Thyristor

Transistors turn on and off as the voltage at the base rises to a level enough to trigger the component. This can be useful if you wanted a light to turn on and off as the light levels rose and fell but what if you wanted an alarm that required to be reset once it had gone off? A transistor in a circuit for a bag alarm wouldn’t be much good since as soon as the stimulus (perhaps the bag being moved ) was removed, then the alarm would stop sounding. A thyristor operates in a similar way to a transistor but stays ‘latched on’ once the stimulus – the trigger voltage, has gone away. Now we can have an alarm or a ‘steady-hand’ game that will give a constant output when the circuit is triggered. To turn it off would need the power source to be disconnected or at least shorted out momentarily.