(if you’re wondering why they don’t short, which was my first question, the answer is “magnet wire”, further info in link)
So I’ve got a string with a bunch of modes it can run in; blinking, fading in/out etc. Some of these modes affect all LEDs at once, and some affect alternate LEDs. ie. so odd number LEDs light up while even number LEDs are off, then reverse)
How is that achieved?
There are different ways of doing this. One way is to separate LEDs into different circuits, but this requires more wires. Another way is to put all of the even number LEDs facing in one direction and the odd LEDs facing the opposite direction (opposite polarity across the wires). Apply a positive voltage and the even LEDs turn on. Apply a negative voltage and the odd LEDs turn on. You can combine this with multiple wires for even more variety, but a lot of them use only two wires for cost reasons.
Fancy effects are generally done with pulse width modulation. If you blink an LED fast enough, your eye can’t tell that it’s blinking. This generally happens around 15 to 20 Hz or so for most folks. Not coincidentally, the NTSC frame rate for televisions used to be 30 Hz.
If you blink an LED on and off at 30 Hz and have it on for 25 percent of the time and off for 75 percent of the time (aka 25 percent duty cycle) then the LED will, to your eyes and brain, appear to be on solid but dimmer than an LED that was on/off 50 percent of the time at the same frequency. By varying the voltage both positive and negative at a 50 percent duty cycle would make both the odd and even LEDs appear to be on simultaneously, even though at any given moment only one set of LEDs is actually conducting current in the forward direction of the LED.
To make the LED appear to blink, you just turn it on (again, probably using the same 50 percent on/off duty cycle) for a second, then off for a second, the on again, etc.
You can get a wide variety of effects with some creative pulse width modulation.
As for the wire, it’s probably a bit less confusing for some people to call it “enameled wire” (wire with an insulating enamel coating) than magnet wire, but whichever name you use, it’s the same thing. It’s called magnet wire because it is often wound around a magnet to create things like speaker coils or guitar pickups or the like, basically low-current and low-voltage (so low insulation requirements) applications where you often want to pack the most wire possible into the smallest area. Alternately, it can be used for things like LED strings just for cost and size reasons.
One way to tell if the lights are blinking rapidly are to turn them on the dark and wave them around.
Just to be clear on engineer_comp_geek’s suggestion. Think of the wiring diagram like a ladder. The rungs/LEDs alternate in polarity.
Note that an LED connected to power will burn out quickly if it’s not protected. Two ways of doing this: resistors (which waste power and produce heat) to limit current or by cycling the LEDs on and off. For a single LED a resistor is the cheapest option (often embedded into the bulb itself). But for a string of LEDs doing effects like this, the cycling circuit actually does double duty so it’s a net savings.
The 15-20 Hz that you refer to is (roughly) the threshold for perceiving a sequence of pictures as an animation (though it will still look jerky at that rate). However, the threshold for not perceiving flicker is much higher–somewhere around 50-60 Hz.
Cinemas only have a frame rate of 24 Hz, but typically triple flash each frame, so the flicker is at 72 Hz. The 30 Hz (actually 29.97 Hz for color TV) you mention for NTSC is the frame rate, not the field rate–which is 59.94 Hz. PAL TV, which runs at a 50 Hz field rate, was just on the unpleasant side of being flickery, so the better quality CRTs used a frame buffer to double the refresh rate to 100 Hz.
Even 60 Hz is fairly unpleasant for “sharp” light sources like LEDs. You can’t quite see the flicker staring straight at it, but it is liable to be noticeable in your peripheral vision, or when scanning your vision. You really want something on the order of a kiloHertz to look smooth under all conditions.