555 timer operation
in relation to the monostable and astable operation.
Principle of working of a 555 timer.
1 The output (pin 3) goes high (near Vcc) when
the 555 receives a trigger input
In the monostable this occurs when the switch is thrown, in the astable
when the power is turned on and the capacitor is initially discharged
2 It stays there until a threshold reaches
2/3 rd of supply
This is determined by the capacitor and the resistor(s) in both the
astable and monostable shown above
Note the threshold is always connected to the capacitor for both circuits
The time this takes to occur in the monostable is determined by
R1 and C1
In the astable, capacitor C1 charges, resulting in a voltage drop across
R1 and R2.
When the voltage at the threshold reaches 2/3 of the supply(Vcc) a
transistor inside the 555 turns on and discharges the capacitor.
The capacitor discharges to 1/3 rd the Vcc which in turn then triggers
Not that it is required at this level - but if you at the lest interested
it what is happening inside the 555 then click here
else not then continue
3 at this time the input goes low (near ground)