What is the difference between a tact switch and a tactile switch?

Answer 1

Tactile (tact) switches are momentary action on/off switches. This page is a comparison of three types of tact switch which could be used in the TB-303. This discussion applies directly to the switches used in the TR-606, and more generally to stem-less versions of the same kinds of switches, as used in the TR-808 and JP-8. For more discussion of those switches, and for details of how to modify the Omron sealed tact switches discussed here, so they fit the TB-303 / TR-606 buttons nicely, please see:

../303-mods/

That page also contains a datasheet for the Omron switches.

Since August 2010, we are installing the modified Omron switches in Devil Fishes. As a service to people who want to replace their own switches, we are also selling packs of modified Omron switches. Please see these sections of the main Devil Fish page:

../#pots_switches ../#tact_switch_kits

Since our choice of Omron switches implies a criticism of the use of the original unsealed ALPS switches, or the other alternative - sealed ALPS switches - and since the reliability of these switches is so important, I have created this page to give people some insight into the characteristics of these switches, and why we chose the Omron switches.

To the main Devil Fish page.

An update history is at the end: #updates .

Dust appears to cause tact switch failure

Before mid-2010, starting with the first Devil Fishes in 1993, I installed the original kind of unsealed ALPS tact switch, with a "dust guard" - a thin mylar sheet with holes punched in it for the switch stems and LEDs. (The plastic was from a 3M overhead transparency protector.) This greatly prolonged the life of the switches- I guess by a factor of 5 or 10.

The failure mode of these switches is that they become erratic and "bounce" - more than one open-close transition when pressing or releasing the switch, as detected by whatever de-bounce algorithm which is implemented in the TB-303's CPU's firmware. As far as I can tell, the reason is that dust, I guess mainly 100% protein flakes of skin, get inside the switch and are compacted into thin layers on both the stationary contact and the moving metal contact which clicks down against it. With a badly bouncing switch, the coating is thin and appears to be transparent. It is visible under a stereo microscope, and easily removed once the switch is dismantled. I have seen no sign of corrosion or actual wearing of the metal contacts. The switches, once dismantled, can't be re-assembled, so this is not a method for fixing erratic switches. They must be replaced.

The dust guard reduces the amount of dust around the switch which can enter via the circular gap between the moving shaft and the metal ring in the body of the switch. This lends support to my theory that dust is the cause of failure, since the dust guard can't affect any corrosive elements in the atmosphere, or the actual mechanical movement of the switch. The electrical current in the TB-303 switch scanning circuit is very low -about 0.3mA, with an open-circuit voltage of about 5V. I think this is probably too low to "self-clean" the contacts, by way of a little spark or similar. Perhaps increasing the drive current, by altering the 15k resistors R223, R219, R218 and R48, say to 1k or 2.2k, would introduce some kind of self-cleaning current. I have never tried this. It is difficult to anticipate the long-term impact of this, since perhaps that process would also erode the metal surface, leading to corrosion or other problems.

I was pretty happy with this dust-guard approach, since I think that an intensively used machine without such protection might need its switches replaced after a year or so, and it was clear that the dust guard greatly extended this time.

However, there were some instances of these replaced switches becoming erratic after 7 to 10 years, at least with the machines which were intensively used. Ideally, we would replace the switches and no-one would have to worry about replacing them for decades, even if the machine is intensively used.

Searching for alternativesIn mid-2010 I evaluated two kinds of alternative tact switches. I hadn't been aware that a sealed ALPS tact switch had been available for some years. I purchased some Omron sealed tact switches and started using them. These Omron switches had stems a little too wide for the TB-303 buttons, so I developed an elaborate jig with a small grinding wheel on a high-speed drill press to grind down the four sides of the top of the stem, evenly, about a thickness of human hair. This was difficult and error-prone. Later I devised a simpler technique of cutting the stem twice, to make it springy. This is described in ../303-mods/ .

During this time, I became aware of the sealed ALPS tact switches and purchased 100 of them. I found they had a very poor feel compared to the original unsealed ALPS switches and the Omron switches. There was little travel in the "click" operation, and little difference between the force required to activate the click and that required to hold the switch closed once it was in the down state. (This was not a bad batch of switches. I was later sent some of these switches by someone who sells them, and they feel identical to the ones I tested.)

I kept two of these switches and returned the remainder.

I can't be absolutely sure these Omron switches will last a long time. However, their datasheet specifies their life as 3 million operations. There's no specification for what this means, but this is a high number for a switch.

I think that as long as dust is excluded, and the switches are not exposed to a corrosive atmosphere, then they will last a very long time. The Omron switches are completely sealed against dust and liquids.

Summary of results#summary

The short version is that the Omron sealed tact switches have a very good click feel, at least as good as the original unsealed ALPS switches. They involve a somewhat higher activation force, but I think that is fine.

The sealed ALPS switches which we purchased had, in our opinion, a much poorer click action. The travel was much smaller and there was less difference between the activation force and the holding force: that required to hold the switch closed in the downwards position before it snaps up.

According to the spec sheets, both the original and the sealed ALPS switches have the same travel: 0.3mm. However, our experience is that the sealed ALPS switches have a much shorter travel between the point where they snap downwards, and the downwards position.

I intend to obtain some sealed ALPS switches from someone who is happy with them. When I do, I will update this page to confirm or reject the possibility that the sealed ALPS switches we tested behaved differently from the switches that this supplier, and his customers, are apparently happy with.

If you accept our choice - if you lack a profound curiosity about the inner workings and behaviour of tact switches - then there's probably no reason to read further.

In early September 2010, I did some simple tests on the switches and put them up on the 303-mods page. These tests, of activation and holding forces, were in accordance with our judgment about the clicking actions of these switches.

A few days later, I figured out a way of doing better tests. The results of those tests are detailed below. I removed the earlier test results from the 303-mods page.

The most important results can be summarised, but first a few terms I invented for these tests:

• Activation force. The force required to almost bring the switch to the point where it clicks to the downwards state. This is measured in Newtons, where 1 Newton is the force exerted by the Earth's gravity, at sea-level, on an object whose mass is about 102 grams.
• By the time this activation force is applied, the stem will have moved somewhat from its zero force position. This movement is what I call the "Initial Displacement".
• Holding force. Once the switch is in the downwards state, the force which is required to keep it down. This is less than the activation force, due to the snap-action hysteresis of the switch. Once a force less than this is applied, the switch snaps back to the up position. That new "up" position is not necessarily fully up, as with no force, and it may be marginally higher than when the activation force is applied in the Up state.
• Delta force = Activation force - Holding force. This is how much force must be removed after pressing the switch down with the Activation force, before it will snap up.
• Delta ratio, as a percentage = 100 * Delta force / Activation force. This is the ratio of the Delta force as a fraction of Activation force.
• Click displacement. The distance between the position just mentioned - with the Activation force, or just a little less, applied - and the Down position after the switch has clicked, and a force similar to the Activation force is still being applied. The stem may go down further with greater pressure (probably only the Omron sealed switch would do this, since it involves a thin synthetic rubber sheet transmitting the force), but this "Click displacement" is the distance we feel the stem fall when the switch clicks.

Here is a summary of the most important measurements. "mN" means milli-Newton.

Original unsealed ALPS

Sealed ALPS

Sealed Omron

Activation force

1243 mN

1595 mN

1484 mN

Holding force

605 mN

1305 mN

764 mN

Delta force

638 mN

290 mN

720 mN

Delta ratio

51.3%

18.2%

48.5%

Initial displacement

0.148mm

0.123mm

0.164mm

Click displacement

0.230mm

0.125mm

0.234mm

Please note that these tests were not done in a fancy laboratory and did not attempt to sample multiple batches of switches. The force measurements for the sealed ALPS switches were based on two switches and the displacement measurements were based on one of these switches.

The figures in red show how different the sealed ALPS switches (at least the ones I tested) were from the other two types:

The Activation force of the sealed ALPS switch was higher than that of the other two switches.

The Holding force was much higher - nearly twice that of the other two switches.

The Delta force was less than half that of the other two switches.

The ratio of the Delta force to the Activation force was much lower: about 38% of that of the Omron switch and about 35% of that of the unsealed ALPS switch.

The Click displacement was much less - only about 55% of that of the others.

I tested 10 unsealed ALPS switches for forces, and chose two of them with close to average forces for the displacement tests. I used 10 sealed Omron switches in the same way. I only have two sealed ALPS tact switches, and one of them I had partially dismantled. So I used those two for the force tests, and the non-dismantled one for the displacement tests.

Even allowing for variation between batches, the differences between the ALPS sealed switches and the other two types are highly significant, and explain why we felt their click action was so poor.

Maybe some people like a lower Click displacement and lower Delta force, but we don't.