![]() It is connected to 0V when the timer output is low and is used to discharge the timingĬapacitor in astable and monostable circuits. The discharge pin is not an input, but it is listed here for convenience. It can be left unconnected if noise is not a problem. Input is connected to 0V with a 0.01♟ capacitor to eliminate electrical noise. Usually this function is not required and the control When not required it should be connected to +Vs.Ĭontrol input: this can be used to adjust the threshold voltage which is set internally Reset input: when less than about 0.7V ('active low') this makes the output low (0V), Otherwise the trigger input will override the threshold input and hold the output high (+Vs). * providing the trigger input is > 1/ 3 Vs, It monitors the charging of the timing capacitor in astable and monostable circuits. Threshold input: when > 2/ 3 Vs ('active high') Inputs of 555/556 Trigger input: when 2M. Buffer - an inverting buffer (Schmitt trigger).Bistable - a simple memory which can be set and reset.Monostable - producing a single pulse when triggered.The input and output pin functions are described briefly below and there are fuller explanations This is rarely a problem in simple circuits with no other ICs, but in more complex circuitsĪ smoothing capacitor (eg 100♟) should be connected across the +Vs and 0V supply ![]() Standard 555 and 556 chips create a significant 'glitch' on the supply when their output changes The 555 and 556 can be used with a supply voltage (Vs) in the range 4.5 to 15V (18V absolute Usually just the pin numbers are used and they are not labelled with their function. The circuit symbol for a 555 (and 556) is a box with the pins arranged to suit the circuitĭiagram: for example 555 pin 8 at the top for the +Vs supply, 555 pin 3 output on the right. ![]() The same pin arrangement as a standard 555. Used when specified (to increase battery life) because their maximum output current ofĪbout 20mA (with a 9V supply) is too low for many standard 555 circuits. Low power versions of the 555 are made, such as the ICM7555, but these should only be Show a 555, but they could all be adapted to use one half of a 556. Two timers (A and B) share the same power supply pins. The 556 is a dual version of the 555 housed in a 14-pin package, the ![]() With just a few external components it can be used to build many circuits, notĪ popular version is the NE555 and this is suitable in most cases where a '555 timer' The 8-pin 555 timer must be one of the most useful chips ever made and it is used in many Introduction Example circuit symbol (above) 555 and 556 Timer Circuits Inputs | Output | ![]()
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