The 555 timer IC is an incredibly useful timer that can act as either a timer or an oscillator. In timer mode, better known as monostable mode, the 555 simply acts as a “one-shot” timer; when a trigger voltage is applied to its trigger lead, the chip's output goes from low to high for a duration set by an external RC circuit. In oscillator mode-better known as astable mode-the 555 acts as a rectangular-wave generator whose output waveform (low duration, high duration, frequency, etc.) can be adjusted by means of two external RC charge / discharge circuits. The 555 timer IC is easy to use (requires few components and calculations) and is inexpensive and can be used in an amazing number of applications. For example, with the aid of a 555, it is possible to create digital clock waveform generators, LED and lamp flasher circuits, tone-generator circuits (sirens, metronomes, etc.), one-shot timer circuits, bounce-free switches, triangular-waveform generators, frequency dividers, etc.
The 555 gets its name from the three 5-k res resistors used to implement astable multivibrator. These resistors act as a three-step voltage divider between the supply voltage (VCC) and ground. The top of the lower 5-k res resistor (+ input to comparator 2) is set to 1/3VCC, while the top of the middle 5-k res resistor (- input to comparator 2) is set to 2/3VCC. The two comparators output either a high or low voltage based on the analog voltages being compared at their inputs. If one of the comparator's positive inputs is more positive than its negative input, its output logic level goes high; If the positive input voltage is less than the negative input voltage, the output logic level goes low. The outputs of the comparators are sent to the inputs of an SR flip flop. The flip-flop looks at the R and S inputs and produces either a high or a low based on the voltage states at the inputs.
In the astable configuration, when power is first applied to the system, the capacitor is uncharged. This means that 0 V is placed on pin 2, forcing comparator 2 high. This in turn sets the flip-flop so that Q_ is high and the 555's output is low (a result of the inverting buffer). With Q_ high, the discharge transistor is turned on, which allows the capacitor to charge towards VCC through R1 and R2. When the capacitor voltage exceeds 1/3VCC, comparator 2 goes low, which has no effect on the SR flip-flop. However, when the capacitor voltage exceeds 2/3VCC, comparator 1 goes high, resetting the flip flop and forcing Q_ high and the output low. At this point, the discharge transistor turns on and shorts pin 7 to ground, discharging the capacitor through R2. When the capacitor's voltage drops below 1/3VCC, comparator 2's output jumps back to a high level, setting the flip-flop and making Q_ low and the output high. With Q_ low, the transistor turns on, allowing the capacitor to start charging again. The cycle repeats over and over again. The net result is a square wave output pattern whose voltage level is approximately VCC – 1. 5 V and which on / off periods are determined by the C, R1 and R2.