This circuit is based on the 555 timer. It is used to detect whether someone is touching the tip of resistor R2 or is coming close to it. R2 can be connected to anything metal, for example a small piece of copper or tin foil, etc. When someone's hand comes close to this piece of metal or touches it, it will cause the 555 to trigger on pin 2 which will cause it to output a high on pin 3 and turn on the LED. Not only will it turn on the LED but it will also cause the UJT to oscillate and output an alarm sound through the speaker. The tone of the sound is controlled by the combination values of R4 and C3. The UJT Q1 is a 2N2646 but any small signal UJT will work fine.From : http://home.maine.rr.com/randylinscott/sep21.htm