Touch switch using op amplifier
Here's a touch switch: you can change the status of the circuit by touching the terminal.
Here the operational amplifier acts as an amplifier and a one-shot multivibrator. You can turn ON by a single touch but it is automatically turned OFF about 1 - 2 seconds later.
U1(a) is an amplifier that amplifies the voltage induced by a touch of your finger. The output from this amplifier is rectified by D1 and D2 to produce a trigger pulse. The pulse is then sent to the one-shot multivibrator.
U1(b), as the one-shot multivibrator, works as an electronic switch. When it receives an input, the output stays at low level for 1 - 2 seconds, then lights up the LED again.
Turn power ON and touch the wire J1-J2. If this touch switch doesn't work as explained above, your finger tip might be too dry. See if it works well when you touch it after wetting your finger tip.