Project proposal (ha! it will be funny when you read it)

Well, I'm proposing to my girlfriend. I'm usually pretty creative when it comes to birthdays and such, so I've got to be REASONABLY creative here. I've got an idea that has been bouncing around in my head for a few years.

You know those long thin sticks on a base that folks use for putting their rings on while they do dishes? If not, here's an artsy version:

I had the idea about 10 years ago to put an LED at the top so that there would be a visual cue that the ring was left on it. At the time, the hobbyist market wasn't so hot, and it was expensive to get boards made and the LEDs I wanted were too expensive and blah blah blah. My idea at the time was to have the (likely conductive) ring complete a circuit. Well, this would work, but later I realized that wouldn't be so hot for the ring itself, being prone to current corrosion. At the time I was offhandedly thinking that I would build in some cathodic protection and pray.

Now, I'm not sure at all that that would be practical or even clever.

Back to the point at hand, I've been talking about this project with my girlfriend for a while now, and she's aware that it's a pet 'someday' idea. As a goofy proposal idea, I think it would be cool to build it up, make it WORK for SURE, then when showing it to her, give her the ring to test it with.

Without TOO much commentary on how lame this idea is compared to the time you proposed from a hot air balloon over the arctic sea, I'd like to get some thoughts on how to go about this.

#1: IR. Hollow spike (gotta be hollow to run the LED to the top) with an IR emitter inside that's lined up with a quite small hole that is JUST proud of the base. This would mean that even a very low profile ring would get hit. Just to the side of that emitter, a receiver that would wait for the light to be reflected back. Obviously I'd have to firewall them inside the pole, but otherwise it might just work.

#2: Lasers. See all of that above? Ya, that, except that this base would have a raised rim around the edge. The rim would be big enough to house a laser diode that would be aimed at a photo transistor inside the pole. Interrupt the laser? Boom. To make it last more than 3 minutes on a 9v, I would have the laser pulse once every few seconds for a few milliseconds.

#3: I dunno? I've mucked around with super high sensitivity capacitance measurements, but not on an Arduino, and not for this budget.

#4: Weight. I can built a scale sensitive enough that will do the trick but it will be expensive and picky. I can hack a cheap one digital scale, but that's getting silly.

How would you do it? The main requirements:

1. LED goes at the top of the pole. Constraint: pole must be hollow.
2. It must be battery powered. This doesn't mean that the prototype has to run for more than an hour, but still...
3. The pole has to be thin enough to allow a size 6 ring to slide all the way down to whatever sense method is chosen.
4. It has to be able to detect a ring that might not fit ENTIRELY snugly to the bottom of the base because of its shape, but should also be able to detect a quite small band. (can't only detect the bottom .5mm but can't start the detection 3mm up)
5. It has to fit in a base no bigger than say 10" diameter. That's on the outside, and I don't really see that as being a problem, I'm just saying.
6. The solution has to be build-able without paying a fortune. I can get PCBs made in my shop, pick and place for the heck of it, reflow and work all the way down to 0402. I more mean I don't have a commercial lathe and CNC mill.

I hope you all can help

Thanks!

Look into how metal detectors work. With a small coil, you should be able to get a detectable effect when the metal of the ring is nearby.

Edit: not quite the same but possible even simpler, detect a change in inductance within a single coil, similar to stoplight car sense loops.

I was worried that the LED signal would throw that off if I chucked the coil inside the tube. Do you think I'm being too picky?

I'd put a coil in the stick, wound over ferrite rod or a string of ferrite beads, and detect the effect on the coil of eddy currents in the ring when it is placed on the stick. For example, use the coil as part of the tuned circuit in an oscillator, and adjust the gain of the oscillator so that putting the ring on the stick stops it. Then use a diode/resistor/capacitor to detect the output of the oscillator and feed it to an analog input pin.

put a small bit of foil on ether side. one is Ground the other is Positive and then the ring can complete the circuit

Just thinking out loud… :~

A round clear plastic rod could effectively transmit light from the circuit board in bottom to top.

Place a spiral trace (coil) flat on the upper surface of the board with the led in middle (traces on back side). The trace coil would then align with the plane of ring.

Could this function as a capacitance, eddy current detector?
Rich

You could lathe the stick to be conical, so any practical size ring would fit. Then, take an inch tall section about where the ring would sit, and cut a square chunk out on one side -- depth of the cut would be about 33-50% into the diameter of the cone. Basically like a trigger.

Take that side chunk and drill three inset holes into the inside vertical surface, where it mates against the inside of the cone. Top and bottom two holes would be small springs, like from the inside of a pen. Middle hole is a metal contact. Drill matching holes into the inside of the inset chunk on the cone, to hold the other end of the springs and accept the contact.

Finally, drill a hole through the center of the cone from the base to the top. This is where your LED goes, and the wiring inside. At the point where the hollowed hole intersects (perpendicularly) with the contact hole from the side chunk, cut one of the wires and put two contacts of whatever you can fashion, so when the ring is around the base, the chunk is pressed flush into the cone, and makes contact with the cut wire, thus completing the circuit.

Let me know if you need a diagram.

Qsilverrdc:
Just thinking out loud… :~

A round clear plastic rod could effectively transmit light from the circuit board in bottom to top.

Place a spiral trace (coil) flat on the upper surface of the board with the led in middle (traces on back side). The trace coil would then align with the plane of ring.

Could this function as a capacitance, eddy current detector?
Rich

In principle, that could serve as an eddy current detector (and I like the idea of a transparent rod lit from the bottom). However, the inductance of a spiral trace is low, so he would need to run the coil at a high frequency. A coil of fine wire just below the surface that the rod is attached to might be more practical, because it could run at a lower frequency.

tigerbomb8:
put a small bit of foil on ether side. one is Ground the other is Positive and then the ring can complete the circuit

This runs into the current corrosion issue.

SirNickity:
You could lathe the stick to be conical, so any practical size ring would fit. Then, take an inch tall section about where the ring would sit, and cut a square chunk out on one side -- depth of the cut would be about 33-50% into the diameter of the cone. Basically like a trigger.

Take that side chunk and drill three inset holes into the inside vertical surface, where it mates against the inside of the cone. Top and bottom two holes would be small springs, like from the inside of a pen. Middle hole is a metal contact. Drill matching holes into the inside of the inset chunk on the cone, to hold the other end of the springs and accept the contact.

Finally, drill a hole through the center of the cone from the base to the top. This is where your LED goes, and the wiring inside. At the point where the hollowed hole intersects (perpendicularly) with the contact hole from the side chunk, cut one of the wires and put two contacts of whatever you can fashion, so when the ring is around the base, the chunk is pressed flush into the cone, and makes contact with the cut wire, thus completing the circuit.

Let me know if you need a diagram.

This essentially violates design rule 6. I can go get lathe time at the local hackerspace if I MUST, but alternate methods would be preferred. Also: it limits aesthetic design decisions later.

dc42:

Qsilverrdc:
Just thinking out loud… :~

A round clear plastic rod could effectively transmit light from the circuit board in bottom to top.

Place a spiral trace (coil) flat on the upper surface of the board with the led in middle (traces on back side). The trace coil would then align with the plane of ring.

Could this function as a capacitance, eddy current detector?
Rich

In principle, that could serve as an eddy current detector (and I like the idea of a transparent rod lit from the bottom). However, the inductance of a spiral trace is low, so he would need to run the coil at a high frequency. A coil of fine wire just below the surface that the rod is attached to might be more practical, because it could run at a lower frequency.

dc42, I THINK I'm following your thinking here, but if you had a moment to explain a bit further, I would appreciate it. Since I read your post, I've been investigating the (sadly EOL, but still around) CS209A chip from ON SEMI. There's a sexy way to do it metal detector style, I'd just have to be very aware of how the coil was made.

I'm now thinking more in line of metal detection vs optical, and thinking that perhaps a linear hall effect sensor just inside the tube testing against a weak magnet put somewhere off toward the rim of the base? I will have to test later, but perhaps there is enough of a repeatable difference there? meh.

would a hall effect sensor work?

otherwise one of those IR-slit sensors?

fkeel:
would a hall effect sensor work?

otherwise one of those IR-slit sensors?

I'm going to have to get a good linear HE in the shop tomorrow to see if the process would be repeatable in any solid way. For everyone's reference, we're talking about gold here, so that's a little tougher to catch with normal metal detector style circuits and gets a little picky on the coil/frequency.

PERHAPS on that IR option. Are you just referring to my original post, or are you thinking about a specific thing you've seen?

I have a solid collection of very high quality opto endstops, but I'm guessing that busting one in half wouldn't likely work. I think they're pretty needy on who much light gets through, and I can't count on the ring to reflect THAT much.

I'm going to step back for a moment and re-think my original ring-as-conductor bit.

IF I were to assume gold as the conductor (and I safely can), I can determine the conductivity and be very, very careful to limit the current to the levels that I won't have to worry about corrosion, at least unless the ring is left there for months. If I were to pull the current down into the 10mA range, AND send pulses every second or so, it would have VERY little chance of being an issue.

That has me back to:
Round base. Arduino inside to send / receive pulses. Perhaps some more fun electronics inside for any gimmick I come up with. Hollow pole with wires run up for LED. Since the current necessary to run the LED isn't going through the ring itself, it shouldn't cause the corrosion.
At the bottom of the pole are two pads. Split washers would do it. Heck, conductive fabric wrapped AROUND split washers (for a soft landing) would do the trick.

Any objections?

I was thinking of this bag here:

http://www.goldmine-elec-products.com/prodinfo.asp?number=GP23

the larger black components detect if something is inside its slit. You could set it up, so the ring drops into that component.

alternately this here might work if placed in a smart way:
http://www.sparkfun.com/search/results?term=ir+sensor&what=products
(i actually really love these sensors - they are super versatile much more so than one might expect.)

Can you get her a custom ring? one with a magnet in it?

And now that's my fall-back. I can chuck a small RE magnet fragment behind the main stone and use a hall effect sensor just under. I'm also remembering the small glass rfid ampoules. I've got a huge cache of readers...

i like the rfid. that way you can make it identify whose ring is on the stand, and how fair doing the dishes is distributed between the two of you. Depending on whose ring is on the stand a different LED can light up, and you could maybe have two others which suggest who should potentially be doing the dishes based on brightnes

that way you make sure that you are not telling her that you want to marry because she is good at doing the dishes

ha! If i could find smaller tags, it might just work!

then again - that idea might backfire badly....