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Looking to use a strain gauge as a trigger (Z-stop on 3D printer). I also have some other applications for this circuit. Essentially, I want the output from the op amp (TL072 - because of high slew rate, c. 16V/us) to go HIGH as soon as the tiniest voltage is sensed on the Wheatstone bridge. I'm told having it open like this is the way to go but want to know if this circuit will work. I've included a trim pot to balance/tune the bridge as I presume I want the output to be as close to zero as possible before it triggers. The BF350 resistance varies between 349 Ohm and 351 Ohm so with trim pot RV set to give 0V on the output with no strain, if my calculations are correct, the voltage should push towards ~±2.4mV. I think the Arduino needs about 1.85V to register HIGH on a GPIO input.

Very new to electronics so be kind!

Fritzing schematic for Wheatstone bridge

Edit:

Could I use one of the dual op-amps as an amplifier and the other as a comparator? Ignore the value of R3, it was the default and I have no idea what it should be yet

Proposed solution 2

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    \$\begingroup\$ I had never seen a strain gauge used as stop limit switch. Maybe there is a reason behind that. \$\endgroup\$ Commented Oct 5, 2021 at 12:20
  • \$\begingroup\$ Probably because it's simpler, and cheaper, to use a simple mechanical switch but where's the fun in that? Gluing a strain gauge to the printer and calibrating it would be too much for a cheaply mass produced part. Anyway, this isn't the only application I have for these strain gauges. Wondering whether a Schmitt trigger might better suit my needs \$\endgroup\$
    – R Spandit
    Commented Oct 5, 2021 at 12:27
  • \$\begingroup\$ Use a comparator if you want fast rather than an opamp. Have you considered the effect of noise and drift? \$\endgroup\$
    – Kartman
    Commented Oct 5, 2021 at 12:37
  • \$\begingroup\$ @Kartman, only just reading about comparators - ideally I'd like something that will switch in under 0.1us. I haven't really considered noise and drift yet. \$\endgroup\$
    – R Spandit
    Commented Oct 5, 2021 at 12:44
  • \$\begingroup\$ How far will your 3D printer move the Z axis in 100 nanoseconds? How quickly can the Arduino react to the signal? Is 100 nanoseconds a good requirement? \$\endgroup\$
    – JRE
    Commented Oct 5, 2021 at 12:49

1 Answer 1

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The bridge presumably has a differential voltage close to 0mV +/- some amount with no pressure. U1 has some offset (+/-4mV in the case of the TL072 if it is operating properly, which it is not in this circuit, but more on that later). So you could use an external resistor to offset the input so it is in a predictable state before and after pressure is applied.

If you want to use an op-amp as a comparator it needs to operate on the voltage rail available (minimum 4.5V in the case of the TL072). Also the input range and output swing have to be adequate for the job.

The usual advice here is to use a comparator, however comparators are typically not great at dealing with very low input differences- they are optimized for high speed with relatively large input voltages, so they have relatively low gain and relatively large offset voltage limits.

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  • \$\begingroup\$ I hadn't realised that the TL072 needed such a high voltage. Looks like it might be the wrong component for the job. Can you recommend a comparator with a low switching speed? AD790? \$\endgroup\$
    – R Spandit
    Commented Oct 5, 2021 at 12:41
  • \$\begingroup\$ Maybe something like the LMV76x but the offset voltage is +/-1mV and 225ns prop delay is specified with a 5mV overdrive. Most real mechanical systems are have resonances, at best (i.e. very rigid constructions), in the kHz range so not sure where your 100ns requirement is coming from. \$\endgroup\$ Commented Oct 5, 2021 at 12:47
  • \$\begingroup\$ I need to learn what overdrive means - told you I was new! \$\endgroup\$
    – R Spandit
    Commented Oct 5, 2021 at 12:50

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