The diagram below shows how a piezo transducer can be used to trigger a thyristor. Piezo transducers can be used to trigger common circuits such triggering 555 monostable circuits instead of thyristor triggering.
And here are the tips for the thyristor:
1. When a voltage is applied to the gate of the thyristor it turns on.
2. When the voltage to the gate is removed the thyristor will conduct until the power is removed by shorting between the anode and cathode
