What “The touch plate” is?

The touch plate is similar to a variable resistor. In other words it is a resistor in which the resistance can be altered. 

The resistance of the touch pad is very high when not touched and its resistance reduces when touched. The harder you press, the lower the resistance between the terminals. 
When used in the siren circuit, the lower resistance produces a high tone and when used in the LED flasher circuit, the LED will flash faster when you press hard on the plate.

There are a number of different types of TOUCH PLATES and different effects can be created by the circuit :

1. Touch a set of pads and the project turns on. When the finger is removed, the circuit turns off. The finger can touch the pads for any length of time. We also include the feature where the circuit extends the ON period, so the circuit stays on for a length of time after the finger is removed. This is shown in Circuits A.

2. Touch a set of pads fairly quickly and the project turns on. Touch the pads again for a short period of time and the circuit turns off. This is called the "Flip-Flop" effect. If the finger is kept on the pads, the circuit will turn on-off-on-off at a rate of about once per second. This is shown in Circuits B.

3. Touch one set of pads to turn the circuit on and another set of pads to turn the circuit off. 
This is shown in Circuits C. 

Although we have 3 different categories of touch switch circuits, but also we proceed Circuits type A in this post (3xCircuits) and proceed the other type of this circuit soon if the God wants.

Circuits type A 

Here are a number of circuits that turn on a device when the touch-pad is touched.

First circuit: The circuit above is the simplest Touch Switch. It is called a "super-Alpha pair" and is actually identical to a single transistor with a very high gain. 
Putting a finger on the touch pads turns the top transistor ON and this transistor turns on the bottom transistor. When the finger is removed, the circuit consumes less than a micro amper.

Second Circuit: The 555 can be used to create a Touch Switch. The only problem with this is the 555 consumes about 8mA, at all times when the supply is connected. The circuit above turns on the LED when the finger is applied and pin t becomes "open circuit." This allows the 10u to charge via the 100k resistor and when pin 6 detects a HIGH, the LED turns off. The finger should be removed before this occurs. See below for an ON-OFF touch switch without using a 555.

Third Circuit: The Touch Switch circuit above is a very complex design to do a simple task. It is also a very poor design as the biasing (turn-on) for the output transistor is via a resistor and the output transistor is turned off by taking the biasing current to the 0v rail.  This is a wasteful design if the circuit is to be powered by a battery.

A project with Touch Plate fitted to PCB.

source: talkingelectronics.com