TIMER CIRCUITS

The 555 Timer
The 555 timer is a versatile and widely used device because it can be configured in two different modes either as a monostable multivibrator (one-shot) or as an astable multivibrator (oscillator). An astable multivibrator has no stable states and therefore changes back and forth (oscillates) between two unstable states without any external triggering.
A functional diagram showing the internal components of a 555 timer is given in Fig. (1).
The comparators are device whose outputs are HIGH when the voltage on the positive (+) input is greater than the voltage on the negative (-) input and LOW when the (-) input voltage is greater than the (+) input voltage. The voltage divider consisting of three (5-k?) resistors provides a trigger level of VCC 1 3 V and a threshold level of CC 2 3 V . The control voltage input (pin 5) can
be used to externally adjust the trigger and threshold levels to other values if
necessary. When the normally HIGH trigger input momentarily goes
below CC 1 3 V , the output of comparator B switches from LOW to HIGH and
set the S-R latch, causing the output (pin 3) to go HIGH and turning the
discharge transistor Q1 off. The output will stay HIGH until the normally LOW
threshold input goes above CC 2 3 V and causes the output of comparator A to
switch from LOW to HIGH. This resets the latch, causing the output to go back
LOW and turning the discharge transistor on. The external reset input can be
used to reset the latch independent of the threshold circuit. The trigger and
threshold input (pin 2 and 6) are controlled by external components connected to
produce either monostable or astable action.