Introduction: Sensitive Tilt Trigger
Designed for any purpose that might need a very sensitive, any-direction, do-not-tilt switch. This instruction describes construction of: 1) My initial, hand-built prototype, thrown together in a single afternoon out of spare parts merely as proof of concept; 2) The follow-on, 2nd generation prototype; and 3) circuit board layout for a final design not yet etched and built. For reference I will call them the Mk1, Mk2 and Mk3 tilt trigger mechanisms. During that first hasty effort on the Mk1 I learned a few object lessons. I labored to circumvent these in the Mk2. Ditto from the Mk2 to my as yet unbuilt Mk3 design. I'll describe the lessons learrned so that you may not repeat my own mistakes. First photo shows you the rough-finished Mk1 prototype which I built almost entirely of parts from Radio Shack. Second photo is of the Mk2 which inc ludes some parts from a national chain hobby store.
Parts List for Mk1 sensor:
Two Circuit boards, Radio Shack 276-148
One CMOS 4011 quad NAND gate
Two resistors, 10M
2 sq in of metal duct tape
Some 30 guage wire wrap wire
Additionally the Mk2 requires:
Thin copper foil.
Thin brass wire (stiffer than resistor leads).
Also shown in both photos is some ancillary circuitry tossed into the project just to make demonstration easy. The add on circuit is nothing but a 555 timer in monostable mode with a red LED for visual output to show when the sensor trips. As this timer circuit is can be had from Google (and found in data books going back to since I was in high school) I won't detail that part here. But for reference, these are the parts I used to built that circuit too.
One LM555 timer
One resistor, 18M
One resistor, 560R
One capacitor, 1uF
One capacitor, 10nF
One LED, red
The 1st picture shows the Mk1 sensor and ancillary timer, all in one (crudely built) package. The 2nd photo shows the Mk2 slightly less crudely constructed.
Step 1: The Sensor Circuit
In the schematic above, note the symbol at top left marked S1 and DPST_TRGR. Read the notes next to it describing the function of pin 1 and also of pins 2 & 3. Refer back to the photo at the introduction and I think you'll make the connection.
Just in case it's not so obvious as I hope, imagine the circuit kind of like this. Picture a double headed base drum, one with a soccer ball inside. Imagine that both drum heads are of metal, that they are connected electrically both to each other and to the battery POSITIVE lead. The soccer ball too is metallic. Not only that, but that the sides of the drum are metal bars like a circular jail cell. Those bars are floating with only a tiny gap between the drum heads as metal ceiling and floor. Of that cage, all the odd numbered metal bars are electrically connected to one another and also to ground through a 10M resistor. Likewise the even numbered bars, but separately from the odds, and via their own separate 10M resistor. The metal soccer ball then, being otherwise free to move inside this cage, will naturally want to come to rest somewhere. In doing so, it will make a tri-point contact between whichever two bars and plate are the lowest.
In that position the metal ball will short two adjacent bars to either one drum head or the other. Tip it in any other direction and one of those tri-point contacts will break, at least momentarily. On rolling over one bar the ball must lose contact with the other. That bar's connection to positive being now lost, it now has no path to positive but only its ground path through the resistor. It's signal falls from high to low. That same ball, on rolling away from contact with either drum head lets both adjacent bars fall to low. Likewise if the ball bounces, however minimally.
Sensitivity is therefor dependent on how easily the bearing will jounce from resting upon any given pair of bars connecting them to one of the drum heads to resting elsewhere. A small bearing resting deeply between rather widely spaced bars will, of course, be less sensitive to being perturbed than a much larger bearing resting atop bars spaced more closely together. Regardless of spacing considerations, however, any jouncing whatsoever likewise triggers this device.
So basically, it turns out to be more of a do-not-move switch than anything else. Being somewhat asymmetrical in response a critical application might require more than one such tilt switch. It might require two or even three to be mounted all at right angles to one another with their outputs AND'd together. For my own purpose I will probably go with just one.
Therefor in my diagram, all I have shown is the bearing (symbolically connected to BATTERY POS) at repose on a single pair of bars. As I drew this up in KiCad, some kind of symbol was required (one that I had to create) and this was what made sense to me.
So the path to high is via the bearing. Paths to ground are via resistor. In between we have a NAND gate. With the bearing at repose both NAND inputs are HIGH. They are 1 and 1, which in a NAND gate gets you a 0. For all the other conditions (1&0, 0&1, 0&0) the NAND gate gives you a 1. And this would light up the LED shown.
That's not where I really put my own LED however. Why not? Because I wanted to know just how sensitive my circuit was. So in my prototype I left out the LED shown in favor of one driven by a 555 timer in monostable mode. It's well that I did because it turns out that the circuit is very sensitive indeed.
How sensitive? The prototype turned out so extremely sensitive that barely a touch was often enough. A touch so brief would not have lit the LED brightly enough or long enough for me to notice so I'd have most likely missed it. But the 555 timer lengthened each to about 30 seconds. So long an extension was deliberate as a part of my larger design as downstream circuits may be in sleep mode to save battery power. Because of this the LED stayed on nearly all of the time as I was not able to hold the circuit still enough for it ever once to settle. And as a result at first I thought my circuit was malfunctioning. But there was nothing wrong with the circuit. The ball-and-cage sensor, however, did have some defects of design and hasty construction. I will explain these two you so that you may avoid them.
Anyhow, in the schematic, that 2nd NAND gate is for input to the 555 timer, which takes a LOW for its trigger. With both its inputs shorted, a NAND gate becomes a NOT gate, inverting the trigger circuit output.
Step 2: Making the 1st of 2 Pos Contact Plates
Theory of operation aside, let us now build the sensor. We shall start with a pair of Radio Shack prototype boards, the smallest size they sell. From the first of these we'll make our first metaphorical drum head.
1. Lay one of two single sided prototype boards copper side down.
2. Choose an area equal to no less then twice the captive bearing's diameter.
IMPORTANT: Note that ideally the cage should be a circle instead of a polygon. Why? Because polygons all have corners. And corners can be a problem depending on the size of our bearing. So make very sure that the bearing size you choose is able to repose in a corner so as to touch the corner bar. That is to say, it won't so much rest exactly in the corner as to just either side of it, touching the actual corner pin and either of the two just next to it. If it might not, then you must modify the design. If change is needed, then make it as follows: A) choose a smaller bearing; or B) choose a larger area and only put "bars" (actually cut off resistor leads) into every 2nd hole.
Steps for Mk 1 design:
3. Lay down metal foil over the area you've chosen for your sensor's future cage of bars.
4. Trim as shown, leaving a corner to stick through outside the cage for soldering a lead wire to.
5. Provide both physical and electrical clearance for any sensor bar holes which the corner from Step 4.
6. Solder a lead wire to the end you left stuck out.
Steps for the improved Mk2 design:
3. Transfer maximum diameter for base plate to paper via pencil through the perf board's holes.
4. Cut a circle of thin copper sheet to slightly less than this diameter (I traced around a US penny).
5. Work the copper circle into a very slightly convex shape.
a. Work the roundness by sandwiching the copper between anything round and hard rubber.
b. I made mine a bit too complex by rounding it with a screwdriver handle end.
c. A future Mk3 design would be more billiard ball diameter of convexity.
6. Fill the concave side with solder to stiffen it, attach leadwire and polish the front.
NOTE: Yes, it's true. For the Mk1 design I soldered 30 gage wire wrap wire to aluminum. I used a 700F iron and semi-ordinary 63/37 solder right from Radio Shack. It wasn't easy. Nor is it very likely the best of solder joints. But it worked for a prototype. If I were to do this again, I would instead do one of the following: A) find a source for copper foil tape; or B) use a tiny 1/8", 00-80, round-head screw to physically press my lead wire against the aluminum through one of the perf-board holes; or 3) [as sagely suggested by the 1st commenter] firmly tape down the lead wire relying on physical contact. Personally, though, I won't be doing this so crudely by hand again. I will, instead, etch a custom PCB so that I can make it round instead of an ugly polygon. When that time comes, I'll upload the Gerber files here so that you may do the same. For the Mk2 design I just stuck in the lead wire to the back-filled solder while still molten.
And so you are done with the first of two drum head contact plates for your sensor. Flat was okay very nearly all of the time. But not 100% of the time. And so for the Mk2 design, I decided to make the plates rounded. That worked much better.
Step 3: Making the 2nd Contact Plate
Repeat the process in mirror image for results like those shown above. In the 1st photo above you see the Mk 1. In the 2nd you see the Mk2. Note the addition of standoffs from Radio Shack. These made assembly much easier. And here you can the clear advantage of the Mk2 design versus Mk1. At no point will the captive bearing be inclined to repose in the middle away from all contact bars to be added next.
Step 4: Assemble the Physical Cage of Bars
Okay, here is where you can enjoy a much easier time than I imposed upon myself with the Mk1 design. Note how both of the tiny circuit boards have mounting holes in all four corners but I took no advantage of them. The mistake I made was to think I could solder in all the odd number bars onto one board and the evens on the other. Somehow I had naively assumed that it couldn't be all that hard to mate them together once I was done. It might have been, had they all been perfectly straight. But of course they were not. So that method ended up being way too much work. After all of the individual bar-and-hole wrangling I put myself through, I was left with cage bars that were nowhere so straight as I'd have preferred. Perfectly straight is the ideal. And as you can see I failed to achieve it, or even anything near with the Mk1.
Now look closely at the Mk2. The pins are are all as straight as was possible with Radio Shack perf boards. The two hole patterns did not line up as well as I'd hoped. So the bars are slightly askew. But at least they are all equlally askew and that's a huge improvement. How I did that was, copper sides out, I physically mated the Mk2's boards to each other by a distance just slightly more than the diameter of your chosen bearing plus the height of the two convex plates, as follows:
1. At each corner of one board, run a longish, #6 machine screw from the copper side.
2. Spin a nut all the way down to hold the screw firm at each corner so that it now looks like a tiny table.
3. Spin a 2nd nut onto each screw down a height just a bit more than the bearing's diameter.
4. Now sandwich down the second board so that the patches of foil tape face one another.
Now for the bars. These are just the cut off ends of spare resistors. After a while you always have too few of some and too many of others. So for the Mk1 I sacrificed some 10 Ohm ones that have been laying unused for years. In the Mk2 I tried using brass wire. It was okay, but not what I would call a big improvement. So stick with resistor leads if you like. Just don't cut them off yet, though. Leave them whole until I tell you.
5. Holding your first sacrificial resistor by its body, push one axial lead through any hole just outside the foil from one board, all the way through, and out the other.
6. With only the further tip sticking out, solder that stuck out end in place to its copper pad on that opposite board. Solder it only there, do not yet do the push-in side.
7. Now is when you want to cut the resistor free from the push-in side. Leave only just a bit sticking up. Do not solder it yet.
Now repeat steps 5 through 7 from that same direction for every 2nd hole around the foil tape. Before you have done quite all of them, remember to drop in the bearing. A note here about the ball. For economy's sake (and because the stores were all closed on Thanksgiving) I made do with sling shot ammo instead. As such it was not perfectly round, causing my end result to be somewhat temperamental. Don't stint on the bearing. For that matter, don't stint on the bars either.
Now repeat steps 5 through 7 from the opposite side. When you are done all the cage bars will then be installed, but each one soldered on only one side. There is a reason for this. Now we will get to it.
In the schematic I showed you earlier, the bearing and only two pins were shown, pins 2 & 3. Pin 2 of the schematic is one set of bars left unsoldered on one of the perf boards. The problem is that as yet they are all separate when, at least electrically, they need to be one. So for pin 2 of the schematic, choose either set and treat them like so:
9. Strip off the insulation from several inches of wire-wrap wire.
10. Wrap the end of your now bare 30 ga wire around any one of the cut off, unsoldered resistor lead ends on the chosen board.
11. Solder it there.
12. Route a path to the next unsoldered resistor lead end carefully avoiding contact with the adjacent soldered one.
Note: If you like, somewhere along the way tack it down to an unused copper pad.
13. Wrap that lead end with the wire and solder it there.
Repeat steps 9 through 13 so that every second cage bar is now electrically connected. That completes electrically unifying the cage bars comprising Pin 2 of S1 in the schematic.
Now do likewise, repeating steps 9 through 13 for the other board. When done, you will have then electrically unified the cage bars comprising Pin 3 of S1 in the schematic.
Step 5: The IC Circuit
Again I show you the same schematic as before. I need to recapture this diagram in a future update because now I think I'd be recommending 1M or even 100K for the pull-down resistors. The added current drain turns out to be negligable.
In the prior steps up till now all you have done is to physically construct S1 of the schematic, which I call out as DPST_TRGR for double pole single throw trigger.
I assume you can read at least so simple a schematic as is composed of one chip, three resistors and a single LED. Wired up as shown, the LED will flash very briefly whenever the bearing is jostled inside of its wire cage. The output at Pin 3 of U1A will go high very briefly when that happens. Likewise the output at Pin 4 of U1B will go down. It is a falling signal that customarily triggers a 555 timer monostable circuit. As the 555 timer family of circuits is described ad nauseum elsewhere, I will not detail it here.
Step 6: The Future
I have described the manual construction of two, sucessive design protype tilt sensors. I have in mind to employ its type in a larger future project. The tilt-sensing circuit will be a separate plug-in module for that project. I include here an initial circuit board design toward that goal, probably not my final one. Elsewhere on this website are instructions for how to make circuit boards at home using laser printer on acetate film and a clothes iron. I mean to try that at some point. If you get there ahead of me, add a comment to say how it went.
I hope this instruction has proven entertaining for you, perhaps even useful. It is the least I could do in return for the inspiration I have enjoyed from others I have read on this site. Thank you.
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