Need a magnetic key switch

Thread Starter

MrSoftware

Joined Oct 29, 2013
1,937
I'm happy to report, it works perfectly! I wired one up with parts from my bin, using a reed switch, and it works exactly as expected. The biggest load I had here was a fan that draws 2.5A @14v and the MOSFET stayed nice and cool, I'll have to get it over to the test car across town to try it on the higher load. I also ordered a handful of hall effect sensors and I'll try swapping out the reed switch with the hall sensor to see how that goes too. Thanks again everyone!
 

Thread Starter

MrSoftware

Joined Oct 29, 2013
1,937
This might possibly be the spec sheet. I wonder if this is old stock that someone is getting rid of, or if it's made by a different brand that is still producing them:

https://www.google.com/url?sa=t&rct...0J6sU45eQ&sig2=yzI3pVNzyGQFAySum_Hsyg&cad=rja


How about something like a treadmill "dead man"s switch. e.g. http://www.searspartsdirect.com/par...id=SPDxGPROD&gclid=CO6Swb6S88kCFYQXHwodTIINWg + the other end.
Yes a magnetic treadmill type switch is basically what I'm looking for. The magnet part is relatively easy to find, but I thought for sure there would be an off-the-shelf solution for the other half, but I haven't found one yet? This is the type of thing I want to use as the key:

 

cmartinez

Joined Jan 17, 2007
7,174
This might possibly be the spec sheet. I wonder if this is old stock that someone is getting rid of, or if it's made by a different brand that is still producing them:

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0ahUKEwjeiKPqtvPJAhUKy2MKHcG6BRcQFggdMAA&url=http://www.allegromicro.com/~/media/Files/Datasheets/A3141-2-3-4-Datasheet.ashx?la=en&usg=AFQjCNEx847Z2Pp07jOLRSZAy0J6sU45eQ&sig2=yzI3pVNzyGQFAySum_Hsyg&cad=rja




Yes a magnetic treadmill type switch is basically what I'm looking for. The magnet part is relatively easy to find, but I thought for sure there would be an off-the-shelf solution for the other half, but I haven't found one yet? This is the type of thing I want to use as the key:

It is old stock... I remember now that about 15 years ago bought a bunch of those sensors and I still have them with me.
As for the magnet, just google "treadmill magnet replacement" and many readily available options will show up.
 
Look at these http://www.ptreeusa.com/shop_accessories.htm mounting cups. I have no idea where I got some for smaller magnets.

In reality, you could have someone laser cut with 4 holes some steel and maybe have it powder coated. Put some Scotch 77 or better adhesive on the bottom.

Maybe have some plastic rings CNC'ed or 3D printed. Attach your whatever shaped metal plate using the contact adhesive to something "plastic" . Then from the underside use screws for plastic (hi-low screw)(I have a source) to attach your ring. Thinking something like 1/4" square. It's something easy to make with a lathe, but have it made. You pre-drill for the Hi-Low screw. Depending on the plastic, you can also use contact adhesive.
Delrin is a nice material.

Possibly a better choice is bondable Teflon instead of powdercoating. It's etched on the bottom and can be epoxied. It would give you a nice durable surface that won't scratch. Have that laser cut. You can powdercoat with a Teflon coating.

A hall effect sensor mounted on a PCB works fine. You could use the same bolt pattern for the pcb mounting holes and standoffs. The PCB would contain a connector.

I've used all of this stuff for one project or another.
Bondable Teflon - I use it for large glass bottles in the fridge.
Hi-Lo screws - Fix a mirror and house repairs in plastic
Powdercoating - lawn chairs (parts must be able to withstand 400 F)
You can powdercoat in Teflon too.
I've made Teflon washers in a lathe
I've used the hall effect sensors in a filter wheel design for a work project.
 

hp1729

Joined Nov 23, 2015
2,304
Can anyone point me to a magnetic key switch? So far my googling has been fruitless.

This is for elderly disabled people with limited dexterity. The idea is they walk up to the device they're turning on, stick the magnet on and that turns it on. Basically a magnetic reed switch would be fine, but it needs to have a strong enough magnet to hold the key there until the person removes it. It needs to handle 12v DC, and maybe 0.5A of current (enough to operate a relay), though more robust is fine too. One key can work for all switches, it's not a security device. I can make one, but I'm really hoping to find something off the shelf as we're going to need a bunch of them. Has anyone seen something like this?
Are you set on using magnets? Magnets might erase the strip on credit cards. They collect junk. Carry a magnet in your pocket for a while and see what you think of it.
 
The iButton is used a fair bit: http://search.store.yahoo.net/853111/cgi-bin/nsearch?query=ibutton&Submit=Search&catalog=853111 It's basically a Dallas 1-wire based system. The button is a 128 bit number that's mostly pre-programmed. Large orders can customize somewhat. So, 2 wires can be used to read the button. Usually the use is momentary.

They are used to monitor guard travels. I saw them in a supermarket.

This http://www.nokey.com/diadacodefor.html is sort of interesting.

RFID is another technology. Again, mostly momentary. RFID can be powered or non-powered, The keyFOB in your pocket used for some cars is, in esscence, a proximity system. e.g. http://motherboard.vice.com/blog/yo...rochip-id-tags-with-these-hot-new-accessories


NFC or near field communcations is another.
 

Thread Starter

MrSoftware

Joined Oct 29, 2013
1,937
Thanks everyone for the replies.

Are you set on using magnets? Magnets might erase the strip on credit cards. They collect junk. Carry a magnet in your pocket for a while and see what you think of it.
The same thought had crossed my mind, but I couldn't think of another solution that was similar in both cost and ease of function. I'll discuss with the disabilities expert about possibly inserting a plastic card into a slot, or plastic peg into a hole instead of a magnet, but I suspect those might be too difficult just like a regular key.

The iButton and Digilock definitely look like they would do the job, but the expense is too much.
 

Thread Starter

MrSoftware

Joined Oct 29, 2013
1,937
The Hall Effect sensors arrived, and I didn't realize they were going to have npn MOSFETs as their output stage, hmm... So I either need to invert their output, or switch to a pnp power MOSFET for my main switch. I can't think of anything in my parts pile that I can use to invert signals (except maybe another big power mosfet), so I'll just order some pnp mosfets and try again. That will likely be a better match anyway since I really need to switch the V+ line. More info to come!
 

AnalogKid

Joined Aug 1, 2013
8,705
Just winging it here -

Another approach to this is to make the "key" a piece of non-magnetized metal with a hole in it for a string to go through, maybe the size of a quarter or whatever you think they can handle. The sensor is a small and simple metal detector - a coil of wire next to a magnet like one section of a guitar pickup, or the coils in the road that trigger traffic lights. A bit more circuitry but only $10 in parts, and the key doesn't look like a key and can't foul up anything around it. Cover the sensor coil with a flat piece of plastic with some kind of target symbol drawn on it, and it doesn't look like a key slot and gives no clue about what it takes to activate it.

Oscillator - detuned tank - comparator - output driver

ak
 

Thread Starter

MrSoftware

Joined Oct 29, 2013
1,937
Ok I have a problem with my magnetic circuit. I put it under about a 6 amp load today and the MOSFET got extremely hot and almost immediately fried itself. The gate voltage and drain voltage would have been almost identical, does this mean the MOSFET was not in saturation and just built up too much heat? Do I need an amplifier circuit of some sort to be sure the gate voltage is higher than the source and the drain?
 

AnalogKid

Joined Aug 1, 2013
8,705
What is the MOSFET part number and please post your schematic. The gate and drain voltages should be complimentary - when one is t 12 V, the other is at zero (-ish).

ak
 

Picbuster

Joined Dec 2, 2013
1,024
Can anyone point me to a magnetic key switch? So far my googling has been fruitless.

This is for elderly disabled people with limited dexterity. The idea is they walk up to the device they're turning on, stick the magnet on and that turns it on. Basically a magnetic reed switch would be fine, but it needs to have a strong enough magnet to hold the key there until the person removes it. It needs to handle 12v DC, and maybe 0.5A of current (enough to operate a relay), though more robust is fine too. One key can work for all switches, it's not a security device. I can make one, but I'm really hoping to find something off the shelf as we're going to need a bunch of them. Has anyone seen something like this?
To make it strong and durable mount the reed switch at the bottom of a hole drilled in delrin(pom) eq 5-10mm and cover magnet with polyester and add a key ring.
 

Thread Starter

MrSoftware

Joined Oct 29, 2013
1,937
What is the MOSFET part number and please post your schematic. The gate and drain voltages should be complimentary - when one is t 12 V, the other is at zero (-ish).

ak
Here's the data sheet for the transistor I'm using:

http://www.irf.com/product-info/datasheets/data/irfz44vz.pdf

I definitely had something wired wrong; I repaired my circuit and connected it to the same load and it worked great. The difference was this time I connected it in series with the V- line of my load, and previously I was attempting to use it in the V+ line, even though it's an npn. I thought if I just kept the current flow in the same direction then it would work OK, which still makes sense to me so maybe I just had something connected wrong. I'll try it again and this time pay more attention. :) My reason for trying it in the V+ line is on the final application, the motor I'm attempting to control might be chassis grounded, so I might only be able to control the V+ line.
 

MaxHeadRoom

Joined Jul 18, 2013
21,635
the motor I'm attempting to control might be chassis grounded, so I might only be able to control the V+ line.
Is your circuit the only one using the motor, or is it shared with another means of operation, overide etc?
If it is just unique to your control most Automotive motors have the +ve & -ve leads brought out, not frame ground.
If so you have to intercept the motor conductors anyway, so in this case you can allow the circuit to switch either.
Max.
 
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