Posting Comment for "Pedometer with Morse Interface"

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27th April 2010 03:48

Alan Yates wrote...


You should be able to pick some up on eBay or at electronic component retailers locally. With RoHS in Europe, few actually use Hg any more, rather they use alloys of Gallium like Galinstan which are liquid at room temperature and are claimed to be much less toxic than Mercury. They are often marketed as "tilt" switches avoiding the association with Mercury in the recent chemo-paranoia world.

If you are really stuck the switches are easy to make for yourself once you secure the Mercury. The only real challenge is the glass-to-metal seals required where the contact wires penetrate the glass envelope. Commercially a Nickel/Iron alloy plated with Copper is used, which you can sometimes find on eBay, or you can extract from a vacuum tube device or light bulb. Lots of annealing and blind luck might let you get away with running thin copper (or better copper-weld) wires through the glass. Building your own with Galistan is rather difficult, as Galistan wets glass! You would need to coat the inside of the glass with Gallium Oxide - a fairly non-trivial process - or something else to stop it wetting the glass and permanently shorting the device.

You can find similar devices that do not use a liquid conductor, but rather small metal spheres which bridge contacts. Some are spring-loaded to implement acceleration switches, others are free floating for orientation sensing. Generally the contacts and ball are coated in exotic metals (like Gold, Rhodium or Iridium) and sealed in an inert atmosphere to give good contact quality over time. Actually ensuring a reliable inertial/positional contacting action is fairly non-trivial, especially at low currents and voltages where passivated layers on conductors cause lots of problems. Liquid metals wet the conductors and are far more reliable - although I have noticed in some modern liquid switches the contacts seem completely phobic to the metal droplet - despite this they seem to work just fine - I have no idea of the exact materials these devices use?

Other alternatives to the Hg switch exist and might be worth investigating. The cheaper end of the commercial pedometer market seem to almost exclusively use a spring-loaded lever arm with a mass at the end. The arm's inertia makes it swing with each foot strike and impact a fixed contact. Frequently the really cheap units have just a copper wire hitting the iron arm as the contact, although with the large inertia of the massy arm and weight the contact closure force is high enough that the arrangement seems reliable.

The really high-end units use MEMS accelerometers for continuous acceleration measurements which are read by a microcontroller. They have the advantage of being extremely robust mechanically and can operate in any orientation if a three-axis system and suitable software is used. The extra information beyond simple cadence rates extracted from the accelerometer data can be recorded and graphed in lots of pretty eye-candy ways. There are several commercial products based around this kind of technology now available, some include GPS and use smartphones as the source of both the GPS and accelerometer hardware, as well as the power supply and digital processing/storage.



27th April 2010 00:34

Ondra OK1CDJ wrote...

Hello Alan, you have very nice website !! I'm very interested in

Pedometer with Morse Interface, but i don't know where can i buy Hg

switch ??

Regards, Ondra