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The monostable has only
one permanent stable
state.
When triggered by an
external pulse, it
changes over to an
unstable state for a
time determined by a CR
time constant.
It then reverts to its
stable state and waits
for another trigger
pulse.
OPERATION
At switch on, Tr1 is
forward biased by R3.
This turns Tr1 hard on,
giving it a high
collector current and a
low collector voltage.
This low collector
voltage is cross
connected to the base of
Tr2, turning Tr2 off.
This is the stable
state.
A negative pulse to the
base of Tr1 turns Tr1
off.
The collector voltage of
Tr1 goes high and turns
Tr2 on.
The circuit is now in
the unstable state.
C1 now charges from the
supply rail via R3.
Eventually the voltage
on the left hand side of
C1 will be high enough
to turn Tr1 back on,
which in turn switches
Tr2 off.
The circuit is now back
in its stable state.
The monostable can be
used as a short duration
timer or a pulse width
stretcher.
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