555 Timers: Basics
HERE IS THE PINOUT OF A 555 CHIP:
(Note: A 7555 is the CMOS version of the 555 and uses a fraction of the 555's power. Pinouts are the same.)
MULTIVIBRATOR
555s are configured as a multivibrator by adding a few components to the pins. This is a circuit that oscillates from one state to another over time, in this case, creating a square wave. There are two basic forms of timers, both being multivibrators.Monostable Multivibrator:
A monostable multivibrator circuit has an output that stays in a given state (say +9V) until a separate signal triggers the timer. The state of the output is then shifted to 0V and will remain in that state until a pre-determinded time is reached. The output then goes back to +9Vs, waiting for a new trigger signal to come in. See the figure to the left for a graphical representation of this. This circuit is also called a ONE-SHOT or PULSE-STRETCHER.The timer the output changes states for is determined by a Resistor/Capacitor combination that will be described in the Monostable section of this site.
Astable Multivibrator:
An Astable Mulivibrator spontaneously switches from one state to another (say +9V to 0V to +9V…). The time it stays high and the time it stays low is determined by the values of the external components. This will be described in the Astable section of this site.
©Copyright 1999 * John Adams
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