Saturday, February 27, 2010

Watts the deal?

In my last post I tried to explain voltage, current, and resistance. Not too sure how well I succeeded, but no matter. Ever forward, right?

There's one more term that we'll see frequently - watts. Watts (W) are a measure of power, and are calculated by multiplying current by voltage. So if you have a 100 watt light bulb that runs off 120 volts, that bulb would draw .84 amps (100 watts divided by 120 volts). Pretty simple, huh?

So, what's the benefit of knowing all this, you may ask. Well, for starters you can figure out if you have enough power to run your haunt before you overload your power supply and burn your yard up (that would be bad). Just add up the total amperage draw for each circuit & make sure your power supply has a high enough amperage rating to exceed the total load.

Thursday, February 18, 2010

Just who is this Ohm fella, anyway?

In some of my previous posts I've talked a bit about some electronics projects I've been playing with. I've had some very positive feedback so far, but I've also had quite a few questions. So before I go on I thought I'd explain some of the electronic terms I'm talking about and how they affect each other.

So let's start with current. Current is the volume of electrons flowing through a circuit. If you think of electrons as people walking through your haunt, then current would be the number of people that it would take to keep the haunt full.

Current is measured in amperes, commonly abbreviated as amps. Lets say your haunts pathway is wide enough to allow two people to walk side by side, and it's always full because your haunt rocks. So for the sake of this explanation we'll say your haunt draws two amps.

The width of the path in this example is what limits flow. That limiting factor is called resistance.

Hallways, doors, props, and actors all increase the resistance to the flow of people. Similarly, wires, resistors, caps, microcontrollers, and virtually everything else in an electrical circuit contribute to the overall resistance.

Since you've built such an incredibly awesome haunt you have people lined up for blocks waiting to get in. Those people don't like waiting in the cold so they start pushing on the people in front of them, causing pressure. That pressure is called voltage.

So now we have three basic elements of electrical circuit design: current, resistance, and voltage. Back in 1827 this German guy named Georg Ohm didn't have anything better to do, so he figured out how current, resistance, and voltage relate to each other. What he came up with was Ohm's law (Georg was a bit narcissistic). Ohm's law states that I = V/R, where I is current in amps, V is voltage, and R is resistance in Ohms. (Yes, resistance is measured in Ohms. I'll bet that stroked old Georg's ego.)

So lets say things are running smoothly in your haunt. Then unexpectedly a bus load of German tourists shows up to check things out. They've heard great things about the 3-axis flying crank trash can trauma you've built, and in their excitement they start pushing, increasing the pressure (voltage) on the people going through. In order to keep things running smoothly, you can either a: increase the capacity or speed of your haunt (increase the amperage), b: remove obstacles (decrease resistance), or c: sedate the Germans (lower the voltage).

As long as you know 2 of the 3 values, you can figure out the third.

This diagram will help with Ohm's law calculations. Cover the variable you're looking for, and then perform the remaining calculation.

(Note: I'd like to apologize to any German bus tourists - I meant no offense. I'd also like to apologize for any unintentional snarkyness in this blog post. I'm not sorry for the intentional snarkyness, though.)

Tuesday, February 16, 2010

millicandelas and you!

In my previous post about LED wiring I discussed current ratings and voltage specifications, & how to wire LEDs so they don't catch fire (we hope). What I didn't talk about is what you get for all your trouble when you build a project using LEDs. Will that spotlight be bright enough to light your tombstone? Will your flying crank ghosts eyes be so bright they blind your trick-or-treaters? Hopefully this post will help us figure that out. A disclaimer - I'm no expert. If you're planning on building something critical like your own LED car headlight (please don't) then please look elsewhere for your technical guidance. This information is intended to point you in the right direction, but again I'm not an expert.

An LEDs brightness is measured in millicandelas. A millicandela - commonly written as mcd - is 1/1000 of a candela. A candela is equal to the light output of 1 candle. The light output of a lightbulb is commonly measured in lumens. 1 candela is roughly equal to 1 lumen. There are much more scientific definitions for both, but talk of steradians and 555 nanometer frequencies tends to put me to sleep. What we really need to understand is that your average 60 watt 120 volt incandescent light bulb puts out about 460 lumens:

and this LED puts out about 40,000 mcd, or about 40 lumens:

This is one LED at about 5 feet on a very dark night (sorry for the blurry pic, it's clipped from a video).

When you look at the specs of an LED you'll notice that there is usually listed an angle or viewing angle. This represents the directionality of the light output. Basically, the smaller the number the tighter the "beam" of light (think flashlight). This is important in that the tighter the beam, the smaller the effective useable area. For reference, the LED in the second picture above has a viewing angle of 12 degrees. At roughly 5 feet it effectively illuminates a 26 inch tall tombstone with very little overlap.

So basically, if you're wanting to build some cave eyes the 400 mcd LEDs from Radio Shack would work OK, but if you're wanting to illuminate your 7 foot tall Pumpkin Rot you'll probably want to look for something brighter.

There are some very good mcd to lumen calculators on line.

Saturday, February 6, 2010

Sparkfun free day goodness

I was lucky enough to get in on Sparkfun's free day a month or so ago. I figured this would be a good chance to play with some picaxe stuff, so I scored a few 08m chips & prototyping kits,& an 18m chip & board.

Now to see what kind of spooky trouble I can cause with it...:)