Touch switch with a 555 timer has been built long before. The basic idea is the 555 timer should be configured in monostable mode and the TRIGGER terminal of the 555 timer is left open as TOUCH lead. The discrepancy in using the design is switching both ON and OFF intentionally i.e. once the lead is touched, the output of the timer will be ON for some calculated time and then it will go OFF automatically. Still this type of design finds application in many places. But to switch ONOFF high voltage electrical equipments, the 555 timer has been specially designed in this project.
The 555 timer here also has been used as a monostable multivibrator but with less delay in quasi-stable state. Quasi-stable state is a temporary state of the output of a monostable multivibrator where the output is ON. The output terminal is connected to a D-Flip Flop to change its state each time the lead is touched at the TRIGGER terminal of the 555 timer. The output of the Flip Flop is used to drive a RELAY Switch for high voltage applications.
The IC used here for a D flip flop is 7474, which has two D flip flop inside it. Each flip flop has one data input terminal, one SET and one RESET terminal, one clock pulse terminal, one TRUE output and one complementary output terminal. One of the two flip flops has been used here. SET and RESET terminals are active low, so they should be connected to +5V as we don’t need them. The complementary output is connected to the data input terminal. The output of the 555 timer is connected to the clock pulse terminal of the D flip flop. And the TRUE output is used to drive the relay.
1. One 555 timer
2. D Flip Flop IC 7474
5. Diode (1N4007)
6. Resistors (10K , 2.2K , 4.7K )
7. Capacitors (22 uF , 1nF )
5V regulated DC supply for the ICs and a 12V or 5V for the RELAY according to its need.
Project Source Code
Filed Under: Electronic Projects