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Regarding who uses the 555's in their products... several years ago when I worked for a contract PCB manufacturer, we built the electronics for sirens used in emergency vehicles. The signal generator module inside used 3 of the devices per module and we made hundreds of these units per month.
10 years or so ago I did some repairs on some caulking guns that used 556 dual timer. I particularly remember that the circuit left power on the reset line when the supply voltage to the IC was removed.
It would only work with some brands of 556. Others wouldn't like the reset line being at a higher voltage than the supply.
I'm a believe, about the 1 billion. Several ways that these devises might not be seen. They can be deployed in the form of a die and share a custom package with other die's. They can also be etched onto a custom die where it may share die real estate with other circuitry.
The best example is the 556. It's really two 555's connected by either of the above methods. I don't know witch, maybe both.
The image in this link shows that the circuitry inside the 556 dual timer IC is contained all on one die. Personally, I wouldn't count that as two devices.
Back in the day 555's were more expensive than LM339's so we used 339's instead, and also the 339's where more versatile.
But the number of 1 billion per year is not just a made up figure, actually i have been buying 999,000,000 (999 million) devices every year myself leaving just 1 million for others. Once the mask wears out i'll be the only one left with any 555's and i'll then sell them for $5.55 each
Ok more seriously, i think it is a good question if that number is not inflated or taken from another year where they were sold that much. But the cost has come down from years ago too, so that could be a big factor as it was for us back in the 1980's. Once a part comes down in price you end up finding uses for it that you used something else for that happens to be more expensive now.
Well it is also a matter of stock space and functionality. The LM339 can be used for more things, and if you dont need the full function of the 555 anyway then you can use the LM339 without much more difficulty.
For example, in the typical clock oscillator the 555 might require 2 resistors and two caps, while the LM339 oscillator requires 4 resistors and one cap, but while then the 555 is "all used up", the LM339 still has three more sections not used yet that can be used for something else that probably really needs a comparator too like under/over voltage detection.
The frequency stability of the 555 is very very good too but it's not due to any special design of the chip active components, it's because of the internal connections of the resistors. That means we can build an oscillator that should have frequency stability just as good with an LM339 if we set it up almost the same. It turns out that it works pretty good because the two oscillators are wired almost the same.
One advantage the 555 has over the LM339 is the output drive capability is much better for the 555. The LM339 needs a small driver transistor to drive 100ma loads. I guess the main advantage of the LM339 is that you get four sections to work with instead of just one or two with the dual version of the 555.
The question is what to stock. Stocking both requires more space and more inventory complexity, so decisions are made based on price and flexibility of that part to fit into other circuits.
I liked the LM3905 because I thought it was much more elegant. With the LM3905, I found that if I added two parts, the power up glitch was prevented, but the LM3905 is not an oscillator. The 555 is an ersatz timer and an oscillator. The LM3905 is a Timer.
I forget how long I have been using the 555 and or the 556. Three years ago I designed a built a number of solar powered lights. I used s 556 to detect both dark and light. The ones I used were TS556IN. They are now obsolete. I used them as they were the cmos, and had a operating temperature temperature range of -40 to 125 degrees C. I still use the 555 for frequency generators, and pwm control just to name a few. So my stock is pretty good
Signetics built a Manufacturing Facility close to where I grew up 35 years ago; someone I spoke to who worked at the facility said; the "555" was the heart of their business at the time.
I think any hobby that doesn't have a 555 in his parts bin; isn't much of a hobbyist or just hasn't learned how much fun you can have with them.
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