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LM555: Output stays high (not a pulse)

Joshua David Alfaro
Prodigy 20 points
Part Number: LM555
Other Parts Discussed in Thread: TINA-TI

I am attempting to use the LM555 in monostable mode but the output doesn't generate a pulse. It simple follows the state of the trigger. I captured the attached waveform and it shows that the capacitor voltage (pin 6/7) doesn't disappear (CH3). Therefore, we see that the pulse "works" for the expected period of time but instead of going low, it enters this astable mode. It only goes low when trigger goes back to 1. I am using a digital IO PC card for the trigger manually so I am triggering much slower than the desired length of the pulse.

Note: Output has voltage divider so that measured pulse (CH4) is half the out value.

Datasheet examples always show trigger being a shorter pulse than the output pulse. Is this why? Does the trigger need to be shorter or it will otherwise keep the output on like I am seeing.

  • 0
    TI__Mastermind 41503 points

    Hi Joshua, 

    I can't see your images. Can you post again using the "Insert Media" button. 

    However, it does sound like the trigger input is your problem. The LM555 timer is level sensitive (not edge sensitive) so if the trigger pin is held low, the timer will continue to retrigger until the pin is returned high. There is a way around this though. Take a look at this Tina-TI simulation where the trigger input is AC coupled. 

    TLC555 - Monostable, AC Coupled, One-shot output of 1second.TSC

    As you can see in the simulation, the Vtrig_Safety input signal is held low, but the AC coupled Vtrig_IC signal is brought low to trigger the timer and then returns high after a few milliseconds. You may want to consider adding this to your circuit. You can modify the values of R5 and C4 to adjust how long the trigger pulse lasts.

    Let us know if you have any other questions.

    Best,

    Katlynne Jones 

  • 0 in reply to Katlynne Jones
    Prodigy 20 points

    Hello,

    I have attached my circuit and waveform that I measured. Yes, being level triggered explains why a trigger longer than the desired output causes my output to follow the trigger, i.e not a pulse. Thank you also for the suggestion.

  • 0 in reply to Joshua David Alfaro
    TI__Mastermind 41503 points

    Hi Joshua,

    One comment about your schematic. I see you are switching between resistance values to change the timing. One of your values is 0ohms. That resistor can't be 0 because the discharge pin will try to short your 10V supply to ground. 

    Best,

    Katlynne Jones