Maybe, just maybe, I’m finished writing about buttons for a while.   But first I want to mention a last few groovy pushbuttons.

Hall Effect Pushbuttons

Hall effect pushbuttons are cool because they have a stroke, like a mechanical pushbutton, but can last for millions of cycles.

• ITW Switches has a wide variety of hall effect pushbuttons, including metal and illuminated large panel mount switches, such as the Series 48SS, the 48M-SS, 57M-SS, and 58M-SS.  However, availability is poor; for example at Mouser I only found a few 48SS models (but they were all less than $20).
• C&K has the HP series.
• APEM has the IH Hall Effect Switches.  The IHS models are panel mount switches start at ~$40.  The IHL models are unique: they have a linear 0.5->4.5V analog output over the switch’s 4mm travel, and cost ~$60.  However, I think a T-Bar or one axis joystick would be a lot easier to use, although they would typically be larger and more expensive.

Finally, I have to mention Schurter’s MSM CS series: they are mechanical vandal-resistant switches with a ceramic actuator.  The ceramic material makes for a very cool looking button; see the PDF datasheet for pictures.  Prices start ~$25.

While looking into ESD-safe buttons, I discovered quite a few metal buttons with no moving parts.  These buttons do have some potential advantages including:

1. Easier to use in ESD-safe applications (since there is only one part to ground, and many models are made of conductive metal).
2. Great durability, up to 50 million cycles or more, since there is no mechanical wear.
3. Better washdown and cleaning for medical and similar applications, because they have fewer cracks to hide nasty stuff.
4. Better resistance against vandals (since the exposed part is made from one piece of metal).

Potential disadvantages include:

1. No tactile feedback; great feedback is one of the best features of a good pushbutton.
2. Very limited current switching ability; many mechanical switches can easily handle 10A currents.
3. Potential problems with gloved fingers not actuating the button, or with water or nearby objects actuating the button.  I suspect in most cases you won’t have these problems, but you should verify first, starting with the datasheet.
4. High prices, typically $20-$100 (although a comparably sized mechanical button is typically $15-$30).

I found buttons from Schurter (Switzerland), APEM (France), Grayhill (USA), Texzec (USA), C&K (USA), EAO (Germany) and Barantec (Israel); there may be others, too.  I think it’s interesting that almost all of these companies are either European or American.

There appears to be a limited market for this switch type; several companies have dropped lines soon after introducing them, and ITW Switches sold its ActiveMetal line to Texzec.  I’ll mention some of the “missing in action” lines below.

So here are some of the more interesting switches I found, sorted by sensing type:

Piezo Electric Buttons

• Schurter has the PSE line of piezo switches, available in 16 mm, 19 mm, 22 mm, 24 mm, 27 mm and 30 mm sizes.  Cases are made of plastic, anodized aluminum, or stainless steel.  Illumination options are none, spot (1 LED), and ring.  Prices range from ~$20 (CSE 16 plastic), ~$25 (CSE 16 aluminum), ~$45 (CSE 16 stainless) and up.
• Grayhill has the 37F series of piezo buttons.  Cases are aluminum, and prices start ~$20.
• APEM has the PBA series, available in 16mm, 19mm, and 22mm bushing sizes, with and without illumination, and with anodized aluminum or stainless steel cases.  Pricing starts >$30.
• Barantec has a wide range of piezo buttons in 16 mm, 18 mm, 19 mm, 22 mm, and 27 mm sizes encased in aluminum or stainless steel.  Illumination options are none, point, and ring.  Barantec only sells direct in the US.
• C&K had the KP series of piezo buttons back when they were part of ITT Canon, but they are no longer available.

Capacitive Buttons

• Capacitive buttons use a sensing technique similar to capacitive touchscreens.  They can have problems with gloved fingers; however, Atmel claims that many gloves (including typical household, medical, and clean room types) should work fine.  The buttons can often work through a thin non-conductive layer such as glass.
• Schurter had several lines of capacitive switches, including the CSE16, CSE 15 uG and CSE 25 uG.  The CSE 16 models were round metal switches, while the uG models were designed to be used under glass.  Mouser still has a few CSE16 switches left at >$90.
• EAO had the Series 75 capacitive touch buttons, but they are no longer available.
• APEM has just introduced the CP line of capacitive buttons; as far as I can tell, they are not yet available.  The CP line will be available in 16 mm, 19 mm, and 22 mm sizes with anodized aluminum cases.

Ultrasonic Buttons

• Texzec‘s ActiveMetal buttons use ultrasonic energy trapped in resonant cavities.   Available materials are stainless steel, aluminum, plastic, and zinc alloy.  Sizes include 19mm, 22mm, and 30mm.  As far as I can tell, Texzec has no distributors; however, Newark is selling the last of the ITW ActiveMetal buttons for ~$35 (22mm, zinc alloy).

Optical Buttons

I haven’t seen any metal ones, but there are some plastic models, such as these  from Banner Engineering.

This was supposed to be a quick post on one piece metal buttons.  But it’s spiraled totally out of control, zooming past one post before finally settling down, I hope, on three posts.

I first researched metal buttons because I needed an ESD-safe button, and I couldn’t find one.  Plenty of buttons have specs for ESD immunity, but I needed one that wouldn’t cause ESD (Electro-Static Discharge).

ESD can create a high voltage spark which can kill nearby sensitive electronic circuits.  Everything close to the ESD-sensitive part needs to be either conductive and grounded or dissipative (material with resistance of 10^6 to 10^9 ohms/square so current will flow, but not too rapidly) and grounded.

Normal plastic is especially bad, because it is an insulator, and can be tribocharged: friction caused by rubbing the plastic part can create a large static charge.  You can get dissipative plastics, but I don’t know of any buttons that use them.

Anodized aluminum is also an insulator; for an ESD-safe aluminum part you have to use electroless nickel plated aluminum.  For example, Banner’s ESD safety light curtains use electroless nickel plated aluminum for the bodies and static dissipative plastic for the optical covers (BTW, as far as I know, they are the only readily available ESD-safe light curtains).

You can’t reliably ground through a moving part.  So if a button has a moving button, then the moving part has to be grounded with a ground wire as well as the stationery part.

While there aren’t any buttons that are advertised as ESD safe, there are some that might work.  What characteristics would help?

1. Be able to ground the both the body and the actuator.  A single piece, non-moving body is ideal if it can be grounded and is conductive or dissipative.
2. Everything that an operator could touch must be made of conductive or dissipative materials such as stainless steel.  If the button is illuminated, the plastic lens would have to be made of dissipative material.

So what are some possible solutions?

1. One piece conductive metal buttons, such as a Schurter 1241.2611 PSE16 16-mm stainless steel piezoelectric button (~$45) or a Texzec T01-012203 22-mm stainless steel ActiveMetal ultrasonic button.
2. Two piece conductive metal buttons such as a stainless steel vandal-resistant pushbutton (available from Schurter, ITW, and many others).  As noted above, you’d have to figure out how to ground both pieces.
3. Use an ESD-safe cover: cover a regular pushbutton with a fixed body that captures a moving part (to depress the button’s actuator); the cover parts have to be conductive or dissipative.  One advantage: if you use a clear, dissipative plastic for the moving part, you can use an illuminated pushbutton underneath.  This ESD-safe cover will probably cost substantially more than the pushbutton.
4. Spray on anti-static spray.  Although anti-static spray should help short term, I’m skeptical that it will continue to work well for a substantial period of time.

All of these possible solutions would have to be verified: you will need to verify that all external parts of the button are grounded and that the button will conduct or dissipate any static charges.