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This is a type of
bistable multivibrator,
with two stable states.
Revising transistor
behaviour :-
When a transistor base
is connected to zero
volts, it is cut off and
its collector voltage
rises to the supply
voltage. See red figures
for TR1, and blue for
TR2.
When the base is joined
to the supply voltage
via a low value
resistor, the transistor
is turned on, and
becomes saturated. Its
collector volts drops to
a very low value.
See the blue values for
TR1 and red for TR2.
If SET is taken low
(zero volts) thenTR1 is
turned off, and its
collector voltage turns
TR2 on.
Q is high and Q bar is
low.
If SET is now removed
from the zero volt rail,
the two transistors will
remain in this state.
To make the flip-flop
change state, RESET must
now be taken low.
Tr2 will now be off and
Tr1 on.
Q will be low and Q bar
high.
The flip-flop will rest
in this state until SET
is again taken low
again.
This circuit is known as
a SR flip-flop.
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