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Old Jun 09, 2012, 08:58 PM
working to the closest cm
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Magnetic rx switch

Can anybody point me to a DIY magnetic Rx switch similar to something like this?

http://zepsus.com/magnetic-switch/

Maybe hall effect sensor, a pic chip and a MOSFET switch?

Cheers Jeff
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Old Jun 10, 2012, 10:47 AM
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Here is one used for rocket electronics:

http://shop.featherweightaltimeters....3&categoryId=2
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Old Jun 10, 2012, 02:01 PM
Stuart
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Interesting, but a small NiMh lasting several years ?

Several months maybe.
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Old Jun 10, 2012, 11:43 PM
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As a newby to DIY electronics, I don't suppose there's a sympathetic, experienced DIYer that would could put together a shopping list to make one of those magnetic switches like the Zepsus or Featherweight units ..... please?
Thanks
David
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Old Jun 11, 2012, 01:44 PM
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Looking at the Zepsus switch, if one was starting from scratch I doubt if one could be made for much less than the asking price.
However if it was to be an introduction to the hobby of microcontrollers it may be a good way to start.

If you have access to a PC with a serial port I believe that a surface mount 8 pin Picaxe would be the cheapest micro to program, the programmer is a serial plug and 3 wires. The Programming Editor is free and help is available.

The printed circuit board could be hand cut but that Will take time, practice, a measure of skill. I use a Swan Morton brass knife with the tip stoned square.
Otherwise etchant and resist pen or the toner transfer method. I use software and UV box.

In addition to the parts above, at least one capacitor and three resistors Plus a quality servo extension lead. In the UK the postage to 2 suppliers could cost more than the item.

Busy finishing the world's cheapest hot wire Zaggie foam wing cutter. CNC without PC, uses one 8 pin and one 18 pin Picaxe.

But if you want to go ahead???

Dave
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Old Jun 11, 2012, 03:00 PM
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The big problem would be getting the current drain extremely low. You have to use the sleep mode and/or very low clock frequencies to get a PIC or most other processors into the low microAmp range. Don't know if the PicAxe will do that.

Shouldn't need a microcontroller. You could just use a CD4000 series FF or counter and run the hall output into the clock so it toggled the FF or counter when you passed the magnet over the hall sensor.

Here is a hall sensor with built in amp that is spec'd for up to 6v and only draws 3uA.
http://www.mouser.com/ProductDetail/...Fi5wsPThlQ8%3d

Should be able to just add a CD4xxx FF, a mosfet, a pull up resistor and some filter caps.
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Old Jun 11, 2012, 03:27 PM
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Quote:
Originally Posted by jeffs555 View Post
The big problem would be getting the current drain extremely low. You have to use the sleep mode and/or very low clock frequencies to get a PIC or most other processors into the low microAmp range. Don't know if the PicAxe will do that.

Shouldn't need a microcontroller. You could just use a CD4000 series FF or counter and run the hall output into the clock so it toggled the FF or counter when you passed the magnet over the hall sensor.

Here is a hall sensor with built in amp that is spec'd for up to 6v and only draws 3uA.
http://www.mouser.com/ProductDetail/...Fi5wsPThlQ8%3d

Should be able to just add a CD4xxx FF, a mosfet, a pull up resistor and some filter caps.
^ + 1
No need for any software ... a hardware based design is more appropriate here.
I would skip the clock and go with a simple comparator/latch based design ... can probably be done in the digital domain with flip flop and a schmidt trigger logic gate.
Quick and easy.
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Old Jun 11, 2012, 04:38 PM
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Quote:
I would skip the clock and go with a simple comparator/latch based design ...
Most hall effect devices are only on in the presence of a magnetic field so without driving a clock input, how are you going get an output that changes just by passing a magnet by it? You could use a T FF, or a D FF with the -Q connected to the D, or else just the first Q output of a counter, but either way would need to feed the hall output into the clock so the output would toggle every time the magnet is passed by.

Could do it with just an SR latch, but would need two unipolar hall sensors and then the North pole could turn it on and the south pole could turn it off. There are actually some hall devices with all this integrated into one package, but the only ones I could find draw 5 or more milliamps of current.

PS Not up enough on packaged hall switches to know if there would be any bounce as the magnet is passed by the sensor. If there is, may need to put an RC filter on the output of the hall device to prevent one pass of the magnet from creating multiple toggles.
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Old Jun 11, 2012, 07:30 PM
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Reed switch + SCR.

Maybe add a FET for low on R.

LED indicator.
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Old Jun 11, 2012, 09:15 PM
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Reed switch would work, but still need a flip flop of some sort to get on/off action without having to leave the magnet attached. SCR with a DC supply could trigger, but would never turn off.
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Old Jun 12, 2012, 06:53 AM
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Oops, forgot about shutoff. How about unplug the battery?

Why do people want mag switches on planes? On rockets it helps to avoid
switches sticking in the airstream. For planes a tiny slide switch works fine.
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Old Jun 13, 2012, 10:25 AM
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They seem to have other applications as well

http://www.espritmodel.com/jeti-elec...ey-sps-20.aspx
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Old Jun 13, 2012, 01:26 PM
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Nice, but costly. I think the Featherweight unit can do the same thing, smaller.
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Old Jun 13, 2012, 04:10 PM
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One thing that Jeti unit advertises is that it remembers the state it was in, even if the battery is disconnected. I don't see that as an advantage, and could be a safety problem. With a mechanical switch, you have a visual indication of whether the switch is on or off before you connect the battery. With an electronic switch, the visual indication is an led, and that doesn't work until you plug in the battery. If the electronic switch remembers that it was on, and you don't remember, you would have no way of knowing if the switch was off or on until you plug in the battery and the switch sends power to the output(and possibly the propeller). Think it would be much safer to have it always power up in the off position when you plug in the battery.
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Old Jun 13, 2012, 04:32 PM
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Very true, what were they thinking, why did they do it that way?

Probably the software guys were designing the hardware again...
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