Introduction to the 555

Thread Starter


Joined Mar 24, 2008
The 555 IC

The 555 integrated circuit is the most popular chip ever manufactured. Independently manufactured by more than 10 manufacturers, still in current production, and almost 40 years old, this little circuit has withstood the test of time. It has been redesigned, improved, and reconfigured in many ways, yet the original design can be bought from many vendors. The design of this chip was right the first time.

Originally conceived in 1970 and created by Hans R. Camenzind in 1971, over 1 billion of these ICs were made in 2003 with no apparent reduction in demand. It has been used in everything from toys to spacecraft. Due to its versatility, availability, and low cost it remains a hobbyist favorite.

One of the secrets to its success is it is a true black box, its symbolized schematic is simple and accurate enough that designs using this simplification as a reference tend to work first time. You don’t need to understand every transistor in the base schematic to make it work.

It has been used to derive the 556, a dual 555, each independent of the other in one 14 pin package, and is the inspiration of the 558, a quad timer in a 16 pin package. What few weak points the original design has have been addressed by redesigns into CMOS technology, with its dramatically reduced current and expanded voltage requirements, and yet the original version remains.

Originally conceived as a simple timer, the 555 has been used for oscillators, waveform generators, VCO’s, FM discrimination, and a lot more. It really is an all purpose circuit.


The 555 Timer IC – (An Interview with Hans Camenzind, inventor of the 555 IC)

555 Tutorial

555 Timer IC Encyclopedia Article.
Last edited:

Thread Starter


Joined Mar 24, 2008
I Wrote this for the eBook 11/25/2009 (yep, I'm an old broad). Just reposting it after it disappeared for my blog.


Joined Mar 14, 2008
Your tutorial link doesn't work for me.

Here's my short description of the device operation:

It is a level-triggered latch (flip-flop) with an added DIScharge output to reset the timing capacitor.
When the TRIG voltage goes below 1/3 Vcc, the latch is set (OUT high and DIS open).
If the TRIG voltage is back above 1/3 Vcc, then when the THRS voltage goes above 2/3 Vcc, the latch is reset (OUT low, and DIS connected to ground to discharge the timing capacitor).
It stays low until a low on the TRIG input again sets the latch (if the THRS voltage is below 2/3 Vcc).
From that you should be able to understand the operation of the 555 in it's various modes of operation, such as astable and monostable multivibrator.