NE555: For the NE555D, when the power turns off, I get an output pulse up to 1.27 volts on the Q line, even though the R line is connected to ground.

Hi Craig, 

I am unsure what you mean by Q and R line. Can you elaborate or provide a schematic?

Best Regards, 

Chris Featherstone

The SW_DISABLE switch is closed (pin 4 (R) is grounded), and when the 5V power is shutoff, pin 3 (Q)  goes to about 1.67 volts for about 122ms?

Hi Craig, 

There isn't a way to prevent this during power down because at a certain bias level below the min supply the timer loses full control over the circuitry and the output will not be controlled. 

Best Regards, 

Chris Featherstone