Problems triggering off a magnet strip with reed-switch  - Page 1
Forum section: electronics / projects

author: commerou
author_rank: Contributor
timestamp: March 26, 2013, 06:26:20 pm
content: Hi, I am having a bit of trouble and I thought some smart people from the eev forum might have an answer.I am trying to trigger of a magnet strip with a 2 reed-switches in series. The reason why I am using 2 reed-switches is because it is mounted on a machine and there is a lot of vibrations which causes wrong triggering sometimes so I placed 2 reed-switches in series and one of them was turned 90 degrees compared to the other.The problem I have now is that the magnets is under approx. 30cm of dirt, and it is not triggering all the time.Any ideas how to solve this problem?Thanks in advance.

author: Teemo
author_rank: Regular Contributor
timestamp: March 26, 2013, 11:09:42 pm
content: The reed swithes are sensitive to the direction of the magnetic field. They will not trigger equally well with if turned at different angles to the magnetic field.Also I can not imagine reed switch that triggers from as far as 30cm!

author: daveatol
author_rank: Regular Contributor
timestamp: March 27, 2013, 12:51:49 am
content: If you want to detect when either reed switch turns on, then you'd connect them in parallel.If you want to detect when either reed switch turns off, then you'd connect them in series.If you want long distance triggering, make sure you have a strong magnet, e.g. rare-earth type, and not the common lame old ferrite strip.

author: amspire
author_rank: Super Contributor
timestamp: March 27, 2013, 12:58:11 am
content: If you are concerned about vibration, consider using a Hall Effect sensor do detect the magnet rather then reed relays. It will probably work much better at a distance then the reed relays as well.

author: skipjackrc4
author_rank: Regular Contributor
timestamp: March 27, 2013, 03:35:10 am
content: Quote from: amspire on March 27, 2013, 12:58:11 amThis 100%. Reed switches just aren't that reliable. And I'm impressed that you got them to trigger from 30cm away. Very impressed, actually.

author: commerou
author_rank: Contributor
timestamp: March 27, 2013, 11:17:21 am
content: I have considered using a hall-effect switch, but I could not find any which are sensitive enough. Does any of you guys have any idea on which hall-effect I could use?

author: amspire
author_rank: Super Contributor
timestamp: March 27, 2013, 12:02:17 pm
content: Quote from: commerou on March 27, 2013, 11:17:21 amIf the switches that go down to a few mT trip point are not sensitive enough, then search for linear Hall Effect sensors. You will find many examples at places like Digikey.Definitely, a Hall Effect switch or linear sensor will be very much more sensitive then a reed relay.

author: commerou
author_rank: Contributor
timestamp: March 27, 2013, 10:51:12 pm
content: Thanks for the answers, Im gonna try some different hall-effect sensors and hopefully find the best.

author: codeboy2k
author_rank: Super Contributor
timestamp: March 28, 2013, 07:06:33 am
content: An additional benefit of the hall sensors is that they won't be susceptible to false triggering because of vibrations, so you only need 1 of them, not two, as in the two orthogonally mounted reed switches you have now.Also 30cm is really far for a reed, as others have said. You must be using one hella magnet there.A hall sensor is much more sensitive, you can probably reduce the magnet size, if there's any benefit in doing so.

author: Niklas
author_rank: Frequent Contributor
timestamp: March 28, 2013, 07:34:16 am
content: Some safety related applications must have multiple sensors with compaired measurements and sanity check. Two sensors or more, placed at different angles (as seen from the rotationg shaft) and with different rotation (IC package mounting angle) is a nice approach.Hall effect sensors come in a couple of setups: latching/non-latching, sensitive to magnetic north/sensitive to both polarities, constant on/sleep+latch, analog output/open drain etc. I have used the A3245 from Allegro in a prototype a couple of years ago which is now superseeded by A1126. It was not sensitive to the magnetic polarization and it was on all the time. Some low power sensors go to sleep, wakes up, samples and latches the new value to the output. It works fine for closed lid detection on laptops, but not for precise rotation sensing.Also check the magnet orientation so that the field lines hit the sensor from the right direction according to the sensor's datasheet.