I am working on a wake up light project where i am using nine reed sensors as a creative means to set the alarm clock. The idea is that you can set the alarm time by putting magnets in particular spots on the alarm clock.
The Hamlin 59166-1-T-00-A reed sensor i am using is listed as normally open. I take this to mean that it is a sensor which should be used in situations were a magnet is normally absent.
In my design some reed sensors will be closed most of the time and others will mostly be open.
I am wondering whether the prolonged exposure to a magnetic field of a normally open reed sensor will affect its performance over time. I.e. Is long exposure to a magnet in any way bad for the sensor? Will the leads bend? Will it become magnetized?
I can't say I've ever seen a reed switch contact stuck in it's non-energized state because of long exposure to a strong enough magnetic field, but that doesn't mean it's not possible I guess, I just don't know if that is possible.
I do know that reed relays can be obtained with SPST contacts that are normally open or normally closed. I've also seen a case or two where the SPST contacts on a normally open contact were welded closed from having too high a short circuit current pass through the contacts causing them to melt together.
Yes, the reed contacts are metal and can get magnetized, causing them to stay closed, even if the magnet is removed.
Someone on this forum wrote that he had to de-magnetize his alarm reed contacts every now and then, because they stopped working. Perhaps something is wrong with his reed switches and magnets.
Normally, this effect has been taken into account.
An alarm system uses normal (not very strong) magnets, and there is always a gap between the magnet and the reed switch. So it will safely work forever.