Notes on projects: Switches, Cheap Flamable substances, and Power Supplies
I’ve been working on a lot of projects lately that interweave a fair amount with my work. These are some notes on ideas/designs for some things I will be building in short order, as well as generally useful information for procuring materials on the cheap related to these projects.
Flash Circuit Spark Gap, High Voltage Pulse Supply for a Coil Gun
There are a lot of things you can do with the flash circuit from a camera, since it functions as a charging unit for a 330V 120-150 microfarad capacitor, as well as provides a 3rd high potential electrode for creating a spark gap (which is usually a xenon tube, but if the other two electrodes from the capacitor are closer together, it can go through air as well).
Buying a disposable camera for $4 is probably the most convenient option for getting these circuits, however that starts to add up if you’re experimenting with several or want to put some in parallel for more power. The main advantage to using disposable cameras is that you would get a uniforml, exactly the same circuit every time that you would know how to hook up, as well as a camera enclosure that is easy to take apart.
I’ve read suggestions on various websites that say you should go to a film developing shop and ask nicely, and they may give you the used electronics minus the film they developed for free. However, the three places I went to all refused – even when I offered to pay for them at less than the cost per unit of getting a brand new camera.
Since I’m something of a cheapskate/thriftshopper, I went to my local St. Vincent DePaul’s thrift store and a few Deseret Industries stores and found plenty of flash cameras priced for $1 a piece. I’ve taken apart about 6 of these so far, and each one is a pain in the ass. They all have different placements of screws, use waaay too many tiny screws that are wound in there pretty tight (I had to use some Leatherman pliers and a precision screw driver to get some of them to even start to come out), and some of the cameras have some fancy features related to focusing, red-eye reduction, etc. that means you will find a a few big circuit boards inside that are rather complicated looking. The flash circuit itself is fairly simple and doesn’t take up much space, and sometimes it is on it’s own PCB and can be cut off from the rest. The bare circuit pictured above had two other small PCBs attached and was thankfully pretty simple to figure out. You’re best bet for finding cameras with simple circuits inside is to avoid ones with lots of features (variable ISO, any sort of automatic focus, etc.). Focus free cameras are a good bet. They will sometimes have a motor inside for winding the film, but this circuit is easily disconnected and ancillary to the main circuit board that powers the flash. Just make sure you test out the camera before hand to make sure the flash works (all the cameras I’ve run into don’t need any film in them to do so), and take it apart slowly and observe what the various external buttons and switches do internally to trigger things. If you do this while taking it apart, it’s a lot easier to figure out what wires to cross or put switches on later in an enclosure.
The nice thing about these $1 cameras is that, although they are harder to take apart, they often contain a lot of extra components that you can use later – small DC motors, LEDs, switches or brush contacts, small plastic lenses, and photodiodes.
Spark Gaps
As an atmospheric air spark gap circuit, you can generally get a good zap that can easily scare the shit out of me when I’m not expecting it, and even when I am sometimes. The good thing about such a spark is that it can reliably ignite vapors of alcohol! At work, we’ve been working on a large potato gun that has had trouble firing when we use a BBQ piezoelectric sparker on a long cord (for the safety of the guy with the trigger.. you don’t want to be anywhere near this potato gun when it goes off).
The main problem with using this circuit for an igniter is that you must make absolutely sure there is sufficient spacing between any high voltage wiring, otherwise there will be invisible sparks inside the launching circuit or along the 3+ ft. cable that goes to the spark gap inside the potato gun. However, the piezoelectric igniter only makes a small spark, and corrosion of the electrodes makes it even weaker every time, leading to an unreliable ignition when you push the button. This is partly why I wanted to build this spark gap – it should make a very good igniter for any combustion chamber we can hook it up to.
At first I wanted to use a standard 3-prong extension cord to connect the flash circuit enclosure to the endcap of the potato gun, which has the spark gap built in; however, this did not work. My guess is that the 3rd high potential electrode sparks to one of the other electrodes somewhere along the line before it reaches the spark gap. Using short alligator clip wires from Radioshack worked perfectly fine, so it would appear that there needs to be a bit more insulation for that wire in whatever cable you make, or you need to have it run along a separate cable that can hang freely away from the other two wires that are connected directly to the 330V capacitor. In a prototype I made, the 330V lines run along some speaker wire with thick clear plastic insulation, and the high potential line is connected to a 3rd red wire with similarly thick insulation that is allowed to hang a bit away from the speaker wire. This has worked with mixed success, though the prototype circuit was a bit banged up by this point and wasn’t working that reliably with short wires anyway, so I will have to do some further testing with this.
Coil guns
The circuit can also be used as a power supply for a pulse of high voltage current for a small coil gun. There are several instructables on making such devices, so I will not repeat this information here, however there are a few design considerations for using the same circuit that I’ve been considering.
First is that a coil gun device only uses the capacitor’s electrodes, and there is an immediate problem with this: discharging the device. From what I’ve been reading, and from personal experience, it is probably a bad idea to use a standard small on/off switch on one of the wires connecting the capacitor to the coil, since the sudden high voltage and current is likely to do some welding inside the switch. This requires something a bit different – a relay of some sort where switch contacts come together by a trigger that uses mechanical or electromagnetic means.
Since I prefer to avoid mechanical parts due to the fact that they wear out sooner, I started looking for options. Here’s what I found:
Reed Relays: uses an coil of wire that creates a magnetic field that will either close or open a reed switch (two ferromagnetic contacts inside a hermetically sealed tube). The main consideration here is whether or not you want to have to power a switch as well as the main circuit. If the voltage for powering the circuit happens to match the relay’s coil voltage (the voltage needed to close/open the switch), then this works out just dandy. Of course, this type of switch also necessitates having an extra on/off or momentary switch for the power supply of the reed relay itself.
I found a 5V 0.5A reed relay at Radioshack for $2. The flash circuit I’m using is powered by 3V, so hopefully that spec is a bit off and the reed relay can be triggered by a bit lower voltage. If not, I can just use it for some other project.
Magnetic Reed Switch: This is a reed switch that is triggered when a magnet is placed near it. It’s a bit simpler solution than the electomagnetic alternative since it doesn’t require it’s own power source to work and you can use anything from a ceramic magnet that’s right next to the switch to a neodymium magnet that’s capable of turning on the switch from several centimeters away. I happened to find an LED light intended for drawers that has a default-closed magnetic reed switch inside it. By placing the “magnetic sensor” right next to it on some part of the drawer that will move, when you open the drawer the LED will turn on automatically. I plan on taking it apart and seeing if this circuit can be used as a switch for my higher voltage application. Since I don’t know the specs for the components involved, it’s possible I could fry it, but considering I got it for $1 at Dollar Tree, I’m not too concerned about this.
Resistance Switch : This kind of switch is something I’ve always thought was really cool. You can convert a wall-powered lamp’s metal body into a capacitance switch using a kit that costs about $20 at Ace Hardware – i.e., to expensive for this project. However, I found a small desk resistance switch light with 2 small orb electrodes sticking out at Goodwill today for $2, so I grabbed it. Hopefully this circuit can be used for this type of application as well. We’ll see. I’m not sure how I’ll find more of them, but I sure hope there’s some store online that sells them for cheap because I’ll probably buy a few more if they only cost $5-6.
My hope for this type of switching system is that the switch itself works as an isolated electrical relay, so touching those resistance electrodes doesn’t mean I’ll be shocking myself if I hook up the switching mechanism to something with more than the fairly harmless voltage of a small lightbulb (3-6V).
Power Supplies
Of course, these projects are convenient because they can be run off of batteries – a low voltage, low drain application. But wouldn’t it be better if you could plug it into the wall if you were doing a lot of testing, or you happen to be using it indoors more than out?
Personally, I’d like to make mine to last, and something that can be powered by battery for portable use and wall-power if it’s going to be used indoors much for personal entertainment. So, I started looking around for some AC-to-DC power supplies (wall-warts) that were the appropriate voltages and currents. At Radioshack, most of the standard power supplies will run you about $16-$20 depending on voltage, amperage, or if you get a variable voltage power supply. This is kind’ve steep, so I started looking in thrift stores. I discovered that most cell-phone chargers are 3.7V 700mA power supplies, and a few are 5V 700mA. Also, Deseret Industries has a lot of electronics that are really underpriced for their actual value. I found a few 3, 4.5, 6, and 9V Radioshack branded, in-the-original-box power supplies for $2 each. Since I only really needed 3V for this and 6V for another project, this is what I grabbed.
The one thing that is unfortunate about using standard power plugs/jacks for this is that unless you happen to find an adaptaplug for your radioshack power supply in a thrift store, you’re stuck with buying them individually for $6 a piece or $14 for a set of four. I bit the bullet and bought a size M adapter for a $2 port (also at radioshack) that I could integrate into an enclosure for the circuit.
For another project involving speakers, I’ve been looking for a 12V 3A power supply to be used for the amp system. I already have a fairly nice stereo system that uses this type of adapter, and when I plugged the adapter into some amplified computer speakers I found at the Idaho Youth Ranch, it actually worked fairly well, so I figured I should find another power supply using these specs if I were to have an independent system for taking places. Before noticing that I needed 3A, I bought a 12V 900mA DC power supply at Idaho Youth Ranch for $2. This didn’t provide sufficient power to the amplified speaker system I bought for $4 there, so I kept looking. I stopped by an estate sale yesterday and happened to find an electric train track 12V 2A DC power supply for $5, and grabbed it. When I got home and tested it with a multimeter while it was plugged in, I found that it actually produced about 19V, with a knob for varying the amperage. It’s rated for 2A, but I was able to turn it up to about 5A before the Overload indicator turned on and there was an immediate drop in current. Since this thing is fairly ancient and starts to heat up after a few minutes, my guess is that it’s probably a good idea to not stray too far from the 2A on the box, but I think it could conceivably put out 3A for a while. Nonetheless, I’m not sure I want to hook it up to my speaker system for fear of frying some part of the amplifier circuits or otherwise damaging them by putting through too much voltage. So, I will have to keep looking for a 12V 3A supply or perhaps use a high wattage resistor to lower the voltage drop across the speaker system. I’m not sure if this would work since I don’t know much about electronics, but given that’s what is usually done to change the voltage drop across an LED hooked up to batteries that are higher voltage than what the LED is rated for, I’d guess this would work. Not sure, but I also I don’t have a high wattage resistor for trying this out.
Back to the flames…
As a sidenote, I’ve been looking into some cheap sources of flammable stuff for igniting. Butane lighter refill canisters are a good option, or even butane lighters – the gas can be pretty easily trapped in bubble solution or soapy films. It’s pretty fun to light that stuff inside a cheap wine glass with a lighter – the flame is fairly impressive.
Alcohol isn’t too hard to come by either though. I found a bottle of rubbing alcohol that is 50% ethyl alcohol at a Family Dollar yesterday. I also found some gel hand sanitizer that is 62% ethyl alcohol. The gel lights on fire pretty easily with a lighter, but I’m not yet sure whether it can be lit by a spark gap. We shall see! Either way, it is fun stuff to play with. A glob of it on a metal plate will burn down to a small amount of watery residue in a few minutes. The flame is mostly blue, so I think it is burning very efficiently and produces a fair amount of heat – I could easily burn my hand just holding it a foot above the small flame.
Another Sidenote on Unreliable Spark Gap Igniters: Use Matches
I’ve been working on some prototype spark gap igniters with the flash circuit in simple push light enclosures (the kind that look like a big staples “Easy” button) that can be easily had for a $1 at a thrift store or dollar store. It takes some modding, but it can be done. Best of all, the button for the light can be turned into the ignition switch! It kind’ve has a cheesy, cheaply made feel to it but the switch inside is quite reliable. Just be sure to exchange the switch for a momentary rather than the on/off kind typically in there.
In any case, as I was working with flash circuits and trying to put them inside one of these push lights, the first couple circuits got a bit banged up while I was figuring out the wiring and learning how to solder these things up. They were still capable of producing a smaller spark, but they just don’t do as well as a circuit that hasn’t been messed with quite as much. I’m not going to take the time to rework these and make sure there aren’t any shorts and such, but to get it to at least work for the purposes of lighting some alcohol in an ignition chamber, there is a simple solution: rub off some of the phosphorous from matchheads onto the spark gap electrodes. While the spark isn’t quite as big anymore, it does seem to be enough to ignite the matchhead stuff, which in turn is easily enough to light some alcohol vapors. This trick may be useful for a spark gap using a standard piezoelectric voltage source as well. It requires a bit more work than just having a reliable battery powered circuit, but if that’s what you’ve got and you need a solution in a pinch, this one seems to work fairly well.
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