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Description
The 555 comes in an eight-pin dual-inline package. Inside the chip is a set of circuits that serve as building blocks for a high-speed electronic switch. In all, the IC has 28 transistors. It doesn't function as a timer all by itself, but depends on a handful of external capacitors and resistors; how these outboard components are connected determines what the 555 does. The 555 has a sibling component, the 556, which combines two independent 555 timers in a 14-pin package.
Function Blocks
The 555 contains a handful of electronic building blocks that you configure for different results. It has a flip-flop, which has two stable electrical states, on and off, and when you send an electrical pulse to the flip-flop, it changes states. Two comparators also produce on and off electrical output signals. If an input voltage crosses a threshold, the circuit turns on or off, depending on if the voltage increases or decreases. Finally, the 555 has an electronic output stage capable of driving external devices or circuits with up to 200 milliamps of current.
Components and Connections
A capacitor and a pair of resistors determine the time duration of pulses the 555 produces. The capacitor connects between pin 6 and electrical ground at pin 1. A pair of resistors between pins 8 and 6 set the circuit̵7;s ̶0;on̶1; time; of the two resistors, the one between 6 and 7 sets the ̶0;off̶1; time. The total duration of the 555̵7;s pulse is the sum of on and off times, which can range from microseconds to hours. The chip receives power between pins 8 and 4. It can receive a trigger pulse to reset its cycle at pin 2, and outputs its timing pulse at pin 3. An external control voltage applied to pin 5 shortens the 555̵7;s timing cycle.
Applications
The 555 works in one of several different modes, depending on its connections. In the astable mode, it produces a repeating timing pulse having a frequency set by the resistor pair and capacitor. In monostable mode, an external trigger pulse applied to pin 2 makes the timer generate a single output pulse. It has a bistable mode, where a pulse at pin 2 turns the chip on, and a pulse at pin 4 turns it off. Because the chip produces pulses in the audio range, it can serve as part of a siren or noisemaker circuit. Its rectangular pulse output can drive computer and digital circuits. Used for long intervals, the 555 produces pulses to control time exposure for cameras or scientific experiments.