This circuit is a bistable multivibrator, or flip-flop. Click on the "set" input at lower left to bring the output high (5V). Click on the "reset" input at lower right to bring the output low (ground).

The transistors are cross-coupled in such a way that the circuit has two stable states. Initially, Q2 is on and Q1 is off. Since Q1 is off, no current is flowing through it, and its collector voltage is close to 5V. This allows current to flow through into the base of Q2, which keeps Q2 switched on. Q2 is in saturation mode, keeping the collector voltage close to ground; this prevents any current from flowing into the base of Q1 to switch it on.

If you click the "set" input momentarily, this provides base current to Q1, switching it on, bringing its collector low, which stops the base current flowing to Q2. So the circuit switches to the opposite state. Clicking the "reset" input switches back.

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Generated Wed Dec 7 2016