# digital logic purpose of diode in this 555 timer application

Posted on Feb 5, 2014

Make a 555 timer base circuit such that O/P pin of 555 is held LOW by default at power on, and the I/P pin is held initially at HIGH at power on. The main requirement of my circuit is that until I/P pin is held LOW for say 200ms, then only the O/P pin must go HIGH and must remain HIGH as long as I/P pin is held LOW. What will happen if I pull up the control pin to Vcc by say a 1k resistor and remove the capacitor C3

Is the functionality of this circuit affected in some way then I don`t remember where but I definitely saw such a circuit which have this pull-up resistor. What if I keep Threshold pin left unconnected and rest all is same in this circuit - ie anode of diode D1 is now connected only to Discharge pin, R1, C1 and not to Threshold pin! Will now the circuit work to fulfill my purpose Is the condition R2xC2 < R1xC1 need to be hold true here This is in reference to this question actually. Any suggestions for values of R2, C2, R1, C1, and R4 On some tutorials it was specifically mention to use electrolytic capacitors for C1 and C2 here in schematic even for 1uF values (though they did not say that you must use electrolytic caps but they had drawn schematic with electrolytic caps. ). Is it necessary Will it make any difference Why can`t we use ceramic capacitors here for C1 and C2 I mean to say that what things I need to consider so that this circuit functions reliably and accurately. Absolute accuracy is not required - a tolerance of few tens of ms will work. Whether I should choose higher capacitance for C2 and lower resistance for R2, or higher resistance for R2 and lower capacitance for C2, for 0. 4 x R2 x C2 to be = 1 second typical and 880ms minimum Well, I thought that this update will be better to put as an answer rather that as an edit/update to Question. Also it would take too much space in Question. So I`m putting it...

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