Discrete 0.5-300 ms Monostable Multivibrator

Monostable multivibrator circuit produce a fixed pulse-width when the input is triggered.  The input is fed to CR1, which should be an open collector source since the TR1 would short the input to ground during the active state  appear on this circuit’s output.  The output of this circuit is tapped from the collector of TR2. This monostable multivibrator output pulse width can be varied from 0.5 to 300 milliseconds, adjusted by a 1M trimmer potentiometer R5.  Here is   the schematic diagram of the circuit: