver of some sort i can´t see any way to measure a diference in transmited power when pointing a beam of light to something distant.

As I previously suggested, and as Esteban confirmed, the IR transmitter is the receiver, just as many PI detector search coils are both the transmitter and receiver. It is not necessary to measure SWR, only to attach oscilloscope probes to the power leads of the PI coil and to the IR LED to see the transmitted waveform is changing for both of these detectors. You could also read my previous post to Max above, and build the simple circuitry I described to monitor the amplitude of the transmitted power. See here: http://www.geotech1.com/forums/showp...6&postcount=39
Esteban posted the information of how to do it, and I re-posted it in an easy to understand form. In fact, the dynamics inside the IR LED circuitry that Esteban described sounds strikingly similar to the dynamics inside a typical PI detector. All that is changed is the IR does not have a timing scheme to lock in on an expected counter emf signal from the target. He is only watching for a change in the overall power or average amplitude at the IR LED power leads. Apparently the property of a long time buried object that causes the transmit wave form to vary has not been identified, nor has circuitry been built to optimize the capture of the effect.

If I wanted to experiment with IR detection, I may choose a PI experimental project board such as Carl's hammerhead PI that allows a lot of adjustments to be made. This would give me the same square wave that is needed, but with adjustable widths, and lots of room for adjustable timing to look at any part of the pulse cycle from the rise to the fall and beyond. Perhaps this would be a good way to study how the IR reacts when different long time buried targets are in range.

The new example Esteban is now giving will confuse his earlier posts, because he is now talking about a different method of using a receiver rather than simply monitoring the transmitted power as he previously stated.

In his new example, Esteban is stating that the change in the transmitted power going into the single IR LED can also be detected by using a separate independent receiver to listen to the transmitted 400 Hz carrier sent by the transmitter. This is his way of monitoring the IR Tx power without connecting a probe to the wires inside or using the comparator circuit I earlier described. A small hand-held radio receiver can be converted for use in the field instead of carrying an oscilloscope or building a comparator and beeping section inside the IR LED detector.

Best wishes,
J_P