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SA556: Pin 3 not going LOW after 8 seconds

PCBenthusiast
Prodigy 50 points

Other Parts Discussed in Thread: NE555, SA556

I am working on a new project which requires an LED light to turn ON for 8 seconds and turn off automatically after 8 seconds. This is for a seat belt minder. During those 8 seconds, flashing LED every 0.5s would also be nice to have, but optional per the safety standard. So I tried making a circuit using the NE555 as attached to e generate a single shot 8s timer.  The model I have should theoretically produce 8s ON (no LED blinking) but the output waveforms don’t represent that.  Something is wrong in the circuit. Any suggestions on why i'm not seeing an output that stays ON for 8s as soon as powered and then should turn off itself even though the power to the IC remains. Per the waveforms, the output stays ON until the power is removed (not just 8s).

I understand that i have to use 556 dual timer if i wanted to do the LED blinking functionality as well. but for now, i just want to troubleshoot this one shot 8s timer.

  • 0
    Prodigy 50 points
    The part # in question is NE555 not SA556. i dont see an option ti edit the post after posting it!. also i am still in the TINA spice world, have not prototyped this circuit yet.
  • 0
    TI__Guru**** 174306 points
    Aarthi,

    In the first circuit , the time is 1.1 * 10k * 10uF = 0.11 seconds. LED1 is backwards and needs a current limiting resistor.
    In the second circuit, it is blurry, so I think time is 1.1 * 73k * 100u = 8 seconds, However the trigger input needs a cap to start low and charge up slower than the CONT pin. You can even make the trigger time 0.1s or 1s. If Vcc start slowly then trigger needs to be slow too.

    Be sure to look at the power up to make sure trigger is working. In the both sims the output is low, it never triggers.
    Can you attach the TI-Tina circuit?
  • 0 in reply to Ron Michallick
    TI__Guru**** 174306 points
    Aarthi,

    If the previous advice was helpful, then please mark the post as answered. If you need further assistance, provide the information requested and I will help you further.