Found @ CRASH Space No 5. – Tactile Swiches

Tactile momentary switches are magic.

Think of all the giant heavy toggle switches and spring-based momentary push buttons on some of the older electronics you’ve seen and we still have. Crazy satisfying to play with but heavy, clunky and big.

Now picture tactile switches, or tact switches, that still manage to have a physical snap and yet are tiny.  Their physicality comes from the deformation of a metal dome that may or may not also serve as one of the electrical contacts. The clicky seal-intact indicator on jar lids is that kind of spring.

These switches represent a huge step in miniaturization and production automation.  The technical drawing (shown middle left) was captured from the oldest patent I could find using this technology. Everything else I saw from before this before was using coil springs. Current patents filed are all about improving reliability, manufacturing and size.  There are some others in  F@CS No. 5 Flickr set and I’ll be posting more.

The reason I’m writing about them for this series is because someone kindly donated a handful of 12mm tact switches to our parts bins. I enjoy poking around to see what new stash has shown up in the unlabeled drawers.  So this is a little how to on how to use them in finished projects (not just exposed on breadboards or raw pcbs) and a bit of a buying guide if you want to source your own.

Using Tact Switches

A good example of tact switches in the wild can be found here on this very website in Take Apart Tuesday No. 8: Saw III Digital Voice Recorder. The fewer buttons a device needs, the more likely designing a membrane switch pad wasn’t worth it.  Also they can be pretty sturdy in terms of press counts and come available in dust and moisture resistant sealed packages.  I’ve found them as  the eject buttons on external optical drive, under printer buttons, in radios, in digital cameras…  Once you start taking things apart it becomes hard not to try and imagine what kind of button mechanism is under the case.

I suspect the ones I found are the E-Switch 1100 Series, but I couldn’t quite identify the cap accessory.  It is very useful to have the snap-top / plunger for cap type like these are when you are going to make your own buttons. The round caps can be slippery. The human pressable operating part must line up with the circuit below.  Anything other than perfectly centered right angles can cause electrical noise and decreased life span of the switch.

I only found one cap on Thingiverse, and it is a good example of how to make your own illuminated tact switch caps.  If you want to make a 3D printed part, manufactures sometimes supply cad files. Omron has theirs up on their own site’s CAD library and via Trace Parts Online

My example project for this was to make a little slide viewer out of laser cut wood and sanded acrylic. The button is bass wood from a hobby shop. The box was started by an online box generating tool and then modified.

It works well, is stable and didn’t take long.   No soldering even.

Tools Used

• Laser Cutter
• Small screw driver as a chisel (don’t tell)
• Wire strippers
• Hack saw

Supplies Used

• Sand paper
• 1/8 inch plywood
• 1/8 inch acrylic (sanded)
• 10 mm LED (white)
• 1 Ohm resistor (or 24 feet of 26 AWG wire, your pick)
• 2 AA Battery holder
• 2 rechargeable AA batteries
• Small breadboard
• Bass wood dowel
• 26 AWG wire

Buying Your Own

The TL;DR

• The easiest thing to do is to go to either SparkFun or Adafruit and buy what they stock.
• If you want a switch like in the inventors kits: 12 mm, Through Hole,  NO, SPST, top actuated, standard or round actuator style, 160gf, 0.05A @ 12V or 24V, expect to pay between $0.50 and $1.00 if you are only buying 1 or 2. AdaFruit sites theirs as being the Omron BF3 series
• The one I used in the slide viewer: Mostly the same but with an actuator style called “snap-top” or “plunger for cap”
• One like used to be on the Arduino: 6mm, Through Hole, NO, SPST, top actuated, standard or round actuator style, 160gf, 0.05A @ 24V, expect to pay $0.37 or less for small quantities (Listed as Digikey SW400-ND)
• The one that is on the Arduno Uno / Leonardo boards is like before but the mounting type is surface mount (gull wing style) with a grounding pin.

First, if you haven’t bought a lot of switches and abbreviations like SPDT (single pole double throw) and NO (normally open = broken = off) mean nothing to you, go watch the Digikey product training manual called Switch Basics provided by them and NKK. The Digikey PTM‘s swing between really informative and marketing schlock.  This one explains circuits and package types, etc. with minimal “NKK is so awesome love us” side comments.

Going from the Mouser and Digikey websites, these are some of the things to consider:

Manufacturer: Is there someone who’s stuff you like or that your company has a relationship with? Some of the big ones include C&K, NKK, ESwitch, Omron, and ALPS which hilariously has trademarked the phrase (caps included) TACT Switch™  Looking at manufacturer sites can be educational/ Putting their names into Google Patents or Free Patents Online with key words about the product you are planning to source from them can also be kind of entertaining. I put some of those pictures in the F@CS No. 5 Flickr set as well.

Operating Force: How hard do you have to press the plunger to make it move. Choice of 160± 50 gf or 260± 50 gf is what the data sheet says for our switch.  gf stands for gram force, or the effect of “standard earth gravity” on a gram of material. You can internalize this as how many pennies would you need to stack on the switch to get the plunger to move.  If you prefer Newtons here is a conversion chart.

Actuator Type:  Really describing the packaging for the metal dome or lack there of.  There seems to be a lack of consistency in words to describe them. Mouser refers to the one we have as “Snap Top” in Digikey it falls under “Plunger for Cap.” The bare metal dome can be purchased if you want to try building your own mechanism. There are square buttons and round buttons… you get to pick.

Current Rating, Voltage Rating AC/DC, Power Rating: All of these are descriptions of how much of an electrical load will the switch be required to carry. Generally tact switches are used with a micro-controller, so the power loads shouldn’t be too big of a deal.   If you are looking for a push button for a higher current application that is exactly what you should be looking for a pushbutton switch.

Contact Form (Mouser) / Circuit (Digikey): This is where knowing what NO SPDT means comes in handy. Go watch the module. It has graphics.

Switch Function:  Mouser seems to just go by whatever the manufacturer put in this field without a lot of QA.  Digikey has it pretty clearly. MOM stands for momentary,  ON-MOM is for Normally Closed, Push to Break switches.  OFF-MOM is for what you usually think of as a button: Normally Open, Push to Make switches.  OFF-MOM-MOM is for DT or oddly SPST x 2. Again, watch the module.

Mounting Style:  How will it be attached? Where’s the solder going to go? Through Hole has legs that go through a hole on the board and can sometimes fit in breadboards. Surface Mount/SMD/SMT are all the kind that rest on the board with no holes. I don’t love that for something that will be pressed often or hard. There are some panel mount tact switches (i.e. the switch will be mounted to the CASE and lead wires will be attached) but those are exclusively the ones that come with built in illumination.  Digikey allows you to indicate some orientation here, too.

Mounting Direction/Actuator Orientation: In your project’s design will the button be pressed from the top (board is parallel to the case surface) or the side (board is perpendicular to the case) or do you need the button to mount to the actual side of the board on the edge?

Other Options: You can buy caps in different colors and shapes, choose the height of the plunger (i.e. stem), if you want the case to be grounded for added protection that is also a choice.  RoHS is the certification that is intended to reduce hazardous materials like lead in electronics.

Price Comparisons: If you are looking at two buttons and they look exactly the same but one is way more expensive than the other look at their data sheets. One may have higher quality metals in their contacts. One maybe better sealed against dusty or wet environments. One may have more clicks per-life-time.  Going back to the manufacturers website and looking at the description for the product series can also give you more insight on what some of the price raising features might be.