Making Electrical Switch Contacts

Chaps,

I'm making an experimental semi-automatic morse key (like a Vibroplex) and need some advice about how to make reliable electrical contacts for it.

The key has two contacts. One hits a vibrating arm to make dots. The other is a simple on-off used to make a dash. When contact is made a 5V control voltage is earthed to key a telegraph relay or radio transmitter. The switching current is of the order of a few milliamperes.

At the moment the contacts are made between an adjustable M3 screw and a strip brass spring.

The arrangement works but brass on brass is far from ideal, especially given that my wetting current is low.

I own an ex-military morse key with tungsten contacts. Tungsten appears to be readily available as 2.4mm diameter welding rod. Has anyone had any success machining this? My Mini-lathe and I are intimidated!

Alternatively, has anyone tried silver plating brass at home. Is it possible without using Silver Nitrate and Potassium Cyanide? I doubt either is available 'over the counter'!

I see 2mm diameter silver rod can be bought on ebay. This looks like something I could work with but none of my practical books cover silver and I have no experience with it. Am I cruising for a bruising?

Finally, does anyone have any other suggestions? The cheaper the better!

Thanks,

Dave

pinch the contacts out of a relay, preferably not a working one. I've got some in a box somewhere, if you want them send me a PM

I have used bits cut off a TIG electrode for electrical contacts, cuts OK with a slitting disc in a Dremel and can be silver soldered onto you brass.

Alternatively Hemmingway sell points contacts, bottom of page

what about just a 'dob' of silver solder (hi silver content i guess is best).

Google 'Tungsten contact rivets',

Neil

I'v got a wee jar with 3/16" x 1/32" tungsten discs in it, in the last 15 years I'v used two of them.

Ian S C

I would concur with the relay contact idea for the fixed part suitably riveted on to the strip and as a viable alternative for the adjustable contacts why not just drill the ends insert a short length of silver wire with a little solder paste and then heat and polish.

regards Martin

You might try liberating the contacts out of a set of small engine points. These are made to last for lots of operations and are very cheap for being very high quality stuff. JD

Don't be put off by machining silver. I used to operate a quantometer that used a silver electrode which needed to be skimmed clean each time, and have never met a metal that was easier to turn. No need for any lubricant, just a sharp tool and a steady feed to produce a literal mirror finish. I would expect it to take a thread well, although it would be easy to strip.

Brian

How about gold off an old PC board or the spring connectors off a high quality PCB board socket.

Silver plating kits are available but I'm not sure if silver is a good idea in contacts for low current as the oxide makes a diode doesn't it?

How about platinum wire as has been used in a few designs for electrical clocks in ME.

The ideal metal for contacts is Cupro Nickel, you will find these on relays, battery contacts etc. Easily machined and just polish the contact surface.

Clive

I am not sure that silver is a good contact material for this job, because it oxidises so easily. As far as I know no one uses silver by its self for contacts. Its got the best conductivity for bus bars and the like, but relay contacts are gold, platinum, rhodium and mixtures of various metals. basically that do not corrode and some do not splatter when switching high voltages (not relevant ).

For the screw I would drill the end out and soft solder a gold plated pin in it, say from a D type connector. The flat spring connection is more difficult, a complete contact and leaf from a relay would be best (gold plated).

Frank

Need source for Cupro Nickel? Older modern coins were made of this material, Good old fashion coin snipping?

I thought the tarnishing of silver contacts was normally silver-sulphide and that it was pretty much as good a conductor as silver itself. Is that wrong?

Kwil, I scavenged the cupro nickel contacts off Ni-cad batteries. PM me if you want a few. I will have to find them first.

Clive

Lots of good ideas here thanks. Now I'm spoilt for choice!

I'm going to experiment. I'll start with a relay contact and a pin stolen from a D-connector. Then I'll try some silver wire from a bangle. If that doesn't work I have plenty of cupro-nickel in the shape of old florins and dud nicads.

I've gone off tungsten because further research indicates it's more suitable for high-voltage / high-current applications.

I have always believed that silver oxide is a good conductor. Now I'm not so sure. I couldn't find much on the internet.

Thanks again everyone.

Cheers,

Dave

Bazyle, you may be thinking of copper oxide, which was used to make rectifier diodes. My understanding was always that silver oxide is as good a conductor as metallic silver, which is why it was used so extensively for plating electrical contacts.

George

Old spark plug electrode ? Colin

Colin: good idea especially as I have some nice new plugs that don't fit my current car.

Francis IOM: It's a fair cop guv! My design rationale for building the key is far from logical. I like retro technology and making mechanisms. Brass and steel are sexy. In my twisted mind an interrupted beam would be the thin edge of a wedge leading to a fully electronic or computerised solution. I enjoy that kind of project too, but we ME types have to justify all that expensive metal bashing kit in our workshops somehow!

Cheers,

Dave