The diagram below circuits the piezo transducer which is used to trigger a 555 monostable circuit. Instead of triggering a monostable circuit, a piezo transducer commonly be used to trigger a thyristors.
it is deformed, a piezo transducer will generate a voltage. If the
piezo is tapped it will generate a small voltage, which is enough for
triggering a circuit instead of the uncased piezo (which makes a good