# Reed Switch Motor Speed Sensor for Tachometer Wiring



## CFreeman54 (Jan 14, 2009)

After reading a number of posts in the forums, I decided to power a digital tachometer for my Fiero with a nylon magnet collar attached to the tailshaft of my WarP 11, and a Reed Switch Sensor.

Having not done this before and lacking an electronics background, I am have a few questions I am hoping you more experienced people can answer. 

One, I was surprised by the small size of the reed switch and its wire leads; can I really pass 12V off the car's 12V battery through this thing without vaporizing its wires? 

Some people mentioned resistors in the circuit; how do I know if I need these? 

Two, since the switch is sensitive to magnetic fields, isn't the motor's EM field going to effect it? Maybe I should wrap the reed switch in steel foil or magnet shielding foil?


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## Tom Sines (Mar 24, 2010)

Hi, Have you tried a hall sensor, its cheap, and it hardly ever brakes.


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## Homer (Aug 4, 2009)

I'd suggest using the WarP Speed sensor, just bolt it onto your motor and hook up to your tach.

http://www.rechargecar.com/product/warptm-speed-sensor

It was easy to install and hook up.


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## Thaniel (May 25, 2008)

CFreeman54 said:


> After reading a number of posts in the forums, I decided to power a digital tachometer for my Fiero with a nylon magnet collar attached to the tailshaft of my WarP 11, and a Reed Switch Sensor.
> 
> Having not done this before and lacking an electronics background, I am have a few questions I am hoping you more experienced people can answer.
> 
> ...


I used a reed sensor on a car project of mine to sense speed. It only worked for about a year and wore out. I replaced it with an inexpensive hall effect sensor that worked better (cleaner signal) and wouldn't wear out. So I'd vote go for a hall effect sensor or maybe even a gear tooth sensor (no need to add magnets on a gear tooth sensor)


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## Dave Koller (Nov 15, 2008)

http://www.saturn.offical.net/node/326

might give you some ideas..

or mine:

http://www.diyelectriccar.com/forums/showpost.php?p=151836&postcount=37


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## ZipZap (Oct 24, 2009)

Along the lines of this, I have a problem that the tach works fine up to about 1000 - 1500 rpm then falls back to zero. Any one with an idea? Is it the mags, or perfhaps alignment??

Jon


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## Tom Sines (Mar 24, 2010)

Hi ZipZap, The reed switch probably can`t turn on and off fast enough. Get a hall sensor.
Tom Sines


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## Dave Koller (Nov 15, 2008)

Tom Sines said:


> Hi ZipZap, The reed switch probably can`t turn on and off fast enough. Get a hall sensor.
> Tom Sines


I second that - and it does not produce a clean waveform....


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## Homer (Aug 4, 2009)

The Warp speed sensor must be a 'Gear-tooth' type hall sensor since it uses steel set-screws as targets.


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## CFreeman54 (Jan 14, 2009)

Well, over the weekend I took the Edisonian approach to speed sensor wiring...and fried two reed switches. They looked good and showed a nice signal when tested for continuity with a multimeter, but as soon as I put 12V through them the switch welded closed!

See attached pics.


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## CFreeman54 (Jan 14, 2009)

What did you guys using hall effect sensors do to protect it from the magnetic field of the motor?


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## DONEAL (Sep 10, 2008)

I used an all the shelf tachometer and the sensor You can build one yourself, I did. I had problems using the hal-effect sensor. It could have been a bad sensor, not sure, I just had the one sensor to use. Never did get it to work.
I did get the glass reed switch part # rsw-27 .50c each min order two to work. I also purchased Press fit magnet switch # ms-11 at 2.75 each and it worked as well. both from http://www.allelectronics.com/
I bench tested both, I decided to use the magnet switch.
Purchase magnets from K&J magnets inc. ¼” dia. X ½ “ axially neodymium cylinder magnet n42 at .70 each. 
Purchased ¼ “ x 2” npt brass nipple at 1.29 local hardware store.
Purchased two nuts and ¼” Brass collar at 1.49 local hardware store.
Purchased two ¾ “ 10-32 nf allen set screws both at .98 local hardware store
Aluminum Collar made from scrap left over from adapter plate 
Aluminum 1 ½ x 1 ½ x 2 “ long angle from scrap
Steps to build
1. I used a drill with a 2 ½ “ hole saw to cut out the alum. Collar ( size was what the scrap piece allowed)
2. I used a drill with a 19 mm hole saw to cut out the inside hole of the alum collar. (the size of the tail shaft on my motor)
3. I used a drill with a #21 drill bit for the two set screw holes. Then tap them for the ¾”set screws. ( could have used shorter set screws, but didn’t want to take any chance of striping out the threads when I tighten them.)
4. I use a drill with a ¼ “ drill bit for the two magnets. Used glue to hold the magnets in the hole, I did turn magnet poles, one north and the other south. ( really didn’t mater, the magnet switch would work with the magnet poles in either direction.)
5. For the alum. Bracket I use a drill with a ½” drill bit to make the hole for the brass nipple ( note the O.D. of the nipple is ½ “ so I had to woller out the hole a little so the nipple would fit)
6. Pushed the press in magnet switch into on end of the brass nipple then filled the other end with silicone rubber sealant. 
Cost to build 14.21$
Time spent to build about 3 hours


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