The Three Faces Of The 555

In these days of cheap microcontrollers, it is hard to remember there was a time when timing things took real circuitry. Even today, for some applications it is hard to beat the ubiquitous 555 timer IC. It is cheap, plentiful, and reliable. What’s interesting about the 555 is it isn’t so much a dedicated chip as a bunch of building blocks on a chip. You can wire those building blocks up in different ways to get different effects, and [learnelectronics] has a video showing the three major modes you typically see with the 555: astable, bistable, and monostable.

The 555 is really only a few comparators, a voltage divider, one or two transistors, a flip flop and an inverter. The idea is you use a capacitor to charge and the comparators can set or reset the flip flop in different ways. A reset input or the flip flop can turn on the transistor to discharge the capacitor.

The astable mode just generates a series of pulses. The bistable mode really just exposes the internal flip flop. The monostable reacts to an input by producing a fixed output pulse. Granted, you can make an Arduino or other microcontroller do all this but at the cost of complexity.

We’ve had plenty of 555 projects, some quite unique. We’ve also looked at the history of this venerable chip.