I am new to wsn development. i am very much interested in learning how to build wireless sensor networks.
i would like to develop a very simple system which is defined below
1. The system has a sensor unit that must be able to communicate with a Host PC
Should be no problem, most arduinos have an USB-port to communicate with.
2. The sensor unit must be attached with LED.
Most arduinos already have a "test"-Led onboard attached to pin 13, you could use that, but with a current limiting resistor you could drive a led using any output-pin. If you would like the led to glow
at different light-levels instead of plain on/off the so called PWM-pins of arduino are interesting.
3. when a hand is placed on sensor, LED shud glow
Easiest way to do that is using an Light Depending Resistor or LDR. When dark it has its highest resistance and the more light shines on it, the lower its resistance is. Used with another "fixed"resistor one can create a voltage divider. Measured by one of the analog inputpins of the arduino these two will tell you how much light is shining on the LDR.
In it's simplest form you would require an Arduino, an LDR and a fixed resistor.
Driving a Led can be done several ways, the Blink, blink without delay or Fade-example will show you how.
Unfortunately I couldn't find an example of working with LDR on Arduinos-site fast, but Ladyada has
a page with good info.http://www.ladyada.net/learn/sensors/cds.html