Of FSRs and thermistors You've seen the photo,

3D Printing
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1

Of FSRs and thermistors

You’ve seen the photo, I have the FSR’s installed and tuned (show 80k ohm normal, with 300g they show 10k-15kohm)

I have a voltage divider, but the wireing harness they include won’t plug right into the Z_min endstop. Remembering that a member mentioned using a thermistor input, I jacked the leads into T1 and updated the firmware.

But I’m missing something important (like a boolean input from an analog source)…how do I set a threshhold that tells the system ‘yeah, the nozzles contacting the bed.’?

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“Buck up and make a harness for the voltage divider” is an acceptable answer.

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You can’t set a voltage threshold on a digital input, it’s done in hardware. You would have to use a digital pin to do that.

You can, however, adjust the ratio of voltage to pressure on the FSR by adjusting the strength of the pull-up resistor. I posted a suggestion to use a potentiometer as the pull-up for this purpose, but nobody seemed interested.

4

Well, the problem with directly using a digital input is that they don’t neccesarily have a one single voltage where the registered input suddenly goes high (or low) - for TTL compatible parts, voltages below 0.8V must register low, inputs above 2V must register high, but that whole area between those two voltages is not defined - it can high or low at any point and it would even be acceptable for the logic to oscillate between states.
So what you need for the FSRs to work on a digital input is basically a comparator (http://en.wikipedia.org/wiki/Comparator), or preferably a schmitt trigger (http://en.wikipedia.org/wiki/Schmitt_trigger) with a fairly low hysteresis. Add in @Whosa_whatsis ’ potentiometer and you can even set the trigger pressure level to any value you’d want.

When you’re configuring the firmware to use a thermistor input, you’re basically also turning it into a digital input.

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@Thomas_Sanladerer You’re right that there is a zone of uncertainty in FSR readings, but because they go from ~5MΩ to <50Ω over essentially zero travel, I think it’s pretty safe to assume that the time it takes to cross that zone will be pretty short, likely shorter than the sampling rate, assuming the right pull-up value is chosen.

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I’ll throw the voltage divider in there this evening. I was just hoping there was something in the firmware that would treat the analog input as a switch…something like ‘if z_min_temp < 40 then z_min_endstop = 1’