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. READ MORE...