Latch Switch

  
In this circuit a non-locking push switch is used to activate a load. The load remains switched on until power is removed from the circuit. The load is represented by R5 and D1, but could be a lamp, a relay or another circuit. S2 breaks power to the circuit but could be omitted altogether. If S2 is left out, then reset would be by disconnecting the power; this would mean unplugging the battery if battery powered or disconnecting from the electrical outlet.
Latch Switch - schematic

When first plugged in (or S2 is operated) C1 charges via the base emitter junction of Q1 and hence a brief positive pulse is applied. Q1 will switch on and be saturated, its collector emitter voltage being close to zero volts. Q2 is therefore off, and the



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