Thursday, 23 August 2012

T9 Hall Magnetic Switch review

Several pilots on the UK F3F circuit are now using magnet-operated switches. The benefit, of course, is that you don't need physical access to turn your radio on or off.

The device we're going to look at is called the "Hall Magnetic Switch" from T9 Hobbysport.



Hall effect Magnetic Switch

What's the difference?
The Hall Magnetic Switch performs the same function as a traditional electro-mechanical switch. However, instead of a sliding conductor, the magnetic switch employs a Hall-effect sensor which responds electrically to a magnetic field. A microprocessor monitors any movement, and opens or closes an electronic switch.

The actual device consists of a small double-sided PC board. One side contains the Hall sensor and an LED. On the reverse side are a couple of components, presumably the microprocessor and a Mosfet. The components are protected with heat-shrink tube.

The magnetic switch comes with leads for connecting to your receiver, battery and charger. The leads on my unit are terminated with Futaba J connectors.


Printed circuit board is double-sided

Hover and swipe
On connection to a battery, the switch initialises itself to the 'on' state, indicated by the LED lighting up. Current can now pass from the battery.

  • To switch off, you hover a magnet over the sensor for approximately three seconds, at which point the LED goes out. The unit goes into 'idle' mode, i.e. the device is live but cannot pass current. 
  • To switch on again, just swipe the magnet over the sensor a couple of times.
Since the default state is 'on', if you forget to bring a magnet you can still control the electrical connection simply by connecting and disconnecting the battery lead. (I have heard that of at least one other magnetic switch on the market does not default to 'on' reliably.)

Now you're probably wondering if the device can switch itself off spontaneously. As one might expect, the manufacturer claims this is impossible, as long as you don't install the unit right next to a powerful magnet. I've no grounds to doubt this claim, as the magnets supplied are stupidly powerful for their size, and it's hard to see the magnetic environment being replicated without some determined effort.

The switch can be driven by a variety of power sources including a 2S lipo. With a current rating of 20A continuous (50A peak) it should easily pass enough current for F3F purposes. Range is approximately 8mm on my unit, sufficient to operate the switch through a typical nosecone.

Idle current is 50µA, which equates to just 35mAh lost battery capacity per month. You could get away with keeping the switch connected between sessions, however I feel it's a good idea to disconnect at the end of each flying session, just as a matter of good practice.

Installation and use
Having satisfied myself that the unit worked properly on the bench, it was time to install it in a model. And what better than to put it in my number one F3F ship, the Sting.

First job was to clear the decks by removing the old switch, receiver, and battery. The new switch was then glued in place using silicon to hold it in place. Replace receiver and battery... and voilá... we're almost ready to go!

Old switch removed and new switch siliconed in place

Completed installation. Mag switch is space efficient - note rx aerial in space
previously occupied by old switch.

Just one more thing to do: as the magnets are supplied bare, you'll want to make some holders for them.  I drilled a hole in some clear acrylic sheet and fixed the magnet with cyano. Some people are using sawn off  biro caps as holders.

Finally, the location of the Hall sensor must be marked on the nosecone (the sensor is 21.5mm from the LED).


Home made magnet holder from acrylic sheet
Magnetic switch in use in the Sting. Cross represents sensor position.

Summary
There's nothing like some numbers to illustrate a point, so let's take the recent Welsh Open F3F competition which takes place over three days. Had I been using the old mechanical switch, I would have had to remove the nosecone around thirteen times. Instead, using the Hall Magnetic Switch, it was only necessary to remove it twice (to alter the ballast). That saved an awful lot of hassle and fiddling with tape!

Another unexpected and useful benefit is that the magnetic switch is easier to install and takes up less room than a conventional switch.

Any draw-backs? Well of course you have to remember to bring the magnet, but even if you forget it's not the end of the world - you can still operate your radio. Also the power indicator LED is bright enough indoors, but don't expect to be able to see it through a nosecone on a bright day (though this ceases to be a problem after a bit of practice). And finally, you'll want to make a holder for the magnet.

These are minor niggles though. The T9 Hall Magnetic Switch is so much more convenient when access to a normal switch is tricky. I'm very pleased with mine, and it's now a permanent fixture in the Sting.

For more details go to T9Hobbysport.

Specifications
  • Supply Voltage 3V -8.7V
  • Dropout voltage (Input vs output) 100mV @10.2A
  • Continuous Operating current - ~ 20A
  • Peak Output current - ~ 50A
  • Standby Current (Off State) - ~ 50μA
  • Operating temperature range -40°C 80°C
  • Dimensions 27 x 11.5 x 6mm (1.06” x 0.45” x 0.23”)
  • Total cable length Approx 280mm (11.0236”)
  • Weight Including all cables - ~ 9g (0.32oz)
Web links:

T9 Hobbysport - Hall Magnetic Switch
Wikipedia: the Hall effect

1 comment:

Anonymous said...

I've one of these... and it's nothing short of a pain in the backside! Sometimes it will switch off.... Switching on is not a problem - simple swipe.... but to turn off I've used numerous magnets and it can take anything from 4 to 50 secs to turn off. On the weekend past, it decided to switch on continuously, if I switched it off... it turned back on!!
The only way I could get it to stay off was to unplug the battery. Not very good bit off kit!!