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enter image description here

Above is the schematic I simulate in LTSpice. The input signal is a 8V pulse train with 100us ON time and 2250us period. The signal is fed as an input to the timer 555(NE555 in LTSPice).

I'm trying to stretch ON time to around 600us. So according to math R1 = 560 ohm and C1 = 1uF for this ON time.

But when I simulate in LTSpice whatever the R C combination is the output is the same: goes on with input's falling edges and goes off with input's rising edges.

Am I doing something wrong here?

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1 Answer 1

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A typical one-shot mono stable triggers if only the input is as such i.e. HIGH-LOW-HIGH

Hence the monostable triggers on falling edge of the input. If you invert your input signal you will get desired output.

Here's a example input-output waveform from NE555 Datasheet. T = 1.1 RC So 560 ohm and 1uF cap will give you 616 uS on time. enter image description here

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  • \$\begingroup\$ ohh i see many thanks it works now! but just a question, isn't my original input HIGH-LOW-HIGH as well? \$\endgroup\$
    – user16307
    Commented Dec 11, 2015 at 11:56
  • \$\begingroup\$ The thing is, your off time was 2150uS and on time was 100uS, hence your input width was more than output width. So yeah and i would be glad if you mark this as correct answer; it would help me :) \$\endgroup\$
    – ammar.cma
    Commented Dec 11, 2015 at 12:00
  • \$\begingroup\$ so the input high time should be always greater than the expected output width? this wasn't written in any tutorial i read. \$\endgroup\$
    – user16307
    Commented Dec 11, 2015 at 12:03
  • \$\begingroup\$ Let me draw a graph for you. imgur.com/wi2Ih2h That's your input signal. Now compare this to the input signal on datasheet. You'll get your answer. \$\endgroup\$
    – ammar.cma
    Commented Dec 11, 2015 at 12:13
  • \$\begingroup\$ u re right since the circuit works. but i still dont understand what is the difference with my first input and yours. Mine was HIGH LOW HIGH as well. and is the point trigger HIGH time here? should it be greater than some value? where is that written in the data sheet? \$\endgroup\$
    – user16307
    Commented Dec 12, 2015 at 3:49

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