Bistable mode or Schmitt trigger :
A Bistable Mode or what is sometimes called a Schmitt Trigger, has two stable states, high and low. So in bistable mode 555 can operate as a flip-flop. Taking the Trigger input low makes the output of the circuit go into the high state. Taking the Reset input low makes the output of the circuit go into the low state. Remember that the discharge pin (pin 7) is not connected and no capacitor is used. Uses include bounce-free latched switches.
WORKING OF BISTABLE 555
In bistable mode, the 555 timer acts as a basic flip-flop. The trigger and reset inputs (pins 2 and 4 respectively on a 555) are held high via Pull-up resistors while the threshold input (pin 6) is simply grounded. Thus configured, pulling the trigger momentarily to ground acts as a ‘set’ and transitions the output pin (pin 3) to Vcc (high state). Pulling the reset input to ground acts as a ‘reset’ and transitions the output pin to ground (low state). No capacitors are required in a bistable configuration. Pin 5 (control) is connected to ground via a small-value capacitor (10nF) pin 7 (discharge) is left floating.
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