If so, could it be the IC that has been used for the longest time? Or do modern circuits just have timing built into a large chip.
Yes, very much so !
Probably not ( a quick search reveals that the 741 op amp chip has been around
longer ! )
Modern circuits will often use programmable chips (eg arduinos) which can replace
a lot of discrete components - including timers - with software. But the 555 isn’t
likely to go away any time soon.
Agreed that the 555 timer IC is extremely popular; by at least some measures the 555-family of timers including the 556 and 558 it is the widely most produced microchip design in the world. However, outside of cheap household devices that need a simple timer I don’t think it is really in much use in electronics manufacturing today. With the advent of easily programmable low power embedded systems designing separate circuits for just a timer isn’t really useful, and the costs of having to package and mount even a small 555 just aren’t worth it. That being said, the 555 timer IC will remain a mainstay of basic electronics hobbying and education for decades to come because it teaches basic principles that can be built upon and that you won’t learn by programming an Arduino or Raspberry Pi.
Stranger
Even though a modern microcontroller can be almost as cheap as a 555 (and a lot more flexible), you don’t need a computer and software to use the 555.
To be fair to the microcontroller (which, thanks to miniaturization, includes timers and other hardware), while you may need a computer and software to program the non-volatile memory, just like you may need a socket and soldering iron to attach it to your circuit board, it may then work for possibly years with no intervention necessary.
I can’t say that I’ve ever encountered a 555 IC in all my various electronics teardowns. Mystery microcontrollers are a dime a dozen (almost: the cheapest I’ve seen is more like 38 cents per dozen). They’re easier to use and understand than a 555. They don’t even need any passives.
I’m not even certain they’re all that great pedagogically. It’s an analog device performing a digital task. Analog electronics are still useful for doing analog stuff, but mixing the two seems strange at this point.
Digikey (the first place I checked) has about 8,000 of them in stock, so yes, 555 timers are still being produced and sold. There are also low power versions of the 555 available.
I personally wouldn’t use one these days, but I tend to stick a microcontroller into just about anything I design. You can integrate a lot of functionality into a microcontroller in addition to the timing. This keeps the parts count low, which reduces cost and helps reliability (the fewer parts you have, the fewer parts can fail). For me, programming the microcontroller is no biggie. Getting the part pre-programmed during board fabrication is also easy enough to do. At my job we rarely program parts in-house except during initial development.
The 555 is a cool little analog chip that can be used to make a monostable (one-shot) or astable clock. It’s super easy to use and doesn’t require any code. But because the period depends on the absolute resistance value of a resistor and the absolute capacitance value of a capacitor, it is very prone to drift and thus not precise. It becomes especially problematic when using it to generate very long periods. Crystal-based clock sources are much more precise, and long periods can be generated using a counter. And if you already have a microcontroller or μP in the circuit, then you already have a precise timer at your disposal.
So today the 555 is mainly used for educational purposes and by DIY hobbyists.
Although with a microcontroller it is easy to fool yourself into thinking that your system is all digital, electronics are fundamentally analog in nature. Modern microcontrollers are so reliable and stable that you can spend a career programming them to do digital output tasks and treating them as a black box without every considering anything going on inside (and I doubt many users have spent any time learning about them beyond the interface) but if you are using them to control something that is analog like a stepper motor, or taking into data measured by analog instrument, understanding the design considerations of error and bias in analog systems is very important. I have too many examples to count from TVC and battery controllers to breakout boxes and test sets where a failure to correctly understand the practical behavior of analog devices controlled by a digital microcontroller resulted in “anomalous” behavior even though the anomaly was just that reality isn’t binary.
But, as @Crafter_Man notes, it isn’t something you’d use in modern industrial electronics because it isn’t very precise, and even for commercial applications microcontrollers are so cheap and readily available, and at this point so much easier to program than earlier generations, that it would not be cost effective to use a 555 IC even if there were some reason to need a standalone timer. But that lack of precision is actually useful for illustrating certain principles, and wiring a 555 timer IC on a breadboard and watching it drive a countdown circuit is viscerally more satisfying for a hobbyist than writing the few lines of code to do that on a Raspberry Pi.
Stranger
I’m not at all opposed to teaching analog electronics. Any engineer should be exposed to some of this. If nothing else, I think it’s valuable to help expose the analogies (!) between analog electronics and other physical phenomena like water flow. It helps to reinforce abstract reasoning, where sometimes the governing equations stay the same even when the underlying details are different.
It’s just the particular case of timing that I don’t think mixes well with analog. It’s been long established that the best way to do timing is to establish a stable, high-speed oscillator and count ticks. The oscillator may be a quartz crystal or a cesium-133 atom or something else, and the definition of “high-speed” depends on the context (a quartz watch uses a 32,768 Hz crystal; a computer uses something much higher and a PLL to increase the rate further). You could even use the 555 itself as the oscillator, though I don’t know why anyone would want to. At any rate, it is the tick counting that makes timing this way a digital task.
Fundamentally, a 555 is a bang-bang controller. It has a low comparator, a high comparator, and a flip-flop. Hit the high point, bang, it flips the switch off; hit the low point, bang, it switches back on. It’s the same thing as a thermostat or a fluid level controller.
Bang-bang controllers are common and a useful abstraction to learn (like other controllers). It’s just that tying it to timing specifically is weird to me, since there are more straightforward examples of these controllers, and timing is better handled by other means. At least when I played with them, I didn’t feel like I was actually learning anything fundamental–I was just learning how to configure a 555. There are a few common configurations and you looked up resistor values in a chart to get the duty cycle and such that you wanted.
It turns out I went to high school with the daughter of the inventor of the 555:
I only found this out decades after graduating.
Which is why I felt I.could open this thread without feeling stupid. 
About five years ago, I subbed for a high school electronics class for a couple of weeks, and darned near every project the students made included a 555.
But yeah, nowadays it’s much easier, for practical purposes, to just program an Arduino to do whatever it is you want. A few months ago, some folks at my church were trying to replace a very specific but imprecise timer, and the only ones they were finding were hundreds of dollars, and I realized that a $30 Arduino could do the job just as well.
(if anyone’s wondering, our nativity set is illuminated by two lights, one yellow-white and one blue, and it alternates between the two every 5ish minutes to simulate a day-night cycle. The duration of the cycle could vary by a factor of 2 and not be “off”, so long as one light shuts off at the same time the other turns on. By the time the discussion got to me, my uncle had already cobbled something together with a microswitch and a cam wheel, but when that wears out, it’ll probably fall to me to do the next replacement.)
$30???
What’s wrong with a $1 or $2 microcontroller?
The $30 Arduinos contain a $1 microcontroller. However, they’re nicely integrated on a board, have a USB port (with serial->USB converter), power supply, an LED, a button, a crystal, and other stuff. You’re also supporting Arduino educational outreach programs and the development of the Arduino IDE.
But personally, I buy clone Arduino boards for $3. They generally work well enough, though sometimes the serial->USB converter is dicey. Sometimes even the microcontroller is a knockoff! But for the most part they still work well enough and for a simple light controller they’d be totally sufficient. I’ll pay the extra $2 to have the USB port, the power supply, etc. already integrated.
My favorite hobbyist board is the Raspberry Pi Zero W, though. $10 for a real Linux system with WiFi. Even my hobby clocks use them simply because I can debug them via SSH and they can sync their time via NTP. I’d use a lot more of them if they were actually in stock anywhere, though…
An Arduino comes with a nice programming interface and a lot of accessories to easily let it control just about anything you can think of from a simple robot to a home automation system as well as creating an accessible user interface. I would have used a Raspberry Pi for that application but it’s really six of one, half a dozen of the other.
Stranger
It’s true that I already have stuff to flash a small controller with, as well as uncountably many electronic parts like LEDs, switches, relays, transistors, diodes, wires, 555s, op-amps, etc. already lying around. If one was just starting out, then those Arduino kits, some of which include multimeters, breadboards, and batteries, start to make a lot more sense.
ETA one project I was tangentially involved with a long time ago ended up with both a Raspberry Pi and one of those cheap Arduino clones inside the box. The reason IIRC was that the Pi lacked an ADC so the $3 Arduino was used to drive the sensor board, perform an FFT and send the results to the Raspberry Pi which took care of all further processing and the user interface. Don’t worry, it was not an industrial or laboratory project…
I mix and match all the time. It makes plenty of sense.
My tweeting litterbox used a RasPi and Arduino. It needed a sensitive ADC (to measure the load cell voltage), and the ones I had had a weird synchronous protocol. I tried using it with the RasPi, but there was too much background stuff going on, and if you missed a bit you’d get the ADC completely out of whack. I could have loaded an RTOS I guess, but I wanted to keep that part simple. So instead I used an Arduino to interface with the ADC and sent the data to the RasPi via UART.
My current project is a giant 7-segment LED clock, but the LEDs need pretty high current (~20 mA) and a higher voltage than the RasPi can handle (technically I only need 3 V, but 3.3 V isn’t quite enough margin for the limiting resistors IMO), and more importantly I need 29 GPIOs, which the RasPi Zero doesn’t have. Oh, and I’m out of RasPis and I can’t seem to find more. So instead I’m using an ESP8266 as the main controller and two Arduino Nano clones to drive a pair of digits each. The Nanos can run at 5 V, have plenty of pins, plenty of current per pin, and since they’re independent I can do some SW PWMing for smooth transitions. For simplicity, I’ll probably just use the UART in “broadcast” mode; i.e., communicate with both Nanos at once and use a destination address to distinguish them. I found an NTP library that works with the ESP8266 for time syncing.
I still have a PICKit programmer and some other junk, and if I really needed to I’d fab some custom PCBs (I did that for my Nixie tube clocks), but mostly it’s just easier to cobble stuff together with Arduinos and such.
My niece used a 555 just last week for a school project (a RC oscillator using a 555 driving a piezo speaker via a MOSFET power stage, for ultrasound mist generation in a greenhouse).
I used one a few months ago. All I needed was something that waited a while after it was triggered, then produced a beep. The simplest way to do it (for me) was a 556.