Replacement for a strain gauge that simulates a mouse click

Hope someone can help…

I have a controller (possibly a Nano) which I have not had any input in developing or coding for - so suffice to say, in reality it’s just a black box to which I do not have access (until I go to install the modification*).

It has other inputs but for this modification I am just interested in the strain gauge connected to the controller which simulates a mouse click when pressed.

I need to replace the strain gauge with a slotted optical switch for improved reliability.

I know I will need power for the optical switch and hope to take it from the controllers PSU and furthermore, I believe that it is highly likely that the strain gauge is connected to an analogue input and registers any applied pressure as a ‘mouse click’ when the resistance of the strain gauge goes low enough.

Can anyone assist me with a possible circuit solution? I have thought about using an SSR driven by a transistor but TBH I do not know if this is the way to go and if there are other potential solutions with other SS components.

  • I do not have access to the physical box and controller (but will have when I retrofit the modification) - its a six hour drive away and thus expensive to get to…

Many thanks in advance…

So you are trying to devise a modification to circuit that you don't know that is running code you don't have?

Sounds like a fool's errand. Is it possible to have somebody else snap some pics of the circuit and send them to you?

You really need to know the response characteristics of the strain gauge if you hope to replace it with something that will behave in a similar manner without changing the code.

+1. And I'd say rebuild the device woth your code, that's easier.

Thanks for the replies and guidance.

The strain gauge is part of a PC based stand up and interact game played by young and old with user activated mechanisms for the inputs (to move the cursor on screen and mechanical 'trigger' to 'click' the mouse.

The problem is that the original implementer spent only a few pence in designing the holder for the strain gauge and it literally gets 'hammered' for 8 hours a day and only has a 2mm rubber cover for protection - so they don't last long (and why I have been asked to find a solution that does not use strain gauges...).

I also know that:

  • the strain gauge was chosen in the misguided belief that a strain gauge has superior operation reliability over a mechanical switch and its sole purpose in life is as a switch for mouse clicks (not click and hold - just a momentary mouse click) which on a busy day can be clicked upwards from 10,000 times
  • if the wires are shorted it registers as a mouse click (I know this because of damage to a previous strain gauges that I have had to replace over the last year or so when I noticed that it was randomly creating mouse clicks when moved within its holder).

Given that a shorted strain gauge causes a mouse click I could introduce another controller with a relay output (and input trigger based on information from this website but I am sure that there is probably a more elegant solution hence my original post.

Looking forward to a few more words of guidance... :O)

Thanks in advance...

What you're asking for is an education.

The easy way: measure resistance when no pressure applied. Measure resistance with pressure applied. get a switch and 2 resistors ... I think you can figure out the rest by yourself :slight_smile:

...and I thought I would post on this website and have a nice conversation picking the brains of a group of friendly knowledgeable people... :confused:

..What did I say? Was my initial posting and ask for help/guidance all wrong? Are you always so tough on anyone that want to join the clique?

The reason to go with slotted optical switch is because it will be non-contact switch and space is very limited... I would try a magnet and reed switch with a 1k resistor or just a little less (which is about the resistance on clicking/pressing) but space will not allow the magnet to move far enough away from the reed switch.

As to a small mechanical switch - it is not contactless and thus there will be a certain amount of wear of the mating surfaces (no way to get perpendicular travel) and given that one moment it could be a child activating the switch and the next moment an Arnold Schwarzenegger wannabe I can't fully remove the potential for it to be 'hammered' which would shorten the life of any switch and brings me back to square one.

I will experiment and see what I can come up with.

Thanks for ALL of your help, guidance and boundless wisdom.

TTFN.

The "problem" with your first posting was

  • the only valid answer was "just do it and come back when you face a problem"
  • didn't you look uo the datasheet? Nobody will do that for you
  • did you do at least 5 minites of searching befor posting?

Harko:
The problem is that the original implementer spent only a few pence in designing the holder for the strain gauge and it literally gets 'hammered' for 8 hours a day and only has a 2mm rubber cover for protection - so they don't last long (and why I have been asked to find a solution that does not use strain gauges...).

Looking forward to a few more words of guidance... :O)

One approach would be to design a better holder/protector for the "strain gauge" rather than changing horses.

But judging from your JPG, it is an FSR, not a strain gauge; if so, you may get some advice on creating a more durable system by talking with the folks at FlexiForce Load/Force Sensors and Systems | Tekscan

When I experimented with piezo disks as force-sensing buttons, the suckers were sensitive down to a very light touch yet also took being banged on with a screwdriver handle hard enough to make the table jump. I literally had to set the disk up to fail before one did.

If you have 2 metal surfaces separated by small distance and one is grounded while the other has voltage, that makes a capacitor. If the distance between the surfaces shrinks, the capacitance goes way up and the time to fill it from ground LOW to digital HIGH also goes way up. This is how membrane keys work, look inside of most PC keyboards and you will see a membrane.

Problem with these is you need to code for them. They aren’t weird switch replacements and TBH without knowing what code reads those force sensors — if it was me I’d try things just to find out what does make what differences.

DaveEvans:
One approach would be to design a better holder/protector for the “strain gauge” rather than changing horses.

Unfortunately my hands are tied on this one. I am restrained to a small cylindrical space of about 20mm dia and 30mm long. if I could somehow make more space I would…

DaveEvans:
But judging from your JPG, it is an FSR, not a strain gauge…

I think you are right. (Electronics is neither a job or hobby…I do mechanisms/PCs/software and modular stuff - not electronics. I was told it was a strain gauge and didn’t know any better.)

GoForSmoke:
… piezo disks as force-sensing buttons…

…If you have 2 metal surfaces separated by small distance and one is grounded while the other has voltage, that makes a capacitor. …

Thanks for the info. I didn’t know about piezo discs… I need to do some research. With regards to implementing a capacitive based switch, at this time I really believe I have no way to establish a good grounding to give me a good reliable switching so i am going to rule this one out…

Now…

After searching FSR’s, on the Pololu website I have found what looks like the sensor fitted… and (to quote from the website):

“In our tests, the resistance exceeded 1 MΩ with no applied pressure and ranged from around 100 kΩ to a few hundred Ohms as finger pressure varied from light (a few dozen grams) to heavy (pressing as hard as possible).”

If the resistance varies from >1MΩ to 100KΩ (or lower) then I should be able to simulate this with a variable resistor (so I can fine tune it to simulate an average press on the FSR) with values from say 0Ω to 200KΩ in series with a relay output of a new intermediate controller.

I would still like to go the route without the additional controller but this way seems to be the best (ATM)…

…Now, does anyone have recommendations for a good, quick and reliable 3D printing service? I need to get a holder made for a slotted optical interrupter.

PS> I have found an old photo of the FSR assembly (attached)…

Capture.JPG