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NE555: NE555 to form a timer and output high when times up

Zhizhao Niu
Intellectual 2315 points
Part Number: NE555
Other Parts Discussed in Thread: SE555

Hi Team,

I want to use NE555 to realize the timer function. When the timing time is not up, the output is low, and after timing 2s, it outputs a high level to the subsequent circuit, which is used as an indication to the subsequent circuit. I have designed a circuit as shown below.

But I think this circuit is not prefect. Because at the beginning, the output is high which is similar with the output when times up. It may cause some error to circuit. Could you please comment on which circuit could realize the function as I mentioned above? Thanks a lot!

B.R.

Zhizhao

  • 0
    TI__Mastermind 25433 points

    Hi Zhizhao,

    Do you want the timer for 2s to start when the NE555 is powered on? What is the function of your switch in the real circuit? It looks like you close the switch for 2s and then the timer is low for about 14s which means the period of your timer is about 12s (not 2s). The output starts high because there is a low voltage on the THRESH/TRIG pin when the NE555 is powered on. If you start with the switch closed then the voltage on the THRESH/TRIG will start out high until you open the switch and the 47uF cap discharges. Will this solution work for you?

  • 0 in reply to Katlynne Jones
    TI__Intellectual 2315 points

    Hi Jones,

    Let me make it more clear.  I think this solution is work for me!

    Function：

    After power-on, I need NE555 output a low level signal and NE555 starts timing, such as timing 14s. After the timing ends, I need the NE555 output a high level signal. This high level output  will be used as a trigger signal to the subsequent circuit. 

    Do you means at begin I need to close switch first and then open the switch, then the output is what I want, right? 

    Is there a solution that does not require switch operation?

    Thanks!

    B.R.

    Zhizhao

  • 0 in reply to Zhizhao Niu
    TI__Intellectual 2315 points

    And one more questions that what is the difference between NE555 and CA555? Could I use NE555 to realize this function? Thanks a lot!

  • 0 in reply to Zhizhao Niu
    TI__Mastermind 25433 points

    Hi Zhizhao,

    According to the CA555 datasheet it is directly interchangeable with the NE555 and SE555, but it looks like the part is obsolete and TI does not sell it. You can use the NE555 for this design. If you double click on the timer and select the 3 dotes next to "Type" you can change the timer type:

    The timer will start on powerup with no switch if you use this configuration:

    C3 starts low (triggers the timer) and charges through R1 and the period of the timer is set by 1.1*R*C, the values I use have the timing set to about 14s. The output is inverted to get the waveform you want using an NPN transistor. The power has to be cycled for the output to be able to return low again.