Transistor as a one-time switch
Simplest circuit is by using a SCR (Silicon Controlled Rectifier), these are pretty common components, but not as common as transistors.







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To sound the alarm, it is enough to disconnect the gate from ground using the TRIGGER switch. A small current will flow through R1, to the gate and latch the SCR.
Once an SCR is latched, it can only be reset by interrupting the current through it (by pressing the RESET switch).

You can replace a SCR with two BJT's:







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