Looking for comment on strain gauge to digital board - Page 1
Forum section: electronics / projects

author: tobiasbp
author_rank: Newbie
timestamp: June 17, 2022, 05:28:48 pm
content: Hello electronics gurus...I'm working on a circuit to register the presence of a human (Or anything heavy) on a plate resting on 4 strain gauges (A weight). I'm not interested in weight of the human, only the presence. The circuit is to turn on a load (Like an LED strip or an inductive load like a motor) connected to the board, when presence is detected. It should also present a logic level output (Active high), as well as activate an optocoupler for interfacing with other circuits. S+ and S- are the input from the strain gauge.Before I design the board and place an order for it, I'd appreciate any comments and/or suggestions for changes. Please take a look at the attached pdf (Schematic from KiCad).

author: Benta
author_rank: Super Contributor
timestamp: June 17, 2022, 07:46:05 pm
content: Sounds an expensive solution to me. Why not just use a pressure mat? Supermarkets have them at the entrance to count (or at least approximate) the number of customers in the shop.

author: tobiasbp
author_rank: Newbie
timestamp: June 17, 2022, 09:08:46 pm
content: This board is for a gaming application (Dancing game, DDR). I'm not sure pressure mats have the needed accuracy? Also, some load (Like LEDs) need to be activated, such a function is probably not a feature of retail pressure mats? Please post a link to an example of what you are suggesting.

author: Benta
author_rank: Super Contributor
timestamp: June 17, 2022, 09:25:46 pm
content: I'm sorry, but you've lost me. In your OP you ask about presence detection only, and now suddenly about needed accuracy?There are pressure mats for supermarkets, home alarm systems, senior citizens etc. They all detect "presence".

author: tobiasbp
author_rank: Newbie
timestamp: June 18, 2022, 07:59:47 am
content: Thank you for spending time on this . Would you be able to comment on the schematic I have included in my original post?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 22, 2022, 06:42:21 pm
content: You could start by running the annotation, so all schematic symbols have a proper RefDes which makes communication much easier.Second thing to do is to get rid of ERC error arrows, they don't belong in a PDF. It's in:Schematic Editor / Inspect / Electrical Rules Checker / Delete Markers(in the left bottom of the window).The diode from the spice library is also disproportionately ugly, you can use the simple "D" from the device library (which is open) or "D_ALT" (which has a filled body).

author: tobiasbp
author_rank: Newbie
timestamp: July 02, 2022, 03:39:53 pm
content: Quote from: Doctorandus_P on June 22, 2022, 06:42:21 pmHello. Thank you for your inputs. I have updated the schematic as you suggested (See attached PDF). If you have the time, any other comments you might have, would be very appreciated.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 02, 2022, 04:50:05 pm
content: LM324 has an offset voltage of upto 5mV. If you multiply that with 47k/100 you already have 2.35V of offset. That's a lot. Your wheatstone bridge is powered from 5V, so U1A may have a 2.5+2.35 = 4.85V output, and U1V an 2.5-2.35= 150mV output just from the offsets. (but it's extreme). The strain gauges themselves also have tolerances, so it could be even worse. I suggest to add some kind of offset compensation, or increase the value of R2 to get a bit less amplification. (What is the raw differential output voltage of your strain gauge bridge?)The differential amplifier U1C only works properly if the resistances in both branches are balanced. I don't know the value of RV1, but I'd probably add a 47k resistor between R5 and RV1. If you want to keep VR1 in this position (see below) then use a 100k resistor, which means it's approximately 47k of resistance when the wiper is in the center.AMS1117 is a not very great voltage regulator, and I think it needs a lot of capacitance on it's output, something like 22uF. Check the datasheet for that. Your circuit is also relatively low power. Your voltage regulator needs a minimum output current for it to regulate properly, and I do not know if that condition is met.Maybe add a resistor in series with D10 to tame the peak detector a bit.You can put D40 in series with the IR diode of U40, which reduces current consumption and shaves a resistor of your BOM list. (voltage drop of IR diodes is typically 1V).Why Q40 and Q50? Why not use a single fet for both functions?I like the way you numbered your parts, with all the "30'ies" around the comparator etc, but I do not like the boxes around sections much. They do nothing that a bit of empty space does not do equally well, and such "dress-up" tends to soak time during the initial drawing and further revisions.I am not entirely sure your "instrumentation amplifier" will work as intended (because of those offsets).I would build this on a breadboard before making PCB's (Or at least simulate it).An Idea that might work better:1. Replace RV1 with a 47k resistor to GND so you have a "normal" instrumentation amplifier.3. Put a big resistor (Maybe 1Meg) between pin 3 of U1A and pin 2 of RV1.With this modification you can pull your bridge a bit sideways (either up or down) to compensate for both offsets in the bridge itself and for offset voltages of the opamps. You can calculate the value of the "big resistor" for the adjustment range you need.The whole setup is not very accurate, but you don't need that, but you do have to check if changes in temperature may throw your circuit off balance.It may be convenient to add some test points to your PCB and some mounting holes. I normally add the mounting holes to the schematic. This ensures they never get deleted accidentally from the PCB.KiCad V6.0.6 was released about a week ago. There is approximately an update with bug fixes (which increments the 3rd digit) once each month.

author: Ian.M
author_rank: Super Contributor
timestamp: July 02, 2022, 07:04:01 pm
content: Most strain gages have resistances in the hundreds of ohms to low kiloohm range. That's low enough you don't need an instrumentation amp (especially not one scratch-built out of low spec OPAMPs and discrete resistors), as the lower input impedance of a simple OPAMP differential amplifier is still many times higher than the source resistance of your strain gage bridge. OTOH the disadvantage is you may need to trim to match the +in and -in resistor pair ratios to get acceptable CMRR with commodity resistors. An instrumentation amplifier IC with its precision factory trimmed resistor network avoids this. You'll still need to trim for bridge balance but that's probably easier to do with a low-ohm variable resistor in one arm of the bridge, and a balancing resistor of half its value in the arm on the other side.Your peak hold circuit is rather iffy, the major problem being the Vf drop of the diode vs its If current which will make it highly sensitive to the width of the peak. Its probably preferable to go straight to the comparator stage, use a real comparator IC and if nessercery stretch its output pulse with a retriggerable monostable, to get much the same result, but with greater threshold precision.