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This circuit uses a 555 timer as the bases of the touch switch. You can learn more about 555 timers in the Learning section on my site. When the plate is touched the 555 timer is triggered and the output on pin 3 goes high turning on the LED and the buzzer for a certain period of time.

The time that the LED and the buzzer is on is based on the values of the capacitor and resistor connected to pin 6 & 7. The 10M resistor on pin 2 causes the the circuit to be very sensitive to the touch.