The LM555 timer and its twin brothers the LM556 are cornerstones of model railroad electronics but the sensitivity of the trigger input gives rise to many false triggering problems. The addition of a 470K ohm resistor and a 0.1uF capacitor at the TRIGGER input (Pin 2) will provide a delay of approximately 1/20th of a second from the time the input goes to zero volts until the trigger threshold of 1/3Vcc is reached. This short delay can eliminate false triggering in most cases and if the problem persists the value of the
SET TIMING CALCULATORS FOR THE LM555
capacitor or resistor can be increased as needed.
The following schematic shows two additions to the basic 555 timer circuit. One reduces the trigger sensitivity and the other will double the output pulse duration without increasing the values of R1 and C1.