ICS You Should Know : LM324 Quad Op Amp (chip with 4 operation amplifiers in 1 package)

lm324 pinout-note Vcc (current IN) and Vee (Ground)

The LM324 is a incredibly useful chip, perhaps more useful than any others except the venerable 555 timer, 386 amp, etc. 
That it's still being made today after more than 30 years of production is all the proof needed of its usefulness.
I'm NOT going to describe what uses an Op-Amp can be put to in and of itself, because I'll cover that in another IC weekly on something like the 386 or 741.
In this case the real importance and usefulness of the 324 comes from the fact that it has FOUR (4) Op-Amps in one package.

This also makes it easy to memorize the Pinout (which is the same in the SMD and the Through-Hole versions).  The 324 has 14 pins, 7 on each side.  This may sound complicated but all you really need to remember is the middle and outside pins.  For the middle, simply remember the Vcc (current IN, +) is the 4th pin and that Vee (Ground, - ) is the 11th pin.  For the outside, simply remember that EVERY CORNER is an OUT for the amps.  Everything else is an input.  This means all you have to really remember is 3 pins: Vcc, Ground, and Out (4 corners).  Simple.

Since the 324 has been in production as long as it has, its VERY very cheap. There are knock-offs, NOS (new old stock), vintage, overstock, copies made in China, Russia, Phillipines, etc as well as inexpensive new TI (Texas Instruments) ones.  You can find good ones for as little as 10 Cents a piece (.10 USD).  Can't beat that, can you?

I use it most often for LED audio meters (LEDs light in response to loudness of sound) where some of the OpAmps are used as comparators, some to boost/amplify the signal; Lo-Pass Filter modules that I often add to guitar synth pedals, synths, and other audio circuits, and simple mixer circuits.

The thing to keep in mind is that ANYTHING that employs any opamp like the 386 or 741 can also be used with the 324 -and then combined with another circuit employing the remaining opamps in the 324 (since the 386 and 741 both have fewer opamps in their packages)

Below is an example of a filter schematic I've used employing the 324 which requires 4 opamps:

multiple mode filter with multiple outs and pots to sweep frequencies with