Connect the circuit as per the circuit diagram and apply the 5 volt . To turn “ON” the device, touch the “ON” plate with your finger and to turn OFF the device , touch the OFF plate.
When power supply is applied to circuit, the device connected through the relay (we have connected a light bulb) remains OFF. Now, if we observe the circuit diagram, Pin 2 is pulled HIGH and Pin 6 is Pulled LOW.
When we touch the ON plate, voltage at Pin 2 (Trigger Pin) of the 555 IC becomes LOW. As Pin 6 is already LOW, the output at Pin 3 becomes HIGH. Since this is connected to the Relay Module through the Transistor, the Transistor will be turned ON and it in turn activates the Relay. As a result, the device gets switched ON.
At this point voltage at pin 6 is zero as it pulled LOW by default and voltage at Pin 2 is HIGH. Now, when you touch the OFF plate, the Pin 6 is supplied with +5V for a brief time and as a result, the output of the 555 Timer IC will become LOW. This will turn off the transistor and also the relay. Hence, the device will be switched OFF.