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Newbie Question Regarding Reed Switches/Sensors

Kap23

Oct 31, 2018
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Oct 31, 2018
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Good Morning,

Apologies if this is in the wrong section but I am looking for some advice for a project I am currently undertaking. I am building a box which will hold a revolving platform/LED light. The platform/light operates of 3xAA batteries which I belive is 4.5v. At present it is operated by a toggle switch which I would like to replace with a reed switch/sensor. I have sought advice from a guy I know who works in car electronics and he has told me that a reed sensor wont work as they are only capable of taking small amounts of power. He has suggested a micro switch but they don't appear to be as discrete as a magnet switch. Ideally I would like to mount the switch as discretely as possible so can someone please point me in the direction of a reed switch/sensor/magnet switch which is capable of handling 4.5v or another alternative. I have included a picture of the box so you can see what space is available. I am based in the UK so links to UK sites would be appreciated as it saves waiting 2 weeks for delivery.

Thanks in advance.
 

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kellys_eye

Jun 25, 2010
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A reed switch can carry up to 500mA (half an amp) but we don't know the specification for your motor.....

Either way, the reed switch could be used to power a switching transistor (or MOSFET - logic level mosfets would work) that could drive your motor. This reduces the current through the reed switch to almost zero.
 

kellys_eye

Jun 25, 2010
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Those Farnell parts are still only 500mA. Motors are notorious for 'variable' power consumption. Stalling them can make it an effective 'short-circuit' situation and the current is only limited by the motor winding resistance which is usually very low so current can be very HIGH.

A MOSFET can be found in a package that is only some 5mm square - is that an issue size-wise? It would be a lot smaller than the reed switch you wanted to use!

You could even use the reed relay to drive a 'proper' relay (subminiature) with contacts rated for a few amps. Sub-miniature relays can be found in packages as small as 10mm square.
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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Jun 21, 2012
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Hey,

Thanks for the reply.

I don't know the motor specification. ...
How do you expect to successfully design anything without specifications? Get a multimeter and measure the motor current and the motor voltage applied by a set of fresh AA dry cells. Make the current measurement with the device spinning normally and the LED illuminated. Then restrain the device (stall the motor) and measure the "locked rotor" current of the motor, as well as the voltage now supplied by the AA power pack. These two current measurements represent the "best case" and the "worst case" current switching requirements. Size your magnetic reed switch accordingly.

...
Your option sounds good but I would need to find space to accommodate the mosfet. ...
Well, doh! If you can "find space" to accommodate a reed switch, a few millimeters more for a 2N7000 or similar MOSFET switch should be a no brainer.

...
Ideally I was looking at something like this:

http://www.farnell.com/datasheets/4...6.1093999511.1540985988-1462398609.1539251151

but not sure how to tell if it would handle the power?
It is required to handle the current NOT the power. A reed switch dissipates zero power when closed. It also dissipates zero power when open. MOSFET switches behave in very much the same way except for a small amount of power dissipated when the MOSFET is closed, i.e., conducting. In both cases, MOSFET or magnetic reed switch, it is the current rating that is important.

I doubt you can produce enough current from three fresh AA alkaline cells to damage either a reed switch or a MOSFET switch, but go ahead and measure (with the multimeter) the short-circuit current from your battery pack to make sure...
 
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