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NE556: 556 timer not working; but works correctly when powered separately

PCBenthusiast
Prodigy 50 points
Part Number: NE556
Other Parts Discussed in Thread: NE555

Attached is a circuit with 556 chip (i believe NE, SA, NA all should work). I have this circuit in a breadboard as well as on a PCB. The timer 1 should provide a one sot 8 second timer and timer 2 should provide a 0.5s ON and 0.5s OFF timer. The circuit as is where OUT1 is connected to RESET2 should provide a timer of 0.5s ON and 0.5s OFF for a total of 8 seconds INSTEAD i'm only getting a 1 seconds ON of LED and then it stays turned off. I checked this setup with 2 555's (connected their OUT to RESET as well) and individually tested the 2 timers inside 556, they work properly! when i connect them together, they just give an output of 1 second ON. Any thoughts?

  • 0
    TI__Guru**** 164616 points
    Hello Aarthi,

    It should work. I calaucate one shot at 6.8 seconds, but could be 8 seconds if C3 is on the high side of it's tolerance.
    1st thing to check is 14V to make sure it is clean. Then check 1OUT/2RESET to see if that is 6 to 8 seconds high pulse then low.
    If it is good debug the right side. if it is bad debug left side. 1THRESH can be watched with a DMM or a real time oscilloscope and 2THRES would be better watched on an oscilloscope. Because large caps 1uF and 10uF are being discharged quickly make sure ground side of U1, C3, and C2 stays at 0V.
  • 0 in reply to Ron Michallick
    Prodigy 50 points
    Well, the left side is not staying high for 6-8 seconds. it is only staying high for 1 second. I have been using a 12V portable battery, do you think i should use a power suppky instead? How would i make sure the grounds are at 0V?
  • 0 in reply to PCBenthusiast
    TI__Guru**** 164616 points
    Aarthi,

    Try adding a VCC to ground capacitor close to pin 7 and 14, 100nF is good. The NE555 and NE556 have a lot of shoot through current when switching so a bypass capacitor is mandatory.

    For a ground checks, use a oscilloscope with ground lead at pin 7 of timer. then use the probe on the other ground nodes and look for voltage spikes.
  • 0 in reply to Ron Michallick
    TI__Guru**** 164616 points
    Aarthi,

    Were you able to get the desired output?
  • 0 in reply to Ron Michallick
    Prodigy 50 points
    Ronald,

    sorry for the late response. I was travelling for work and now back to my desk. adding a capacitor from v+ to GND along with reducing the R4 resistor to 1kohm from 20kohm helped. the addition of cap along did not help however. thanks for the advise!