SCRAP - Articles

Article

Triple Stern Cannon Test Circuit

This little circuit allows you to test and tweak a set of triple stern cannons individually, and fire them simultaneously on the water. It's a fairly simple little circuit that bends the mind a bit, but gets the job done.


	The diagram for the cannon test circuit.

Before getting into how to build one, let's discuss the basic concept first. The concept of this circuit is based around the interesting uses of the SPDT (single pole, double throw) switch. This switch allows you to take a "common" (C) output and switch either of two inputs to that common output. One input is the "normally closed" (NC) input. When the switch is not pressed, the NC input is always routed to the C output. Conversely, the "normally open" (NO) input remains open until the switch is pressed. (Just like a normal SPST switch...)

That said, let's look at this circuit as though it were a single cannon test switch. The first thing you'll notice is that the - (black) input goes directly to the solenoid. This is the common ground. Second, you'll notice that the common output from the switch goes to the other lead of the solenoid. Now, if we just consider the "NC" portion of the switch, you'll notice it's attached to the green, switched input from the radio box. As a result, when the switch is not being pressed, the normal radio controlled signal can pass through the switch to the solenoid and fire the cannon. So, basically, until you press the switch, it acts as though it's not even there. Now, the magic part of this arrangement is the red wire, which goes to the normally open part of the switch. When the switch is pressed, it disconnects from the NC (radio) input, and switches to the NO (positive) input, bypassing the radio box, and sending a direct voltage to fire the solenoid. And (of course) when you let go, the switch is released, and returns to the NC (radio) input. Great, isn't it??? Best of all, you'll notice that this circuit will work just as well with one switch for one cannon, or two switches, for two cannons, three, or even four switches for four cannons.

That said, there's one little detail to explain. The funny triangle thing below the switch is called a diode. A diode is basically a one way valve for electricity. It lets it flow one way, but not the other. The reason there are diodes in this circuit is because of one of the funny properties of solenoids. A solenoid is basically an electromagnet, and the funny thing about electromagnets is that they use electricity to generate magnetism. Well, that's not the really funny thing. The really funny thing about them is that when you turn them off, the magnetism takes a very brief instant to go away. During that brief instant, the coil of the electromagnet generates a backwards electrical charge. If this backwards voltage spike is not dealt with, it can eventually damage your switches, or even your radio, and can definitely cause radio interference. Enter the diode. If you buy some diodes (I use the 50V, 3A variety), you'll notice that they have a black (or silver, or other contrasting color) band on one end. This corresponds to the crossbar on the triangular "diode" symbol, and indicates the direction that current is allowed to flow. As a result, if you look at the diagram, the diodes are placed so that when the + side is +, and the - side is - (as it should be), the diode is inactive and preventing current flow. On the other hand, when the solenoid generates one of those nasty reverse voltage spikes, the diode allows the current to harmlessly pass through it, saving your switches (and radio) from damage and interference.

To build this circuit, you merely have to pick up some diodes and SPDT switches from Radio Shack, if you want to, or you can order fancier stuff from your favorite electronics supplier. For those of you who want to get fancy, the switch I like to use is C and K #8121SHZGE, available from Allied Electronics, and their stock # is 676-0400, at a cost of about $5 each. Also, if you really need details for the diodes, I like the 1N4004 type, which you can get from Allied, #568-0144. Or, you can deal with the tedious catalog search tool for your favorite electronics dealer.

Anyway, once you have all those fiddly parts, solder it all together like in the diagram above. You might just want to test it with a multimeter and clip leads before doing anything dangerous. Then, once you have it all put together, connect your solenoids, and away you go. Hope it works as good for you as it has for me!