1
\$\begingroup\$

I am trying to solve a work related issue. Long story short, I have a circuit that monitors the resistance between two electrodes. When an object passes by these electrodes, the resistance reduces drastically, the circuit then measures this and activates another circuit.

I want to replicate this, mainly due to a couple factors, one being that the circuit doesn't work properly due to some external issues but also to provide a temporary solution till we get a fix!

I read somewhere on here that it can be done with a microcontroller, but no further explanation was given. I've got a lot of electrical experience but less so in electronics or programming, so some heavy advice and guidance or even something to reference so that I can learn more, would be greatly appreciated! I figured I'd post this to try and learn at the very least!

EDIT: (at the request of some comments I read)

I don't have specific values as I don't actually know them, I only really know the principle of operation which is, really quite difficult to explain, the above is as simple as I can make it with my current level of knowledge! I'm sorry!

\$\endgroup\$
9
  • 1
    \$\begingroup\$ Welcome to the EE.SE site. Please edit your question including minimum info to make it clear and non-ambiguous (e.g. values of resistance, time to detect change, principles of operation of other equipment). A schematic may help too: the schematic tool that you see when you edit your question is easy to use. Include also your attempts and ideas for the solution. \$\endgroup\$
    – andrea
    Commented May 10, 2021 at 10:30
  • \$\begingroup\$ Is there a detectable change in current when the object passes through? Can an industrial current sensing unit be used? Or can a voltage change be detected? If so, and because it is for an industrial application, I would recommend using industrial 24 V modules and a micro PLC (Siemens LOGO!, A-B, Mitsubishi, etc.) to do the control if you don't already have a PLC. This way you will have support and spares from any automation engineer rather than trying to debug a hobby electronics board which is not industrial hardened and uses 5 V GPIO instead of opto or relay isolated 24 V IO. \$\endgroup\$
    – Transistor
    Commented May 10, 2021 at 13:28
  • \$\begingroup\$ @Transistor, you bring up valid points regarding opto isolation. the original board has this and is far more professional and safe for the application. however the way this project has ran on it has lost alot of internal support, I am just looking to provide a temporary solution, which will work in practice. I am not sure if its measuring a change in resistance, or a change in current, I just know it monitors the resistivity between 3 electrodes, and when something passes the first, it activates 1 section of an and gate, and when it passes the second it activates the same and gate to provide- \$\endgroup\$
    – Diesel
    Commented May 10, 2021 at 18:02
  • \$\begingroup\$ - an output, what I want to replicate will do the same thing I hope? I want it to monitor the change in resistance by a hopefully, measurable amount, even if its current I assume its very similar? again so far out my depth but I want to at least try and learn for the process! \$\endgroup\$
    – Diesel
    Commented May 10, 2021 at 18:04
  • \$\begingroup\$ I think you're going to have to reveal more details of the product, the sensing and what it is that you're doing that can't be done with standard industrial sensors such as reflective or through-beam sensors. Why do you need to detect conductivity? And why does it need three contacts rather than two? \$\endgroup\$
    – Transistor
    Commented May 10, 2021 at 18:10

3 Answers 3

0
\$\begingroup\$

Arduino can do this (microcontroller board). You will want to make a voltage divider, where one resistor is fixed and another one is the one which changes resistance. The output of this voltage divider will therefore change as the resistance changes. Arduino can poll that value all the time and notify or do whatever else (e.g. activate smth else) when you want.

Difficulty: minimal

Required skills: most basic arduino skills, namely GPIO, ADC, nothing more. This one you can figure out in a pair of days.

You will need to watch like 5 Arduino videos for that, the very intro, nothing more really. Unless you need to activate smth complex there, but it's probably nothing one can't do with an arduino.

\$\endgroup\$
4
  • 3
    \$\begingroup\$ Alternatively you could learn microcontroller programming proper, starting from scratch with no pre-made libs. Takes much longer, but you end up as an embedded systems programmer as a result, with the actually ability to trouble-shoot microcontroller problems. \$\endgroup\$
    – Lundin
    Commented May 10, 2021 at 11:07
  • 1
    \$\begingroup\$ Alternatively, you could learn how processors work, design your own risc-v core in your own microcontroller and suit it to your own needs. The question is, is it really necessary in this situation? :D (besides, Arduino is a fine start anyway, it only takes some will to move forward from it, if necessary) \$\endgroup\$
    – Ilya
    Commented May 10, 2021 at 11:15
  • 1
    \$\begingroup\$ The problem with Arduino is that lots of people get stuck with it, then use it for all manner of DIY applications it was never intended for. They become carpenters with one tool. Cut a board? Use a hammer! Screw a screw? Use a hammer! Drill a hole? Use a hammer! Yeah... you probably can use a hammer for those things, but it wasn't intended for that and the end result will be somewhere between bad and terrible. Also for this metaphor to make sense, we probably have to picture Arduino as one of those blown-up hammers you get as reward at amusement parks for winning in whack-a-mole... \$\endgroup\$
    – Lundin
    Commented May 10, 2021 at 11:32
  • \$\begingroup\$ WELL! I think I understand what a voltage divider does now! thats awesome! I assume Arduino is something like a raspberry pi? Honestly not familiar with it but I can look into it easy enough! but what you've described sounds like exactly what I want to do, so once the value changes, which I think it reduces resistance, I would just then tell it to activate the next circuit, which is essentially going to be some form of counter/notification to say its happened. at least thats my initial plan, I feel so far out my depth but I want to get good points from work for trying! :D \$\endgroup\$
    – Diesel
    Commented May 10, 2021 at 17:46
0
\$\begingroup\$

I think you can fix your issue by using an "obstacle detection" project. If you insist to do it with any resistance obstacle detector, you can use LDRs. Ligh Dependent Resistor, for example if something which you want to detect is moving on a convoy rail or something like this, you can use a LDR, and a light bulb at the other side of it, and when your object passing between the lamp and the LDR (LDR is connected to a microcontroller like Arduino) you can send a command to your next step circuit (like using a relay in arduino's output).

You can find the code and schematic at many websites like https://www.instructables.com/ or https://hackaday.io by seaching "object detection" , Arduino object detection" or "Arduino object detection usind LDR" & etc. .

Feel free to ask more or for clarifying this answer.

Best of luck

\$\endgroup\$
1
  • \$\begingroup\$ Thanks for the swiiift reply! It has to be a change of resistance as, its detecting an "Object" under water. the original circuit has been designed using just microcontrollers in a circuit but, I am unsure how to properly interrogate it and I don't have the original program. so its a bit risky and might be easier just redesigning my own, simpler circuit \$\endgroup\$
    – Diesel
    Commented May 10, 2021 at 17:42
0
\$\begingroup\$

The basic idea about measuring resistance is to supply the observed resistor with constant current source what creates voltage drop proportional to its resistance. Lets say if you supply 5k Rsense resistor with 425uA the voltage drop on this resistor is 2.125V. If you change this resistor to 2k the 425uA creates 0.85v voltage drop.

The simple current source can be made with two transistors called current mirror, in both legs (Rset and Rsense) flows equal current so you can set Rsense current changing the Rset resistor. The Rsense must be less than Rset for proper operation.

enter image description here

\$\endgroup\$
1
  • \$\begingroup\$ It now turns out that he's detecting fish underwater. \$\endgroup\$
    – Transistor
    Commented May 10, 2021 at 23:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.