There has been controversy regarding oscilloscope waveforms associated with square waves. There are THREE types of electricity, as - reversing AC, straight constant DC one direction, and on + off DC pulses. The AC + Pulsing DC with duty cycle have various frequencies. The output as tested with a probe for NE555 Timer GENERAL circuit is called on and off DC. This is NOT a square wave as AC. The signal goes in one direction only, pulsing, either positive or negative. Users must see where the zero base line is on oscilloscope. The on + off DC will be only above the base line. AC signal will be across both sides of base line. The 555 Timer can be adjusted for various frequencies, as also used for the self therapy Dr. Clark Zapper. That has what's called a pull up resistor to raise up the voltage higher to Vcc, with also 1/4 volt positive offset above the base line. The IC can also be configured for an actual AC output signal if desired. The other problem is that hand meters testing voltage will register on both AC and DC settings and different voltage. That's why you have to verify which waveform you are needing. (See my square wave generator - doesn't mean anything) The AC type output can be used with a voltage doubler circuit, but not the pulsing DC type. The NE555 is more stable than LM555. The CMOS 555 uses less power to operate. The 556 IC has (2) 555 inside it. There are reference books on the 555 timer with circuits. China sell cheap hand held oscilloscopes on Ebay, about $35, contrary to the large box scopes as used in old TV repair shops. Did you know the electrical human body has tissues with tiny cells that communicate with each other using electrical square waves? That's why the therapy Clark Zapper output is accepted to the body. The woman actress Karen Allen, in movie Raiders Of Lost Ark had cured her mosquito Lyme skin disease using her cheap Zapper with copper handles after spending $8000 on doctors. What do you think about the common 555 Timer?

 

What do you think about the common 555 Timer?

Fantastic, versatile integrated circuit. Wish I had designed it.

 

I think it's biggest bonus is a fairly high Output-Current, vs, most common Op-Amps or Comparitors.

But, my biggest personal discovery was FET-Gate-Drivers,
they are super-fast, with a high-Impedance CMOS-Input with hysteresis,
they both Push and Pull all the way to the Rails equally well,
and they provide substantial Current capabilities.
A lot of them are even available in a quasi "Comparitor" configuration with 2 Inputs.
And, they are perfect for driving heavily-Inductive-Loads such as Solenoids or Motors.
.
.
.

 

I think it's biggest bonus is a fairly high Output-Current, vs, most common Op-Amps or Comparitors.

But, my biggest personal discovery was FET-Gate-Drivers,
they are super-fast, with a high-Impedance CMOS-Input with hysteresis,
they both Push and Pull all the way to the Rails equally well,
and they provide substantial Current capabilities.
A lot of them are even available in a quasi "Comparitor" configuration with 2 Inputs.
And, they are perfect for driving heavily-Inductive-Loads such as Solenoids or Motors.
.
.
.

And you can configure them in all the traditional 555 circuits (with a schottky diode to provide the "discharge" function)

 

@russwr You have painted a rather simplified picture of what the 555-tiimer IC can do. It is much more versatile than that.