555 Timers
It is not normally our policy to quote other web pages, we just link to them. This may be reverted to just a link. What do other admins think? --russ_hensel 01:46, 18 January 2010 (UTC)
The 8-pin 555 timer is one of the most useful Integrated Circuits (ICs) ever made and it is used in many circuits. It is also one of the cheapest and most-rugged chips on the market costing anywhere from $0.20 to $1.20 depending on the quantity and distributor
With just a few external components it takes about 10mA (even when the output is not driving a load) and will operate from 4v to about 16-18v which can be used to build many circuits, not all of them involve timing! A popular version is the NE555 and this is suitable in most cases where a '555 timer' is specified. You may have heard of a 556 timer also, this is simply a dual version of the 555. The dual version (556) is housed in a 14-pin package, the two timers (A and B) share the same power supply pins.
The 555 relies on both analogue and digital electronic techniques to perform its functions, but if we consider its output only, it can be thought of as a digital device. The output can be in one of two states at any time, the first state is the 'low' state, which is the voltage 0V at the '-' (black) connection of your power supply. The second state is the 'high' state, which is the voltage Vs at the '+' (red) connection of your power supply. Vs is commonly 5V when using digital ICs. For convenience, these states are sometimes referred to by other names:
Low is also known as 'space', or 'logic 0'. If a digital device is said to be 'off', its output is normally 0V. High is also known as 'mark', or 'logic 1'. If a digital device is said to be 'on', its output is normally Vs.
The most common types of outputs can be categorized by the following (their names give you a clue as to their functions):
Astable - producing a square wave Monostable - producing a single pulse when triggered Bistable - a simple memory which can be set and reset Buffer - an inverting buffer (Schmitt trigger)
Original source from 555-timer-circuits.com.
