Make a one-way switch with a BJT or relay?
You can't create a latching circuit with only one transistor. You can make one with a thyristor and you can make a thyristor with a PNP and NPN transistor.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. (a) A latching circuit using a thyristor. (b) The same circuit made of transistors.

On the second circuit when ON2 is pressed Q2 receives some base current via R4 and this turns on Q2. Q1 now turns on as base current is drawn through the emitter and out through the base into Q2. Q1 then maintains Q2's base current and so the circuit latches.