# Dirt cheap way to detect change in resistance over tiny changes in wire length?

#### Tonyr1084

Joined Sep 24, 2015
7,247
then measure the voltage that the motor is generating.
The TS used a "Train" as a mere example of a moving "Something". Basically he's building a loop circuit with two wires, one going up each side and something bridging those two wires, creating a loop.

Here's what I perceive the TS intention to be. For whatever reason ? ? ?

#### Captain Bob

Joined Nov 9, 2022
7
The TS used a "Train" as a mere example of a moving "Something". Basically he's building a loop circuit with two wires, one going up each side and something bridging those two wires, creating a loop.

Here's what I perceive the TS intention to be. For whatever reason ? ? ?
View attachment 280344
Tonyr1084's diagram is exactly what I am going for. There is no motor in the train. The train would just be physically moved by a person. What I am ultimately trying to create is a "red light / green light" game where the game itself will be the judge of whether or not you are moving during a "red light".

Thank you to everyone for your responses so far.

#### joeyd999

Joined Jun 6, 2011
4,696
Tonyr1084's diagram is exactly what I am going for. There is no motor in the train. The train would just be physically moved by a person. What I am ultimately trying to create is a "red light / green light" game where the game itself will be the judge of whether or not you are moving during a "red light".

Thank you to everyone for your responses so far.
Just slap wireless-enabled accelerometers on all the kids!

#### Jerry-Hat-Trick

Joined Aug 31, 2022
232
How about a quadrature encoder turning with the wheels, presenting different resistor values across the tracks? The turning torque maybe too high and if it's sitting on the edge of a transition it may give a false 'movement' reading so you'd need to process the measured value to detect when both channels are changed.

Or using a small dc motor, turned by the wheels, which will generate a measureable voltage?

#### WBahn

Joined Mar 31, 2012
28,172
Tonyr1084's diagram is exactly what I am going for. There is no motor in the train. The train would just be physically moved by a person. What I am ultimately trying to create is a "red light / green light" game where the game itself will be the judge of whether or not you are moving during a "red light".

Thank you to everyone for your responses so far.
So where does the 2 mm criterion come from? That seems pretty small for something that is being moved manually.

Electrically, there is going to be all kinds of changes in the contact resistance as the thing is moved. That might be useful as your indicator of movement -- depends on just how the object connects to the track.

#### Tonyr1084

Joined Sep 24, 2015
7,247
With the introduction to the idea of detecting a kid moving 2mm; breathing alone can deviate the location by that much. And I have yet to see a kid that can sit still. Unless he's fast asleep or down with fever.

Is this something the child moves by hand? Again, the more detail you can give the greater detail we can give you in return for a solution. Even if we fail to come up with a solution at the very least we can point out flaws in your project.

I'm now wondering if maybe something tapered with tapered contacts and a non-conductive insulator that as it moves it can change a capacitance, which can then be converted into an audio signal. You won't need a speaker, but rather a F to V chip (Frequency to Voltage). Then as the object is moved the capacitance changes, changing the frequency which is then converted into a voltage value. THAT can be easily detected by a meter.

You can take that a step further with a window comparator who's reference voltage is buffered by a capacitor and resistor. As the value slowly changes the comparator sees the minor changes but because the change is so slow you don't pass the upper or lower threshold, thereby not triggering an alarm. But if the child exceeds the thresholds then the comparator outputs a signal triggering an alarm.

To be honest, it would take me a month to figure all that out. Someone else here might already know a solution. But I toss that out there as a seed. Perhaps someone else may take that in a whole new direction.

#### Tonyr1084

Joined Sep 24, 2015
7,247
As the inner core moves back and forth the spacing between the copper plates and aluminum foil either opens up or closes down, changing their capacitance. If you read that change you can detect movement. Then by means of electronics you can determine what is acceptable movement when someone is supposed to be still.
If you've ever seen how some people build a crystal radio using a cardboard tube wrapped in foil and slipped over a smaller tube also wrapped in foil you can see how the capacitance changes. And that's not even tapered. It's just putting more material closer together by changing the amount of mass in proximity to a stationary mass.

#### Tonyr1084

Joined Sep 24, 2015
7,247
From time stamp 0:35 to 2:25 the YouTube video shows how to build a variable capacitor.
From time stamp 6:04 to 6:15 the video demonstrates how sliding the tubes changes the capacitance.

#### Captain Bob

Joined Nov 9, 2022
7
Yes, the 2mm is arbitrary. I am hoping to detect "slight" movements, where "slight" is open to interpretation.

My thought was that the "train" would be a conductive disk just resting on the wire rails via gravity. Movement of the train would come from a kid's hand.

#### Ian0

Joined Aug 7, 2020
6,983
Yes, the 2mm is arbitrary. I am hoping to detect "slight" movements, where "slight" is open to interpretation.

My thought was that the "train" would be a conductive disk just resting on the wire rails via gravity. Movement of the train would come from a kid's hand.
It makes it much simpler if it doesn't supply power. If you can supply a voltage to both ends of the same rail, then measure the voltage on the other rail, it will eliminate most of the noise. (The "metre bridge" principle.) Stainless steel is an alloy which is readily available and has quite a high resistivity (10 times that of copper), and the added bonus that it doesn't corrode!

#### panic mode

Joined Oct 10, 2011
2,401
if you do want to use resistance then ... use resistance.
use PCB as one or both tracks, split the copper into desired number of segments and put resistors between them. the disc or whatever is a wiper of your custom "linear potentiometer".
dirt simple to design and cheap to order if you want nice looking boards, with some mounting holes, can be ordered assembled too. just one resistor type is needed and they are cheap as dirt... make it double sided so that resistors are on the bottom side (not side contacting disc).

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#### WBahn

Joined Mar 31, 2012
28,172
if you do want to use resistance then ... use resistance.
use PCB as one or both tracks, split the copper into desired number of segments and put resistors between them. the disc or whatever is a wiper of your custom "linear potentiometer".
dirt simple to design and cheap to order if you want nice looking boards, with some mounting holes, can be ordered assembled too. just one resistor type is needed and they are cheap as dirt... make it double sided so that resistors are on the bottom side (not side contacting disc).
Using the original 2 mm spec, that would be a lot of resistors (18 in / 2 mm) = 229 segments (can get by with just one track being segmented). Also, the PCB has to be laid out so that it can work with segments that are on the order of 0.1" long. Then there's the matter of ensuring that the disk, whatever it is, makes adequate contact with the copper tracks, which probably precludes the use of a solder mask on the board.

#### Jerry-Hat-Trick

Joined Aug 31, 2022
232
I'm now wondering if maybe something tapered with tapered contacts and a non-conductive insulator that as it moves it can change a capacitance, which can then be converted into an audio signal. You won't need a speaker, but rather a F to V chip (Frequency to Voltage). Then as the object is moved the capacitance changes, changing the frequency which is then converted into a voltage value. THAT can be easily detected by a meter
This triggered a similar idea. Using strip board, where the copper strips are like the railway sleepers with alternate strips shorted together and the “train” has the same on its base, with thin insulation between, so the (series) capacitance varies as the train slides over the rail with cycle length of 2.5mm. Need to sketch this to be sure of my logic but I’m pretty sure it would work

#### MisterBill2

Joined Jan 23, 2018
14,230
The cheap and simple way to detect very small changes in resistance is to use the "Kelvin Connection" approach. For that to work the conductors must be a material that does not develop a surface resistance, so I suggest some alloy of stainless steel, or else some variety of nichrome wire or rod. These must be insulated from everything, but there is no shock hazard.
The system will use a constant current generator to pass a current through the conductors that flows up one, through the ball of whatever, and back through the other conductor.. The regulated current should be stable and not noisy.
A second set of wires connects to the same ends as the current source, and displays the voltage developed across the resistance. Sensing a change in that resistance can trigger a comparator to announce any movement.. OR, if an audible detection will work, the amplified voltage can control the pitch of an oscillator so that a change in tone will be simple to sense.

An arrangement that will allow even greater sensitivity, and also allow the use of aluminum, or any other material, rods or wires, would be a device often called a "String Pot", although the correct name is closer to "Linear Displacement Sensor". IIT consists of a potentiometer connected to a spring return reel that a flexible cable rotates as it is pulled off the reel. Those are available in standard ranges up to many feet. They are far from cheap.

#### joeyd999

Joined Jun 6, 2011
4,696
The cheap and simple way to detect very small changes in resistance is to use the "Kelvin Connection" approach. For that to work the conductors must be a material that does not develop a surface resistance, so I suggest some alloy of stainless steel, or else some variety of nichrome wire or rod. These must be insulated from everything, but there is no shock hazard.

The system will use a constant current generator to pass a current through the conductors that flows up one, through the ball of whatever, and back through the other conductor.
Yup. Works well with N gauge rails.

#### Kjeldgaard

Joined Apr 7, 2016
476
If there is access to the far end of the "rails", then this idea might work as four-terminal measurement:
Code:
Local end of "Rails"
Cur1  Mes1
|     |
|     |
|<--->| Moving  "Wagon"
|     |
|     |
|     |
|     |
|     |
|     |
Cur2  Open
Far end of "Rails"
Function:
• Requires a relatively high resistance in the "rails"
• Send a suitably high constant current from Cur1 to Cur2
• Measure the voltage between Cur1 and Mes1

#### panic mode

Joined Oct 10, 2011
2,401
Using the original 2 mm spec, that would be a lot of resistors (18 in / 2 mm) = 229 segments (can get by with just one track being segmented). Also, the PCB has to be laid out so that it can work with segments that are on the order of 0.1" long. Then there's the matter of ensuring that the disk, whatever it is, makes adequate contact with the copper tracks, which probably precludes the use of a solder mask on the board.
like everything else, it is just an idea. there is not enough info on what the "train" is and how good contact it is supposed to make, what the budget or deadline are etc.
btw. don't really see why would any of the critique points are a concern. using ENIG finish will minimize problems with contact resistance and oxidation. since the resistors will be major part of overall circuit resistance, train characteristics are less important (material etc.). designing PCB with single array of parts is dead simple and quick. smt parts (either single resistor or array networks) are dirt cheap and next to free. and i am not sure why would anyone want to waste precious time soldering it by hand - just get it assembled by the same place that will make the PCBs and while at it have few spares. compared to time, effort and finished look of tracks fabricated in any other was, \$30 or so for 10 PCBs and shipping is well worth it (to me). and it can be decorative, have some nice patterns and solder mask and silkscreen colors that would make it stand out. and it is easier on batteries as it does not need 100s of mA to operate

#### Captain Bob

Joined Nov 9, 2022
7
In Kjeldgaard's post above (and pasted below), assuming stainless steel rails that are 18 inches long, what would "a suitably high constant current" be?

And then what is the device that measures the difference in voltage? I'm guessing it would need to be something that remembers the voltage when the "red light" is turned on, then compares that voltage to the voltage of when the "red light" turns off, and then if there is a difference in the two voltages a signal is sent to the "you have been caught cheating light".

If there is access to the far end of the "rails", then this idea might work as four-terminal measurement:
Code:
Local end of "Rails"
Cur1  Mes1
|     |
|     |
|<--->| Moving  "Wagon"
|     |
|     |
|     |
|     |
|     |
|     |
Cur2  Open
Far end of "Rails"
Function:
• Requires a relatively high resistance in the "rails"
• Send a suitably high constant current from Cur1 to Cur2
• Measure the voltage between Cur1 and Mes1

#### MisterBill2

Joined Jan 23, 2018
14,230
The current that I used was 100 milliamps, but that was for detecting the number of one-ohm fuel injectors connected. With stainless steel or nichrome wires you might need to use 500 milliamps. My current trgulator was an LM7805 regulator connected as a current source. one IC and one resistor and the voltage, I think, was 12 volts.

#### Ya’akov

Joined Jan 27, 2019
7,026
If the game could be built into the puck that is moved, or the puck could be wirelessly connected to the game, a computer mouse would be perfect. In fact, a Bluetooth mouse is all set for the application.