Low force microswitches

I was successful - it turned out that microswitches were of much better quality than any keyswitch I had previously encountered.

The best of the switches I tried was this one:

G3M1T1PUL - Saia-Burgess G3 low force microswitch (dismantled)

G3M1T1PUL - Saia-Burgess G3 low force microswitch, 15g, 7A, 250V ac

G3M1T1PUL - Saia-Burgess G3 low force microswitch (dismantled)

G3M1T1PUL - Saia-Burgess G3 low force microswitch (dismantled)

G3M1T1PUL - Saia-Burgess G3 low force microswitches

The specs of this switch may be found [here].

After some experimentation, I found that a Cherry MX keycap would fit on to the actuator of this switch - if part of its cover was removed - after filing some material off the plastic actuator.

After preloading with a relegendable keycap, the required activation force was 8cN. This is very low - perhaps even lower than is desirable.

The activation travel is small - 1.2mm according to the specification.

The total travel is typically large compared to the activation travel, and the force increases in a pleasing non-linear manner as the total travel is approached.

Also, the tactile and auditory feedback with this switch seem to be of very high quality.

The switches are referred to by their manufacturer as being snap action switches - a reference to their tactile feedback.

The tactile feedback works by magnifying small changes in the activator displacement into larger displacements at the contact points. The catastrophic change results in an audible click, and - significantly - a delicate click sensation being transmitted back to whatever is supplying the activation force.

The click sensation is extremely precise. Almost all other low force keyswitches I've tried (excepting the Omron microswitch below) feel mushy by comparison.

Most tactile feedback mechanisms operate at some risk of transmitting too much energy back into whatever activates it - with the potential of causing cumulative damage disorders. The action of this switch seems to be subtle enough to avoid causing problems - while being dramatic enough to provide the desirable activation signal at a level where it can be detected.

The overall effect is reminiscent of the old IBM "buckling spring" keyswitches - but with a much lower activation force.

The noise created by the switch is noticable. It is increased by the casing of the switch, which acts rather like the surface of a drum.

Switch surgery is possible - and the switch is fairly hackable. The contact points can all be bent - changing the behaviour of the switch. The spring itself can be removed, inverted and reinserted - which makes it offer more resistance.

The activation force can be increased to at least 30 cN - or reduced to next to nothing. The activation distance can be increased or decreased. The strength of the tactile feedback can also be increased or decreased.

There are a number of drawbacks to using these switches in a keyboard:

• Cost: these switches cost a little more than one UK pound each;
• Large size: these switches are huge;
• Too long: these switches are longer than a typical keycap;
• Awkward pins: these switches are not designed for PCB mounting;
• Lifespan is not great - these switches are rated at 1-10 million operations;

However, the activation force is certainly extremely low - and the tactile and auditory feedback seem to be of excellent quality.

Indeed, as far as the feel of these switches go, they are near perfect - I have difficulty in imagining a simple mechanical switch with a better feel. In the far future, servo mechanisms may be used to provide tactile feedback - but until that happens, switches like this one appear to be attractive as keyswitches.

I have used this switch to replace the left button of my mouse.

So far I have constructed three keypads using these switches. See here, here and here for more details. The results are excellent.

I am currently constructing a full keyboard using these switches.

These switches can be obtained from [RS components].

Omron microswitch

Interesting - but not as good as the Saia-Burgess switch - from my point of view.

The switch costs half-as-much again - and it may be more robust. However, the default spring force is greater, the audio feedback is not quite as good - and fitting keycaps looks as though it will be much more difficult.

Other microswitches

I quite like the idea of a lever - it alows a configurable tradeoff between activation distance and activation force to be made.

I bought these switches because the specifications claimed low activation forces. However, these switches were useless to me: the activation forces were far too high. The manufacturers must have been measuring the activation force at the far end of the lever.

I bought all the switches pictured here from [RS components].

Tactile feedback

I hypothesize that direct spinal circuits can be formed to deal with relaxing upon encountering tactile feedback - based on existing withdraw-on-finger-prick circuitry.

By contrast, auditory feedback necessarily has to travel via the brain - which is slower - and thus less effective.

Often good quality tactile feedback seems to require high activation forces.

The G3M1T1PUL microswitches show that very high quality tactile feedback is possible while retaining a very low activation force - provided the activation point is not constrained to be at exactly the same position as the deactivation point.

Saia Burgess G3 part numbers

Links

This site

Saia Burgess

Saia Burgess retailers