By David Herres
The most successful IC, if sales volume is an indication, has been the 555 timer. With production averaging over a billion a year, this simple timer-oscillator sees widespread use in the world of electronics, wherever a blinking light, audio tone or bounce-free switch, to mention some prominent applications, is needed.
The 555 was introduced in 1971 by Signetics, later acquired by Philips. Numerous manufacturers currently market this little IC, with slight internal differences, operating parameters and part numbers, but essentially identical pin outs.
The IC contains all of 25 transistors, 15 resisters and two diodes, and generally it sells for under a dollar. Most commonly, it is seen in an eight-pin dual in-line package. Additionally, there is the 556, a single microchip consisting of two 555s in a 14-pin dual in-line package. Then there are the 558 and 559 devices which are 16-pin dual in-line units consisting of four 555s. Subsequently, a low-power CMOS version, TLC 555, has appeared.
For an overview, here is the pin out of the conventional 555:
The standard 8-pin 555 in a dual in-line package illustrates the simplicity and multiple functions of this remarkable device. As always, pins are numbered consecutively from the dot/notch, beginning with pin 1 and proceeding counter-clockwise:
Pin 1, GROUND: Provides the low level reference.
Pin 2, TRIGGER: A timing interval begins and pin 3, OUT, goes high when this input drops below half of the voltage applied to pin 5, CONTROL.
Pin 3, OUT: This output goes 1.7 V below Pin 1, GROUND, when triggered.
Pin 4, RESET: The timing interval resets when this input is connected to pin 1, GROUND.
Pin 5, CONTROL: Varies the timing of the RC network.
Pin 6, THRESHOLD: Detects 2/3 of rail voltage, making OUTPUT low if pin 6, TRIGGER, is high.
Pin 7, DISCHARGE: Becomes low when Pin 6, THRESHOLD, sees 2/3 of rail voltage.
Pin 8, SUPPLY: Must be connected to the positive supply voltage for the IC to work. The acceptable range is 4.5 to 15 V dc. For digital applications, use +5 V dc.
There are three operating modes, monostable, astable and bistable, also known as a Schmitt trigger. The 555 is a one-shot pulse generator in the monostable mode. When the voltage on the trigger pin is less than one third of the supply voltage, the pulse initiates. Its duration depends upon the values of the resistor and capacitor in the RC network. Until retriggered following the single pulse, the IC remains dormant.
In the astable mode, the 555 is an oscillator. At slow speeds it can cause an LED to flash. At high speeds it can function as a logic clock. In between it creates audio tones.
In the bistable mode (Schmitt trigger), the 555 is a flip-flop. Pin 7, DISCHARGE, is not connected and the external capacitor is not used. An important application is the bounce-free switch, made possible by the latching action of the 555, operating in its bistable mode.
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