Tiny Tuesday: Reed Switch

TRIGGER WARNING: This write-up is a bit geeky!

Our house burglar alarm system is starting to feel its age as it was installed 37 years ago when we moved here. It doesn’t use new-fangled wireless technology like many modern alarms, it’s fully wired. Some of the wires run under the carpets and are therefore at risk of repeated trauma from vacuuming etc: they consist of very fine wires in a plastic sleeve.

Normally the alarm is very reliable which is why we’ve not had it updated. However my adventure underneath the floor on Saturday involved lifting part of the hall carpet and as I replaced it, it became evident that this trivial disturbance had finally revealed a weak section of wire. The wire leads to a reed switch at the front door exit route. So I’ve finally got to what this blip is – it’s the reed switch.

Reed switches are very clever devices which activate when a magnet comes close to them. The tiny switch is located in a small hole in the door frame, and opposite this (when the door is closed) there’s a small magnet buried in the door. When the door closes the magnet comes close to the switch and the switch closes. If the alarm is set and the door is opened the magnet moves away from the switch, the switch opens and the alarm activates (after a small delay to allow you to get to the box and turn it off as you enter).

Bell Labs invented the reed switch in the 1930s and they’re still widely used in many applications. They consist of two ferromagnetic blades enclosed in a glass envelope – a beautifully simple idea which results in them being very reliable, often even capable of billions of activations. The circular plastic housing of this switch is only 2cm across so you can tell how tiny the actual switch blades are.

There’s an excellent further explanation of them at this link which includes an animated diagram:
https://standexelectronics.com/reed-switch-technology/what-is-a-reed-switch-and-how-does-it-work/

Thanks yet again to Debbi (dbifulco) for hosting Tt this month.