QUESTION: "How do the little magnetic on/off switches work and are they reliable?"

Well, maybe it would be a good thing to review the engineering and design behind this technology with our customers here.

KEVIN:
"First, I think a bit of history about magnetic switches is in order. In early days of falcon telemetry, a handful of research telemetry companies produced transmitters with an early version of a magnetic switch. This type of switch (a Glass Reed Switch); constructed just as the name implies, with a hollow glass tube with a magnetized wire (reed) at its center, would make or break connection in the respective absence or presence of a magnetic field. This type of switch, though very simple in its operation, had several disadvantages for use in a falconry transmitter. First and foremost was reliability. Being constructed of glass, the switch was fragile and susceptible to breakage due to physical shock (i.e. high G’s in a stoop, or dropping the transmitter onto a hard surface), they were also [relatively] large and bulky. From a user standpoint, they were also lacking in the convenience department as a strong magnet had to be physically attached to the body of the transmitter to keep the unit from transmitting. It's one advantage was that you did not have to "un-tape" and physically remove the battery on the early push-in battery designs of the day every time you wanted to turn off the transmitter, but you still needed to physically remove the transmitter from the bird and attach a magnet to it to turn it off. So we felt it was just a partial answer.

Luckily, there was a better way. Technology in the automotive, computer, and mobile electronics has been progressing at an astonishing pace over the last 3 decades. One of the amazing technologies to come from this march of progress was the tiny Hall Effect switch. Actually discovered more than 100 years ago by physicist Edwin Hall, the Hall effect presents a way to detect the presence (and strength, and orientation) of a magnetic field by observing the changes of a flow of electrons exposed to a magnetic field. The genius of this technology is that a solid state (no moving parts) detector can be made to sense the presence of a magnetic field. The use of the Hall Effect has become so wide spread in its use, that chances are you are helped by this technology on a daily basis. Almost all automotive ABS and traction control systems owe their very existence to the small, reliable, and rugged hall effect sensor.

Another interesting automotive use of this same technology (for those of you old enough to remember) is the elimination of points in the distributor, replaced with these same Hall sensors. Technology moving away from problematic moving parts, to solid state devices.

In 1997, Marshall Radio first began work on integrating this technology into a falconry transmitter. But to do this, one more technology (also never seen before in the falcon telemetry world) had to be added: the Micro Processor.
Instead of a simple RC or LC timing circuit used since the 1970’s to control the pulse rate of a transmitter, the original Marshall PowerMax of 1998 was the first falcon transmitter to utilize micro-processor control of its higher functions (think of this as the transmitter’s brain).

With this combination, the processor is able to measure the output of the Hall effect sensor (which incidentally produces a voltage when it senses a strong magnetic field nearby (i.e. the presence of the magnetic wand), and turn the transmitter on/off. An additional benefit of using a microprocessor, in a transmitter that allows you to do, is to monitor the state of the battery. The PowerMax was also the first falconry transmitter that had a battery fuel gauge (warning you with a double beep that the battery needed to be changed).

This simple idea, to be able to tap a magnet on the side of a transmitter to turn it on and off, was also the beginning of a totally new way to use telemetry in falconry, since it facilitated the acceptance of new mounting methods as well. No more did the transmitter need to be removed after each flight, instead, a tail mount or a TrackPack mounted transmitter could be left on the bird (reducing stress on the raptor, and frustration on the part of the falconer), and simply switched on before each flight.

As with any new technology, there were limits as to was possible 15 years ago. Microprocessors had not advanced sufficiently to run below 2 volts, so one thing early users of the PowerMax learned was, that with a low battery (one that was ready to be replaced), the magnetic switch would not always function (this being a side effect of the battery voltage droping when the magnet caused the hall sensor to begin conducting current from the battery). In 2002, we upgraded to the latest generation on low voltage processors newly available, and were able to maintain on/off switch functionality well below 2 volts.

Fast forward to 2004, when the programmable RT Plus (and later in 2005 the new versions of the Micro and PowerMax) were introduced. This took telemetry for falconry to another level with what was now possible. Once more utilizing cutting edge technology to push the limits, we introduced a fully programmable transmitter, one that no longer used a crystal (long the weak link in any falcon transmitter due to its susceptibility to physical shock) which increased reliability many times over. Along with this advance, we also upgraded our Hall sensor, further boosting ease of use and reliability of the design. With this generation of our transmitters, we added new logic to the processor. It now takes just 50ms of exposure to a strong magnetic field (i.e. mag wand is held next to the transmitter) to turn it on. But it takes almost a full second of the magnet to turn the transmitter off.

This was done for three reasons:

1) to make it quick and easy to activate the transmitter (important especially if doing it one handed with the falcon on the fist, and a small flat transmitter might actually be hidden by tail coverts)

2) to make it very difficult to accidentally turn it off, and

3) to make it impossible for any AC source (i.e. high voltage power lines that happen to generate a very powerful magnetic field) to ever be able to turn off the transmitter.

Interestingly, with US and EU power grids, the polarity of the magnetic field generated is switching at 50-60 times a second, so it can never turn off a transmitter (we actually tested this on a 400KV power line, which is very exciting BTW).

In addition, we have all of our transmitters default to on, if ever there is a temporary loss of battery power, or if there is a fault with the mag switch. So if ever there was a problem in the field, the transmitter would continue to run (and not switch off suddenly).

Speaking of reliability, with the current mag switch that is in all of our current transmitters, tens of thousands now in the field, we have seen exactly 3 of them ever fail. A failure of this part results in a transmitter that does not respond to a magnet in any way (i.e. it runs with the battery, and cannot be switched off).

Even with all of the engineering that has gone into this simple idea of adding a non-mechanical on/off switch to a transmitter, there are still a number of things that can surprise an end user and make them think that something has gone awry with the mag switch. Most instances that we hear about are caused by a few simple activities that have been handled in a less than thorough manner.

The most common is the falconer believing that they have turned off the transmitter, but they have not in fact done so. This can be something as simple as checking the wrong channel on the receiver after turning the transmitter off (the solution here is to always listen to the transmitter while it is being switched off). Second most common is inadvertently storing the magnetic wand in a place near the transmitter (like in a holster, case, or falconry bag), again the transmitter is designed to be easily turned on, and a random close encounter with the magnet may do just that. Another less common event is if a transmitter is placed within 1cm or so of the speaker on the top or bottom of the Field Marshall receiver (this can happen if transmitters are thrown in the same case compartment with the receiver), the Field Marshall uses a low profile speaker with a strong magnet inside. Solution here is to always keep ‘em separated.

We also hear of a few instances in the field where the transmitter (usually a 2004/2005 model year RT Plus) seems to switch on/off while flying on the falcon. This particular instances is really a maintenance issue. The original RT and RT Plus was capable of running either on the large 1/3N Lithium battery, as well as 2 smaller 389 silver oxide stacked in a low profile lid. This design utilized a current path that passed through the lid. If the lid was not kept squeaky clean (i.e. periodically cleaned with a cotton swab and rubbing alcohol once or twice a season), the battery would not make a consistent connection to the transmitter, and as it was jostled about (either on the falcon, or in a hawking kit), the battery might suddenly go from a poor to good connection, and the transmitter safely start to beep. A simple cleaning of the lid would always solve this problem. This is no longer necessary on 2006 forward RT Pluses as the battery now touches both the positive and negative contacts directly (the tradeoff was that it would no longer work with the short lid or stacked batteries).

On the Micro, we have seen instances of some battery manufacturers making their cells smaller that the minimum specification. In cases where a slightly undersized battery is installed, a slight adjustment of the battery tab may be necessary to assure positive contact. This can carefully be done with an X-acto blade or pocket knife (bending the tab inward another 5-10 degrees).

What it all boils down to is this: The advent of the magnetic switch was a wonderful advancement in the way we use our transmitters. Its reliability has been outstanding. And with proper technique: keeping the magnet from inadvertently hanging out with the transmitter, always double checking that you have switched the transmitter on/off (either with a tuned in receiver, or a Signal Sensor), and making sure batteries are properly installed, clean, and tight; the magnetic switch will work perfectly on every Marshall Transmitter.

The next time you switch on your transmitter, please take a moment to enjoy all of the engineering that went into making a on/off switch that works so simply yet reliably."

- Kevin Harcourt