555 and 556 Timer Circuits

  
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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 cases where a `555 timer` is specified. The 556 is a dual version of the 555 housed
555 and 556 Timer Circuits - schematic

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) because their 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 they are not labelled with their function. 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 100 µF) should be connected across the +Vs and 0V supply near the 555 or 556. Trigger input: when <1/3Vs (`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 >2/3Vs (`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 input impedance >10M Reset...



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