Timer 555

555 and 556 Timer Circuits
Introduction
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Example circuit symbol (above)Actual pin arrangements (below) |
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The 8-pin 555 timer must be one of the most useful ICs ever made and it is used in many projects. With just a few external components it 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 caseswhere a '555 timer' is specified. The 556 is a dual version of the 555 housed in a 14-pin package, the two timers (A and B) share the same power supply pins. The circuit diagrams on this page show a 555, but they could all be adapted to use one half of a 556.
Low power versions of the 555 are made, such as the ICM7555, but these should only be used when specified (to increase battery life) becausetheir maximum output current of about 20mA (with a 9V supply) is too low for many standard 555 circuits. The ICM7555 has the same pin arrangement as a standard 555.
The circuit symbol for a 555 (and 556) is a box with the pins arranged to suit the circuit diagram: for example 555 pin 8 at the top for the +Vs supply, 555 pin 3 output on the right. Usually just the pin numbers are used and theyare not labelled with their function.
The 555 and 556 can be used with a supply voltage (Vs) in the range 4.5 to 15V (18V absolute maximum).
Standard 555 and 556 ICs create a significant 'glitch' on the supply when their output changes state. This is rarely a problem in simple circuits with no other ICs, but in more complex circuits a smoothing capacitor (eg 100µF) should be connected across the+Vs and 0V supply near the 555 or 556.

The input and output pin functions are described briefly below and there are fuller explanations covering the various circuits:
* 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)

Inputsof 555/556
Trigger input: when < 1/3 Vs ('active low') this makes the output high (+Vs). It monitors the discharging of the timing capacitor in an astable circuit. It has a high input impedance > 2M.
Threshold input: when > 2/3 Vs ('active high') this makes the output low (0V)*. It monitors the charging of the timing capacitor in astable and monostable circuits. It has a high inputimpedance > 10M.
* providing the trigger input is > 1/3 Vs, otherwise the trigger input will override the threshold input and hold the output high (+Vs).
Reset input: when less than about 0.7V ('active low') this makes the output low (0V), overriding other inputs. When not required it should be connected to +Vs. It has an input impedance of about 10k.
Control input: this can be used toadjust the threshold voltage which is set internally to be 2/3 Vs. Usually this function is not required and the control input is connected to 0V with a 0.01µF capacitor to eliminate electrical noise. It can be left unconnected if noise is not a problem.
The discharge pin is not an input, but it is listed here for convenience. It is connected to 0V when the timer output is low and is used todischarge the timing capacitor in astable and monostable circuits.

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Output of 555/556
The output of a standard 555 or 556 can sink and source up to 200mA. This is more than most ICs and it is sufficient to supply many output transducers directly, including LEDs (with a resistor in series), low current lamps, piezo transducers, loudspeakers (with a capacitor in series), relay coils(with diode protection) and some motors (with diode protection). The output voltage does not quite reach 0V and +Vs, especially if a large current is flowing.
To switch larger currents you can connect a transistor.
The ability to both sink and source current means that two devices can be connected to the output so that one is on when the output is low and the other is on when the output is...