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Original question:

LM555: One Cycle Pulse Generator

SN74AHC123A: One Cycle Pulse Generator

Ron Michallick
Guru*** 144126 points
Part Number: SN74AHC123A

Hi all,

I want to make a one cycle pulse generator with an LM555 timer IC. The pulse timing after powering up the chip is: high (300ms) --> low (1s) --> high (continue until power off).

Help me make a circuit with the LM555 to meet the required pulse (given below).

                                                                                      300ms high                 1 second low                                           continuoe high till power off

As soon as the IC is powered up, the pulse should be started with the high edge.

Regards,

jitendra                                

  • 0
    TI__Mastermind 44040 points

    Jitendra,

    Assuming the power up is fast, the circuit below would be a 'rough' solution for this only using one IC. 

    I say rough because it won't give you the exact pulse lengths (there are a lot of variables involved). This would also be a circuit you would want to evaluate on bench before implementing in a design to get the timing as close as possible to the desired values. A quick summary of operation. The RC circuit tied to the B input is to provide the 300 ms after power up. The capacitor voltage should reach the input threshold voltage of the IC in one time constant (RC = 300 ms). When the threshold is reached the IC will trigger driving the output low for the amount of time set by the RC circuit tied to Rext/Cext and Cext (RC = 1 s). Then the output will remain high until power off (assuming no additional rising edge on the input).

  • 0 in reply to Dylan Hubbard
    TI__Guru*** 144126 points
    I was thinking about R+C applied to CLR pin on 1st timer set to a short time but greater than supply power up time (power up triggering). 1st timer then runs for 300ms then it triggers 2nd timer in the IC. That 2nd timer runs for 1 second. The only concern is that the second timer might accidentally trigger on power up.
  • 0 in reply to Ron Michallick
    TI__Mastermind 44040 points
    That is also a good solution and I believe there is some protection from the triggering during power up. I would have to dig into it to find out. Each method will have some variance in timing so if accuracy is critical then using a monostable multivibrator may not be the best option. If there is some tolerance they can work with then this will be a good 1 IC solution.