I presume the spring's diameter is close to the inner diameter of the tube? If so, positioning some kind of contact to ride on the spring is going to need difficult.

thankfully the spring seats into a plastic follower which pushes the discs up, and that follower is keyed into the channel... almost like they anticipated this. here's a sketch of the follower:



it's got to be 98% the size of the tube's ID. it has three little nubs at 6, 9, and 12 o'clock to minimize the contact surfaces against the sleeve, and the small protrusion into the slot to keep it from spinning I suppose? the spring is probably 80% of the tube's ID, and I'm hoping I could just use a coiled wire up the center of the spring

 

What are the dimensions of the steel cylinder?
I see the bottom cap is threaded. Are you allowed to drill a hole in the bottom cap?

 

What are the dimensions of the steel cylinder?
I see the bottom cap is thread. Are you allowed to drill a hole in the bottom cap?

the cylinder is about .75" around (I didn't measure that unfortunately). the walls are sheet steel and the base cap feels like pot metal. the cylinder is powder coated or some other robust coating, but the base cap is unfinished (or the finish is gone). drilling is no problem

 

Could th spring itself be the sensor? It would change it's inductance based on the compression or extension of the spring. A simple LC oscillator to measure the frequency. You might need a fixed coil as well for a reference to compensate for drift variations due to temperature, etc.

 

Could th spring itself be the sensor? It would change it's inductance based on the compression or extension of the spring. A simple LC oscillator to measure the frequency. You might need a fixed coil as well for a reference to compensate for drift variations due to temperature, etc.

My father had suggested this, but my concern is that many of these hoppers are older/newer and I can't imagine they've all got the same tensile strength at this point. Even if we replaced them all, I would imagine they'd wander away from the baseline at different rates?

 

Perhaps somebody will talk me out of this:

.2x3mm nichrome ribbon. Would be impossibly easy to drag a contact up and down that. Would this much nichrome be too conductive for my purpose?

 

Perhaps somebody will talk me out of this:

The resistance is inversely proportional to the cross sectional area so if 0.1mm diameter wire has a resistance of 23 ohm over 120mm length then 0.2 x 3mm strip would have a resistance of 0.3 ohm. In practice, with variations in contact resistance with the wiper I’m pretty sure this won’t work

 

That was my supposition, thanks for confirming it. While the nichrome strand sounds easiest, it doesn’t sound as reliable as a resistorized panel. It will be incredibly economical to buy resistors in a 1206 or similar package, unless of course I consider time = money too

 

This was composed yesterday and apparently never saved, so here it is...

Based on the drawing, it appears that it could be possible to attach a pair of pogo pins at the top near the exit from the feed lips that could act as a switch. The disk would short them when it was fed off the stack.

You would need to do some unusually long time constant debouncing, and any method that involves friction would mean the parts in contact would have a definite life (so I would design them to be replaceable.

You could also use a very small snap action switch (microswitch) that could have a low friction actuator for a longer life. Best would be a non-contact proximity sensor which would have no friction issues.

I would also be inclined to use a little 8-pin SMD MCU (like an ATTiny13A) to do housekeeping so basically you would keep the count local and just answer queries about it, or send messages when a count occurs, etc.

 

That was my supposition, thanks for confirming it. While the nichrome strand sounds easiest, it doesn’t sound as reliable as a resistorized panel. It will be incredibly economical to buy resistors in a 1206 or similar package, unless of course I consider time = money too

I am afraid that anything depending on wiping contacts with be unreliable, particularly if you are depending on small changes in resistance for the indexing.

If you do try this, be sure to make your wiping contacts bifurcated.

You might even be better off attaching a very small switch to the bottom of the follower on tab, and relieving the tab so something in the tab's channel operated the switch. You can run small wires down to the bottom of the cylinder, which must have space to accommodate the compressed spring.

If wiping contacts are going to be used anyway, you could make a PCB that fits into the channel and is just two traces with pads at the right interval to be shorted by a pair of contacts fixed to the follower so that it engages those pads. The active parts could be in the base. You could even use your resistor array idea and have the follower contacts complete a circuit at each interval with large enough resistance differences to overcome noise and resistance changes from the contacts.

 

That was my supposition, thanks for confirming it. While the nichrome strand sounds easiest, it doesn’t sound as reliable as a resistorized panel. It will be incredibly economical to buy resistors in a 1206 or similar package, unless of course I consider time = money too

Just to clarify, my post #16 recommended series resistors so you have the advantage of buying a bag full of the same value and you can pick a reasonably large value, say 1K, so each step is sufficiently large for it to be easily distinguishable and the contact resistance of the wiper is less critical allowing lighter force with less wear.

 

I had considered your suggestion for series, and I couldn’t come up with a reason not to do it that way other than slightly more complicated PCB traces… which only matters if I DIY that piece.

unfortunately, I put a caliper on the disc, and it’s actually 4.7(066)mm, which means I can’t rely upon PCB pitch to do the spacing for me

 

I had considered your suggestion for series, and I couldn’t come up with a reason not to do it that way other than slightly more complicated PCB traces… which only matters if I DIY that piece.

unfortunately, I put a caliper on the disc, and it’s actually 4.7(066)mm, which means I can’t rely upon PCB pitch to do the spacing for me

Not with standard 2.54mm (0.1") pitch, but it's relatively inexpensive these days to get prototype quantities of custom boards made - lots of online vendors, typically China based. Best to match dimensions so you can get as many as possible out of a standard biscuit size. Use the opportunity to make the pads the recommended size for the resistors, there is probably no price premium for complexity of single sided boards as they are digitally "masked" these days. In fact, you may be pleasantly surprised by the cost of getting the boards populated with resistors while you are at it.

To check the concept you could still try it with standard spaced strip board with a resistor between each adjacent track - it should work okay for a few steps and then get confused. Rather than check just one disc, maybe measure a stack of 30 in case they vary a bit?

 

good advice, and I did. 4.7066mm is the mean disc size computed from two full stacks of 30, with variance from ~4.4-4.9 on a handful I measured. I don't know how thin PCB can be, but I'm considering a copper clad polyimide circuit because after closer inspection, the "key" on the follower only leaves me ~2mm of space in the channel and there is some slop in the follower's travel

 

If you always start with a full stack of disks, you really need to know only that a disk has been ejected. An optical sensor with a printed strip of alternating light and dark bars can tell you when a disk has been ejected.

Non-contact, nothing to wear, easy to mount the sensor to the plastic piece at the end of the spring.... Plus low cost and easy to install.

 

If you always start with a full stack of disks, you really need to know only that a disk has been ejected. An optical sensor with a printed strip of alternating light and dark bars can tell you when a disk has been ejected.

Non-contact, nothing to wear, easy to mount the sensor to the plastic piece at the end of the spring.... Plus low cost and easy to install.

we do not always start with a full stack as it’s tedious to load them. Most instances require 10-16 discs, so connecting a hopper and seeing if it has enough for the job would be immensely helpful. Imagine the loading supplies on one end of the campus and yourself with the equipment on the other.

after periodic loading sessions, there are numerous partially full hoppers, and even during those loading sessions there’s no way to tell when it’s full other than it being incredibly difficult to stuff another disc in, if you’re starting from a partial load.

 

Maybe you should make a version of this:

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There are many variations on this, but this shows the basics.

The body of the loader fits over the magazine and is free to slide.

At the top, the curved “hood” has a tooth that engages the cartridge then the body is pushed down the magazine driving the cartridge down and leaving a space between the stack of rounds and the top of the feed lips the size of a cartridge.

The protrusion at the front with the groove in it serves as a ramp to guide the cartridge into the magazine.

Once the cartridge bottoms out on the tooth the loader is allowed to return to the resting position, moving the tooth obstacle and the cartridge is then fully seated.

The other thing that comes to mind is witness holes which are a standard feature of magazines:

​

In your application I would prefer a slot to holes since you are modifying an existing part and milling a slot is quicker than drilling a dozen precision holes. If the magazine must be sealed from the environment, I think I would attach an indicator to the follower and put a clear plastic window on the inside wall covering the slot but showing the follower indicator.

 

How about a weight or pressure sensor? The sensor goes underneath the spring and wires are fed out from a hole in the bottom cap. In effect, the sensor would be measuring the compression force in the spring.

 

Great ideas. I do have a few that I took a grinder and added a witness slot to. A speed loader wouldn’t be a bad idea, but it wouldn’t solve the problem of knowing how many discs remain.

 

How about a weight or pressure sensor? The sensor goes underneath the spring and wires are fed out from a hole in the bottom cap. In effect, the sensor would be measuring the compression force in the spring.

I don’t know enough about measuring those qualities to have a real opinion, but I do know that many of the springs have various “springiness” to them and I personally believe that the position of the follower is the most precise gauge