magnet reed switch - Page 1
Forum section: electronics / projects

author: cncjerry
author_rank: Unknown
timestamp: July 03, 2017, 04:20:15 am
content: I am building a vertical vibroplex style key. WR Smith, who designed the key, used an unspecified reed switch inside a brass tube. I have a reed switch that works but the magnet needs to move quite a bit away from the switch after a "make". Consequently, I don't get the action out of the key that I would expect. Bill was getting as many as 105 dots; I get 10 and this requires quite a bit of energy on the dot side of the paddle.So are certain reed switches "faster" than others? Is it common for reed switches to require the magnet to move further away on break? I would assume so as once the contact is closed the gap is smaller requiring less magnetic field to keep it closed.Any ideas? I am using the Meder/Standex type switches.Thanks

author: floobydust
author_rank: Super Contributor
timestamp: July 03, 2017, 04:50:32 am
content: A pic would be helpful. There's many things to look at.Small reed switches are 3-10x more sensitive than large ones (>0.5" and >0.5A) and you may have too strong a magnet or it's too close to the reed switch.I think your magnet's geometry and nearby iron, nickel objects affects the flux pattern.This build used an adjustable spacer:http://watchmaking.weebly.com/telegraph-key.htmlDigikey hasStandex-Meder training modulesshowing the magnet/flux interaction with reed switches. It might help to get your intuition understanding things.

author: cncjerry
author_rank: Unknown
timestamp: July 03, 2017, 05:58:57 am
content: I changed out the magnet and things got a lot better. I had been using a flat magnet about 1/16th thick and .3125" diameter. That doesn't say much because the material makes all the difference but it was a "rare earth" type. I then traded it out for a little ball from one of those outlawed magnetic toys. They are ridiculously strong for a little ball bearing. This one was about .195" in diameter and it worked a lot better. I would say it doubled my dot count. So I am up to about 18 dots whereas the original builder was getting over 100 (not that there is a message that needs that many dots). Seeing that Bill Smith died last year, I can't contact him to see if he was getting 100 dots or better with the reed or with normal contacts as he built one of each.The switch I am referring to is on the end of the red wire in this picture:https://wrsmithclocks.com/product/how-to-make-a-vertical-high-performance-telegraph-speed-key/Mine is pretty complete at this point. I just need to mill the base. I made every part including the screws, CNC rope knurled locking bolts, even drilled the tube for the reed switch instead of purchasing ready made tubing. It was a lot of work as he made it using manual machines so he had you make something and then set the next post, for instance, based on how the prior part was made. I would have just drilled the base plate using CNC, but who am I to question an artist like Bill Smith.The reed switch I used was a single throw with both NO and NC contacts. It has two leads on one end, one on the other. I am going to order another switch with only the "make" contacts (SPST NO) to see if it makes a difference. These are small glass encapsulated switches about .095" diameter. They are installed in a brass tube and then the magnet passes along the tube axis to make the circuit. There is an arc but given the make/break distance of about .25" and the pendulum being about 5" at that point, the arc isn't significant.I also have to look at the mainspring as the material I used, though specified, comes in a wide variety. I think a stiffer spring would help but that has nothing to do with the reed switch, just the energy in the pendulum.Thanks for the input and I'll check out the digikey link.Jerry

author: cncjerry
author_rank: Unknown
timestamp: July 03, 2017, 06:02:17 am
content: By the way, that training module is very helpful. Thanks again.