This is a touch-triggered switching circuit designed to generate a pulse for use by other circuits.  The main component of this circuit is the 555, a versatile timer IC.  It is configured as a monostable multivibrator, a circuit which will output a single pulse if triggered.  It will remain dormant until it is triggered once again to output a single 'shot' of pulse.  For this reason, it is also known as a 'one-shot'.

The input pin of the 555, pin 2, is very sensitive.  Touching pin 2 of the above circuit with a finger is enough to trigger an internal flip-flop to output a 'high' level at pin 3 and initiate the charging up of C1.  Once C1 reaches a certain voltage, the internal flip-flop is triggered once again to change state, pulling down the output to 'low' once again.  The entire cycle basically produces a pulse at pin 3.    

Learn more about the 555 Timer IC and its applications...

See also:  555 Monostable Multivibrator; Flip-flops

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