Rooting around in the fascinating stuff at the bottom of a draw labelled 'Engineering - Junk Miscellaneous'. Delving amongst the delightful...
An Engineer in Wonderland – the glory of reed switches
I was pondering the failure of my car roof light to operate, wondering if newer cars had gone beyond the corrosion-prone two-pieces-of-metal-and-a-bit-of-plastic door switch.
When reed switches came to mind.
For the uninitiated, these consist of two flat flexible magnetic wires – reeds – held parallel and overlapping in a sealed glass tube.
A correctly applied magnetic field induces a N pole on one reed and an S on the other, at which point they bend towards each other and touch.
Operation has in-built hysteresis as, once attracted, the reads don’t unstick until the field has been reduces well below the engagement level.
With very little protection, they will operate for years in almost any climate, or immersed in almost any liquid, because the contacts are snuggled away in an inert atmosphere behind a hermetic seal.
Like Hall-effect switches, they are operated by an applied magnetic field.
And although they are not available with the sort of on-board signal-conditioning that makes Hall switches so versatile, they do operate entirely quiescent current.
Types are available to switch several hundred volts, and others will switch over an amp.
And with nothing to oxidise the contacts, any tiny current can be used to detect whether the switch is open or closed – although, oddly, I can’t find any spec sheets to tell me what the minimum operating current is.
Sadly, life is too short to manufacture a couple of reed switch assemblies to make my interior light work, but the failure did reminded me just how good reed switches are.
There us some more detailed actuation stuff here.
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