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NE555: NE555 out(Pin3) keep High status

Hugo Liu
Genius 5095 points
Part Number: NE555

Hi Sir,

Our schematic as below, if we trig TR(Pin2) then OUT(Pin3) change Low to High,
But why out(Pin3) is keep High status similar Locked, we need Short Pin5 to GND Discharge CB37, OUT(Pin3) unlock to Low status.
Tw=1.1RC not keep High status right? any problem in this schematic?

  • 0
    TI__Guru*** 140166 points
    Hugo,

    If COM1-TX is low or was low less than 52 us ago then out should be high. (actually less than 52us due to diode voltage drop)
    If COM1-TX is high (or open) for at least 52 us then output should be low.

    When the output is stuck high, record voltages on all the device pins (ideally at the NE555 leads).
  • 0 in reply to Ron Michallick
    Genius 5095 points

    Hi Sir,

    We are trig RX(CMO1) to TRIG(Pin2), Q(Pin3) change to High status,

    wait over 5 minute, but Q(Pin3) never change to Low status. 

    we need short CV(Pin) 5 to GND Discharge,  OUT(Pin3) unlock to Low status.

  • 0 in reply to Hugo Liu
    TI__Guru*** 140166 points
    Hugo,

    Thanks for the detailed scope capture. It is very helpful. The trigger voltage input high is only 2.84V so the diode D5 only allows Threshold voltage to climb to 3.48V which is very close to 2/3 VCC (the voltage on pin 5) especially if VCC is slightly higher than 5V. Try forcing pin 5 to 3V as a test to see if that fixes the timing.

    If that works then pin 5 voltage can be lowered, or D5 cathode voltage could be increased.
  • 0 in reply to Ron Michallick
    Genius 5095 points

    Hi Sir,

    the Pin 5 is 3.52V now, this pin is I/O link CB37=0.01uF to GND, How to forcing pin 5 to 3V? short to GND?

  • 0 in reply to Hugo Liu
    TI__Guru*** 140166 points
    Hugo,

    Notice that pins 6 rises to only 3.48V and pin 5 is 3.52V therefore pin 6 never trips the threshold comparator.
    You could try adding 10k resistor in parallel with (or in place of) CB37. That should reduce pin 5 to about 2.65V.
    Keep in mind that also shortens the timing pulse in case you want to make the change permanent.

    Can the input signal to trigger pin have a high input voltage high? That would also fix the application.
  • 0 in reply to Ron Michallick
    Genius 5095 points

    Hi Sir,

    Thank you very much, Q (Pin3) is change to Low status.