I'm trying to modify the board game "operation" (see wikipedia if you don't know it). Basically it's about removing little plastic pieces ("organs") from plastic slots. It is done with metal tweezers and each time you touch the metal borders of the slots (which you are not supposed to) a circuit is closed and the game vibrates and a light bulb indicates the fault.
I want to modify the game to become more interactive with sounds, light, moving stuff and so on and I even feel up to it (at least so far). The big question is, how to detect that an "organ" has been (correctly) removed from its slot.
I considered many approaches and in the end replaced the plastic organs with metal ones. The idea was to have a wire pattern at the bottom of each slot and that the metal organs close a circuit when touching simultaneously the two wires. Very similar to how common push buttons work. I tried different wire patterns but in like 30% of the cases it just doesn't work because the organs somehow do not properly touch the two contacts.
Maybe there's a different and much easier approach to solve this problem and I just don't see it. Or maybe I'm already on the rigth way but need a little more input from somebody.
I would be very grateful for any ideas and suggestions!
st3f0
PS: I hope that this is the right place to ask such a question and that I was able to present my problem in an understandable way. If not please let me know.
I think it is correct that magnetic organs are not an option since they would stick to the tweezers. I also considered optical detection but I do not like the idea a lot, because it seems like a lot of work and material (for each slot) and I'm not convinced about getting it reliably to work.
I do like the idea of capacitive sensing. I did a quick research and apparently a simple piece of aluminium foil below each slot could do the trick. However I'm also here skeptical about the reliability of this detection idea.
Has anybody already experience with this kind of capacitive sensors? How reliable and at which distance do they work? What are the important parameters here? Geometry and material of the measurement electrode and the detected objects? What works best then?
Again I'm very grateful if somebody can share his knowledge and experience with me. Either way I will play around a little bit and report back. And of course I'm still open for other suggestions if anybody has an even smarter idea.
You will have to try this out! Capacitive sensing is sensitive - very sensitive, sometimes too sensitive, but also works with quite small signals.
Distance through air: well, look at a Theremin how far it can detect hands in air.
I have a sensor here that can sense water through the wall of a non-conductive pipe (5 mm is no problem).
For you: most sensible would be two copper traces next to each other, with some non-conductive paint to cover them, and the metal object to be placed in between them. Should be quite easy and reliable to detect. Buy a few of those cheap water sensors with lots of parallel traces and start cutting away, that may be the easiest/cheapest way of building such a configuration (try it out with pieces of film first).
Using non-magnetic tweezers is not a bad idea at all, ardly. Thank you for the suggestion. I googled a bit and there are aluminium and copper tweezers around, but not so many and mostly relatively expansive. However, I already ordered hall sensors just in case and I'll keep that option at the back of my mind.
Capacitive sensing did not really work out in the end, unfortunately. I experimented with different electrode/capacitor geometries, resistance values and different samples/organs. I was surprised how well this works with big/medium sized items like fingers or scissors or so and this might come in handy in one of my next projects. But it looks as if my "organs" were just too small for that kind of sensor.
I also tried that stripe geometry, that wvmarle suggested. I used a standard stripboard for that. I did not get it working, but maybe I did not get exactly how this was meant. Could you please clarify this, wvmarle? Should the stripes be alternately connected to the measurement electrode and to ground? Or somehow different?
Right now I think about going back to the original plan and short-circuit some wires by placing the "organs" on top of them. I will never get 100% reliability there, but maybe there is no easy way of achieving that and I can live with it. I'll experiment a little bit more with it different geometries.
However, I still keep all the other suggestions in mind.
st3f0:
I also tried that stripe geometry, that wvmarle suggested. I used a standard stripboard for that. I did not get it working, but maybe I did not get exactly how this was meant. Could you please clarify this, wvmarle? Should the stripes be alternately connected to the measurement electrode and to ground? Or somehow different?
No, my idea is to cut it so you have two parallel strips left. Nice and small, like your parts. Makes it easier to get the sensors small enough.
A bunch of alternating strips indeed doesn't work. I tried that myself.
Just wanted to let you know which solution worked best. I tried metal parts that close an electrical circuit which worked only in 2 of 3 cases. For capacitive sensing the parts i wanted to detect were probably just too small, so that did hardly work at all.
In the end I opted for optical detection with an IR emitter and phototransistor pair (tcrt5000), as Grumpy_Mike had suggested. I had to use 2 per slot to avoid blind spots and the wiring is then with 26 phototransistors relatively complicated but the reliabilty convinced me. It works 100%.
As a bonus I even can detect when an "organ" is moved and I plan on using that information to trigger flash lights, electro shocks and a sirene as soon as the tweezers are in a slot. It's supposed to be quite difficult in the end.
I want to thank everybody in here for their help! Greatly appreciate it!