not advisable to keep trigger pin open(pin 2) .
connect a 10k resistor to supply Vcc and pin 2 . then pin2 with a push button switch to ground.
otherwise the behaviour will be random.
Bear in mind that floating inputs also present a risk of damage. In simple terms, that static charge you used to trigger the timer could be big enough to damage it. When you touch a conductive surface or IC pin, the charge between you and that object balances out, you become joined and therefore at or very near the same potential. That doesn't necessarily mean you are both at ground potential, just that there is little difference between you. The rate of flow of charge as you equalize depends on how conductive the path is at the point of contact. In a bipolar IC like the 555 the path is quite conductive and within reasonable limits, the charge can safely pass to the device but many circuits use MOS inputs which are far more fragile. They are not conductive and work on a voltage sensing principle, because they can't easily equalize the charge, there is a huge risk of the voltage differences being higher than they can tolerate. The damage to them is instantaneous and permanent. To give you some idea of the scale of risk, on a dry day with low humidity if you walk to your car in synthetic soled shoes, you could easily be charged to ten thousand volts or more, it's what gives you a tiny shock as you touch the car door. The maximum voltage most modern MOS devices can resist before damage is about 3.5V!
Brian.
In a way you are right but the risk is that your connection isn't to ground first then the trigger pin second. If you touch trigger first you discharge into it and potentially cause damage before completing the ground connection. As power isn't a real problem here, maybe something along the lines of an IR LED and a photosensor instead of a push button may work. So you still have to place your finger on a target but in doing so it breaks the light beam.
As far as using a high value resistor as a pull-up, it will work and the higher than value the less resistance to ground is needed to operate the trigger but the risk of not discharging safely also increases. As the 555 is a bipolar device and therefore current operated, there will be an upper limit to the value before it ceases to work anyway and I would guess is is probably less than 1M Ohms.
Brian.
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