Reed Switches versus Hall Effect Sensors

Question

I have an application for speed detection. I desire to have a magnet fixed to a wheel that will pass a sensor. I've narrowed down the sensor to either a Reed Switch or a Hall Effect Sensor.

The reed switch I can wire with just two wires (one if I ground the chassis). A hall effect sensor takes three wires.

Does anyone have a preference in such an application - reed switch versus hall effect sensor?

Thanks,

Sean

jw0752 · Answer

Hi Sean, Speed might be a consideration. With a specific magnetic field, the reed switch will have the time delay for the field to pull the contacts together. As the speed ramps up the magnet will advance at the point where the contacts close. At slower speeds or for your application this may not matter. At higher speeds the Hall Effect might be a better choice. John

mcb1 · Answer

Does anyone have a preference in such an application - reed switch versus hall effect sensor?

Hall Effect everytime.

You have some options with the different types, so be careful choosing the type and the magnet that triggers them.

Mark

dougw · Answer

The reed switch can have bounce which may need debouncing, but it does not require much power to work. The Hall sensor can be nicely debounced with hysteresis and is faster, but it requires more power. Power is important if you use a wireless sensor. I like the Hall sensor because it won't wear out.

Fred27 · Answer

I assume by trike you mean a bicycle (rather than motorcycle) trike. I'd be tempted to look for a commercial bike speedometer. Maybe buy a cheap one and ditch the speedo part. This will help you with neatly mounting the magnet on the wheel and the sensor on the forks. Of course, if building all of it is part of the fun then ignore me! I'm sure you'll find that commercial ones are hall effect based so that would suggest that's the way to go.

Sean_Miller · Answer

Well, I went the hall effect sensor approach and it work perfectly. I found I needed to pull up the "signal" pin as the sensor would normally float and when a magnet comes near, it switches to ground. Using a rare earth magnet, I was able to have plenty of wiggle room on my mounting brackets as it could switch it within 2 inches. I'll soon have the complete build in my final Trike blog for those interested. It'll be published in the next two weeks. See ya', Sean

mcb1 · Answer

What aspects do you like the Hall Effect over reed?

Speed, size, mounting (depending on the particular reed), sensitivity.

Cost may also be a factor, but again it depends on the type of reed.

You can also get Hall effects that will only trigger with North (or South) poles, which means any other magnetic infuence will be ignored.

With a 24inch wheel the outside circumference speed will be high, which means the magnetic flux acting on the reed will be short.

You could mount it closer to the centre to get more time, but it will be shorter between pulses which means accuracy might be less.

Mark