voltage controlled monostable

The 74121 IC is a monostable multivibrator, the pulse length of which may be varied from 40 ns to 40 s. An external RC-network determines the duration of the pulse and if the fixed resistor in this network is replaced by a potentiometer (from pin 11 to supply voltage) then the pulse length of the output may be adjusted accordingly. However the diagram shows how it may be used as a voltage-controlled monostable. The external resistor is replaced by transistor T 1