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enter image description hereFor a hobby project I have bread boarded, I am considering using a pair detectors that operate as a single use physical barrier. When hit by a projectile, a trace is broken allowing +5VDC to to trigger a flipflop that is part of my gate logic for a counter-timer.

I have envisioned this detector as a thin adhesive sticker with a thin copper trace embedded in it that forms a simple pattern to cover the area, zigzaging across the surface. Similar to the passive RF sticker tags used by retail stores for antitheft.

My questions are: 1) Will it be practical to have a manufacturer print a batch of these things for me? 2) I've never used CAD software before, is there a suggestion for a free CAD program, that doesn't have a huge learning curve for this type of project (I'm a linux guy but thinkning W7 is more likely here ) ?

EDIT: Photo diodes are a great idea, and I will likely try them and ask questions in a separate thread. The break-able trace allows for more configurations and keeps slew rates ( and other rates ) small. I plan to use a very small distance to track the velocity of said projectile, like 1 or 2 cm.I have a 10 Mhz clock for my timer.

EDIT2: I should have added a current limiting resistor to the diagram so as not to start a fire with the trace...but I think you get the idea from this diagram.

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    \$\begingroup\$ I do not understand the first part of your question at all. However, a really basic and easy to use PCB CAD software package is ExpressPCB. \$\endgroup\$
    – NickHalden
    Commented Sep 30, 2012 at 3:53
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    \$\begingroup\$ @NickHalden imagine a cheap way to measure bullet velocity. Set up two of these homebrew sensors, shoot through them, and measure the time delay between the input states changing. I don't understand the downvote from someone so I'll just have to upvote it because it's an interesting project. \$\endgroup\$
    – Dave
    Commented Sep 30, 2012 at 5:09
  • \$\begingroup\$ Ah I see. I was not the mystery downvoter btw. \$\endgroup\$
    – NickHalden
    Commented Sep 30, 2012 at 14:42
  • \$\begingroup\$ Don't see any reason you couldn't get a manf to make some for you, but I agree with Dave do it yourself first. You could just get some coper tape and cut the shape you want with an exacto knife. Then stick that on a piece of card stock and give it a try. \$\endgroup\$ Commented Sep 30, 2012 at 14:51
  • \$\begingroup\$ Oh and for something this simple you can use the demo versions of real tools like: Orcad PCB Editor Lite (which is basically Allegro). Mentors Pads may let you do it as well, probably most others as well since this is such a simple circuit. \$\endgroup\$ Commented Sep 30, 2012 at 14:53

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Using metal Christmas tree tinsel used to be common for this sort of thing in physics labs; then photo gates got cheap, tinsel feel out of favor as a decoration, and modern varieties may not be conductive anymore anyway.

For a disposable item like this, you might want to investigate some sort of carbon-based conductive printing rather than a metal circuit board process or silver-based conductive ink which costs a small fortune. You don't need anything nearly as conductive as a normal circuit board, as you only need to detect if the connection is still intact or not.

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I would definitely do the proof-of-concept using readily-available materials first. What sort of projectile is it? Can you use 1mil mylar sheets, or do you need something really easily-breakable like cellophane or plastic wrap?

For the traces, perhaps one of those circuit repair pens will work. Or you can pick up a small jar of Bare Conductive's conductive paint and paint it on yourself.

If you have access to a laser, you can cut the pattern out of mylar sheet and use it as a stencil, then paint the conductive paint onto the sensor's plastic substrate.

If you want to go with CAD, the only free ones I know of that are popular are Eagle and KiCad. I haven't used either much. I have used ExpressPCB before for two projects -- while simple enough, you are stuck using their proprietary software, and you have limitations on the board size to get their special prototype pricing. You'll also need to do things like print text with conductive traces because you don't get soldermask on the cheap boards. From what I understand, Sunstone Circuits will take the Eagle format, so if you learn that, it should be a piece of cake to get the boards made. In addition, services like Dorkbot PDX sound awesome for low-cost runs if you're not in a rush.

The only thing I have heard of manufactured that is close to what you're talking about is a flexible circuit, but I don't think the plastic is easily broken.

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  • \$\begingroup\$ I'd like the projectile to be a .30 Cal M1 carbine bullet or faster, but for now it's a .22 Cal Air Rifle pellet. I'll def check out Eagle. Thanks ! The repair pen I tried was a total disappointment. I'm considering conductive spray paint but I have a bad feeling about it. I think the resistance will be uber high and not allow enough current to power my 74HC gated Flip Flips. The FF btw, are b/c I have no idea is bounce will occur with this design. I thought it'd be prudent...plus I have never worked with FF before... \$\endgroup\$
    – wbg
    Commented Oct 2, 2012 at 1:00
  • \$\begingroup\$ Did you look into Bare Conductive's paint, yet? It's not spray paint and you can control the thickness of the traces by doing multiple coats. You aren't limited to using flip flops, and you could possibly use the high-resistance (don't know what it is for BC's paint yet) to your advantage. Create several horizontal paths with the paint, and then pass this to an ADC to watch resistance. When trace(s) are broken, the resistance will change and this can be your trigger. You can submit your project and possibly win some free paint as well! \$\endgroup\$
    – Dave
    Commented Oct 2, 2012 at 12:16
  • \$\begingroup\$ And BTW hey, I am surprised I was right in guessing this had to do with a bullet. :) Do you want me to do a test pass for you on any old substrate (paper?) just to a resistance test? How wide do you want the traces to be? I'll have to get a reservation on the laser to cut a stencil first if the traces need to be narrow. \$\endgroup\$
    – Dave
    Commented Oct 2, 2012 at 12:20
  • \$\begingroup\$ thanks for the offer I'll say yes, b/c I'm curious to know if this will work, but please don't feel obligated to continue the experiment if it starts to get ugly. I'd love to have a die milled too, I tried to call in a favor earlier this summer but no love. Since the projectile is likely to be as small as .223 inches, I'd say half that, 0.11 inches would work with a gap of same. I'm open to your input too. \$\endgroup\$
    – wbg
    Commented Oct 2, 2012 at 23:03
  • \$\begingroup\$ The detection circuit I have built, is just +VCC --> Trace --> ground, and a parallel branch coming off between grd and the Trace, so that when the trace is broken, the FF gets +5VDC, which latches into the 'high' state and sets up one input on an AND gate. The second detector is the same. My 10MHz clock pulse also goes into (4 input ) AND gate and a dummy +VDC. \$\endgroup\$
    – wbg
    Commented Oct 2, 2012 at 23:04
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If you're in the US, buy a Sharpie magic marker, then head off to Radio Shack and get a slab of single sided bare PC board and a bottle of etchant.Yes, they still carry this stuff: I bought some just last month.

Now, draw the pattern you want on the board with the Sharpie. Let it dry and then go over it once or twice to make it a bit thicker. Nail polish also works, BTW. This is your etch resist.

Next follow the instructions on the etchant to etch away the copper not covered by the ink/nail polish. Note: FeCl2 stains everything it touches, including your fingers. Its fumes will also rust steel over time, so don't leave your container of etchant uncovered for weeks.

And now, you should have the pattern you want on the PC board. Clean it up with acetone (nail polish remover) from the local Walgreens and you're good to go.

If this won't work for you, just email me and I'll design a simple board for you (this will take like 5 minutes!) and email the CAD files along with the name of an outfit that will be happy to make you as many boards as you need.

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    \$\begingroup\$ I don't think he's looking for boards, though. It sounds like he wants an easily-breakable barrier. \$\endgroup\$
    – Dave
    Commented Oct 1, 2012 at 16:46
  • \$\begingroup\$ Good point. I think I got distracted by his question about CAD. That said, if you replace the PC board in my suggestion with thin copper foil, he will end up with a more fragile version of the same concept. \$\endgroup\$
    – lyndon
    Commented Oct 1, 2012 at 21:24
  • \$\begingroup\$ Thanks s much for the offer lyndon. I am hoping that I can manage the schematic myself, since it is so simple. It's kinda like "hello world" for CAD :) Haven't gotten around to trying out the various CAD software ideas presented here, yet. \$\endgroup\$
    – wbg
    Commented Oct 2, 2012 at 23:19

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