This circuit is a bistable multivibrator, or flip-flop. Click the "set" input in the lower left corner to set the output high (5 V). Click the "reset" input in the lower right corner to set 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. Because Q1 is off, no current flows through it, and the voltage at its collector is close to 5 V. This allows current to flow through the base of Q2, which keeps Q2 on. Q2 is in saturation, keeping the collector voltage close to ground; this prevents current from flowing into the base of Q1 to turn it on. If you momentarily click the "set" input, it supplies base current to Q1, turning it on and bringing its collector low, which stops base current from flowing to Q2. Thus the circuit switches to the opposite state. Clicking the "reset" input switches it back.
This page is a utility for simulating бистабильный мультивибратор (триггер) online with specified initial values.
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