Find linear relation between stacked iron blocks

I want to find a linear relationship between stacked iron blocks, one by one on top of each other, and some electrical property. Then create a arduino circuit that could precisely determine the number of blocks stacked on top based on the relation. What electrical property should I be measuring? I was thinking creating a EC probe but the sensitivity may not be very precise.

Any ideas on how to get a linear relationship between the number of iron blocks stacked on top of each other and some electrical property?

I have one base iron block which will have the circuit internally built in, and then 5 iron blocks, each should have some identifiable electrical variable as they are stacked.

Load cells would not be possible for this project since the circuit is internally positioned within one of the iron blocks (the base block).

The dimensions are around 15cm (lenght) x 20cm (width) x 5cm (height). The surface black oxide, there is no way to have a transducer over the stack

I think the best way to do it would be to use a coil to find magnetic field pulses, but the coil diameter might be quite small and not sure if there would be sensitivity with more blocks added, unless there are some other design idea with the coil, possible short circular but long vs large circular diameter?

The objective is to count the number of iron blocks up to a total of 5.

• "Load cells would not be possible for this project since the circuit is internally positioned within one of the iron blocks (the base block)." That's the ideal position for the load sensor - at the bottom of the stack. What is the application? Dimensions of the blocks? Surface coating - stainless, black oxide, nickle plated, rust? Can a contact or transducer be placed over the stack? The question is very vague. Aug 6, 2016 at 14:00
• The circuit is inside the iron block, surrounded by iron, so no physical force is being applied to the load cell... if that makes sense... Aug 6, 2016 at 14:02
• A circuit completely surrounded by a conductive, ferromagnetic material is not going to be able to detect much of anything at all. Internal strain (i.e., a load cell) is one thing that it DOES have a chance of measuring. Aug 6, 2016 at 14:09
• @condo1234: All that information should be in the question so that anyone trying to help you doesn't have to trawl through the comments to piece together your question. You still haven't explained the application. Aug 6, 2016 at 14:12
• It's not clear exactly what you are trying to accomplish with these iron blocks. Do you merely want to count the blocks with this device? Are the blocks identical? I would answer your question if I know these facts. Edit your question to include ALL information. My idea would be to use a surrounding coil, like a metal locator has. Aug 6, 2016 at 14:15

An ultrasonic transducer could project pulses of sound up through the stack. The top of the stack would produce a reflection, and the interfaces between blocks might produce reflections, too.

The speed of sound in iron is rather high — about 5000 m/s — so you'll need to measure time with a resolution on the order of $\frac{0.05 m}{5000 m/s} = 10 \mu s$ in order to count the blocks reliably.

What exactly do you intend to do with this information? How will you power the circuit?

• I had considered this too but reckoned that discontinuity between the blocks (surface finish) and internal multiple reflections would make this hopeless. OP isn't helping much. Aug 6, 2016 at 22:02
• @Transistor: I didn't say it would be easy! :-) It might even require multiple sensors buried inside the bottom block to build up an ultrasonic image of what's above it, and some sophisticated pattern recognition to analyze that image. Fortunately, the blocks have grown somewhat from their initial specification (2 x 2.5 x 5 cm). I'm still puzzled about how he intends to get power in and/or the information out of the bottom block. Perhaps the ultrasound would be useful for that, too. Aug 6, 2016 at 22:11

Oddly, as Transistor has pointed out, a load cell is exactly what you need, but the job is likely to be difficult.

Your comments indicate that you want your circuit to be entirely contained in the base block, completely surrounded by iron, and will keep track of how many blocks have been stacked on top of the base block.

As Dave Tweed pointed out, the outer layer of the block will shield the interior from any electric or magnetic effects, so magnetic sensors simply won't work. What is left is weight. The material of the block will distort (slightly) under the weight of added blocks, and this is what you have to work with. A load cell (or, more generally, a strain gauge) is possible, but you'll need sensitive sensor/amplifier combination. The job is made more difficult by the geometry - your blocks are relatively wide and thin, so the stress on the lower block will be relatively small.

• "Oddly, as Transistor has pointed out ..." You mean I've said something sensible at last? ;^) Aug 6, 2016 at 21:55
• You'll just have to guess. Aug 7, 2016 at 6:06