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Switches with "noble" metal contacts - gold, platinum or palladium - work fine at low currents. Base metal contacts are cheaper, and work fine if you are switching more than the minimum current.

"Dry" reed relays were intended to switch low currents, and were built with noble metal contacts so that they would continue to work, even in the absence of a "wetting current".

-- Bill Sloman, Nijmegen

Robert Scott ---@--- posted to sci.electronics.design:

Basically you need a switch with gold contacts. The anti-corrosion properties of gold allows switches to reliably switch at the picoampere level.

Search on Hall-effect switches...these are ideally suited for low power portable with sleep power down mode and the on/off is used to pull a CMOS input low to wake the circuit up.

unreliable operation

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Wiping action (preferably at a decent pressure) is a necessity. Othewise crap in the air can deposit on the contacts and you'll get unreliable operation even with precious-metal contacts.

Best regards, Spehro Pefhany

Could be a bad choice since it's not only rather low, but usually very poorly characterized. A resistor gives you much better control, and unless the switch is very close you'll probably want some series R as well.

Can you design it so that the switch is open most of the time, as on a remote control?

Best regards, Spehro Pefhany

Yes, that's exactly how I intend to use it. It is a pushbutton for a user interface.

Robert Scott Ypsilanti, Michigan

Maybe you could do something like feed the pullup from a port pin and, if the switch is closed for too long, lower the pullup and go to checking it every now and then.

Best regards, Spehro Pefhany

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Bell Labs invented the reed relay in 1932 for used in telephone exchanges. Please explain what switch they used to protect the dry reed relay switches from having to carry current when opening or closing even though they had to carry some current when closed. Then tell me what kind of switch they used to protect the protective switch ...

In fact, it looks as if we are both wrong - the only reference I could google claims that "dry reed" relays are so called to distinguish them from mercury-wetted reed relays (which really don't need any wetting current).

I happen to like mercury-wetted reed relays - they offer a lower and more stable contact resistance than you can get from any other kind of relay, and they don't bounce. The fact that they have to be mounted within 15 degrees of being vertical can be a problem. People kept on offering orientation-insenstive mercury-wetted reed relays, but nobody evers seems to have mastered the art of manufacturing them reliably.

-- Bill Sloman, Nijmegen

Well, the problem has been solved. It turns out the Tyco tact switch I wanted to use has a minimum current of 10 microamps. My micro weak pullup sources typically 200 microamps with a minimum of 50 microamps, so I am within the spec after all.

Robert Scott Ypsilanti, Michigan

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Or so you'd like to think. You'd be better off accomodating yourself to the idea that you too might have got it wrong, but developing that level of self-confidence does seem to depend on having got it right sometime or other.

No matter how you want to word it, my impression was that those people who had claimed to be able to manufacture orientation-insensitive mercury-wetted reed relays don't persist with the claim, which put me off even trying the next offering when it came on the market.

No - the discussion had moved on to what the "dry" in the name "dry reed relays" actually referred to.

I had though it referred to the capacity of the reed to functon in the absence of significant wetting current, that is when switching currents less than a milliamp. I wasn't able to find any evidence to support this opinion, and the evidence I did find (and posted) claims that "dry reed relays" were so called to distinguish them from "mercury-wetted reed relays" which is equally plausible.

You seem to have got it into your head that the word "dry" in the name "dry reed relay" is related to the use of the word "dry" in dry contact switching, where the relays contacts are carrying no current at all when opened, and see aren't sustaining any potential difference when closed. You haven't advanced any evidence to suggest that this - more rigorous interpretation - of the term "dry" in the name "dry reed relay" has anything to do with dry switching. In 1932, when Bell Labs developed the dry reed relay, the technology didn't exist to let them operate relays as totally dry switches within the telephone switching system.

If you had the wit to understand how to construct an argument, you would also have devoted some time to searching for some evidence to connect the use of the word "dry" in this context to the use of the word "dry" in the phrase "dry reed relay". Without that connection the information you have adduced is irrelevant.

-- Bill Sloman, Nijmegen

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How much time will this switch spend closed? If its closed for a short time (as in a momentary pushbutton for a TV remote control), add a resistor to draw some more current. Heck, that's what those 'kewl blue' LEDs are for. ;-)