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I have an application in which I need to detect a button press on top of a platform which has a load (0.2kg ~ 0.5kg). The press is executed on the load.

Ideally I think what want is a tactile button with a holding force of greater than the load + margin. However I cant find one which such holding force and low profile (which is important for the application).

Tactile switch

So I'm considering building a small mechanism on top of the detection circuitry to provide the holding force (this click mechanism also allows for other detection methods).

The behavior I'm looking for is as follows (ranges and forces are rough estimates): enter image description here

I'm avoiding load cells and other analog reading methods due to energy requirements (device is battery powered with a small battery).

So basically what I want/need is either how to design such a mechanism or if it is possible to source some sort of "non-linear low profile spring" in specific force thresholds.

Wesley Lee
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  • How much movement is the platform allowed to have? – ratchet freak May 20 '16 at 07:52
  • 5mm max, but I'd like to stay way below that (1~2mm if possible) – Wesley Lee May 20 '16 at 08:07
  • You can have extremely low power load cells. Op-amps come in low power versions which use a fraction of a milliamp, and circuits that use a load cell (strain gauge) will use equally low power. – Jodes May 20 '16 at 09:46
  • What if you install a couple springs (or equivalent) to "share" the mass loading? That way the on/off load limit on the switch itself is much less but the assembly performs to your specs. – Carl Witthoft May 20 '16 at 11:16
  • @Jodes Load cells are not out of question yet, I'd just get much more battery life if I find a mechanical solution (which has its cons of possibly more complexity). Op-amp consumption itself is not the main problem, but summing that with polling an analog port in a speed fast enough not to miss presses + post processing, etc... – Wesley Lee May 20 '16 at 11:44
  • @CarlWitthoft That is indeed the easiest option, however I have 2 reasons to want some sort of non-linearity: although the load is relative if compared to the button press I have many different loads, so picking a spring force might not be that trivial. Anyway the "click" mechanism would be nice aswell even if I dont use Tact switches. Some accelerometers have sleep modes with wake-up on impact, which would lead to not needing to "share" a load at all (provided I have the "click") – Wesley Lee May 20 '16 at 11:47
  • Actually when I said op-amp I meant comparator. Use a Schmitt trigger configuration, this would mean no analogue port necessary, and a simple digital input could be used, and better yet use a microcontroller with a wake-up on interrupt on a level change of a digital input – Jodes May 21 '16 at 12:21

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