## Switch that Turn On the Circuit in One Press and Turn off in Next Press

by realfinetime  |  in J K Flip Flop at  04:12

We have already seen Animated Tutorial of 7473, Dual Master-Slave J-K Flip-Flops with Complementary Outputs. In this blog, we will design a push button switch that will turn on the circuit in one press and turn off the circuit in next push.

A push button switch that turn on the circuit in one push and turn off the circuit in next push is an interesting project. Various methods are there to do the same. This project is done by a J K Flip-Flop IC, 7473. Circuit is done as shown in the following circuit diagram.

Input pins ( J, K, CLK and CLR ) of the flip-flop is connected to 5V through pull up resistors. A push button switch is connected in between the CLK and ground. This switch helps to shift the CLK between HIGH and LOW voltages. When the switch is OFF, CLK will get HIGH voltage. Similarly, if the switch is ON, CLK will get LOW voltage. 5V is given from a 5V Voltage regulator. Output ( Q1 ) is connected to an LED through current limiting resistors.

Pinout diagram of 7473

Pinout diagram of 7473 is given below. Each 7473 has two master-slave J K flip-flips. Half portion of IC, above VCC and ground constitutes the first flip-flop and the half portion below VCC and Ground constitutes the second master-slave flip-flop. First flip-flop is used for this circuit.

Truth Table of 7473

From the truth table, we can conclude that:

1. When CLR is LOW, Q will be LOW and Qbar will be HIGH. These outputs will not change, even if the other inputs ( J, K and CLK)  change, until CLR is set to HIGH.
2. When CLR becomes HIGH, if J is LOW and K is HIGH, Q becomes LOW and Qbar becomes HIGH during the next negative transition.
3. When CLR becomes HIGH, if J is HIGH and K is LOW, Q becomes HIGH and Qbar becomes LOW during the next negative transition.
4. When CLR becomes HIGH, if J and K are HIGH, Q and Qbar will toggle in each negative transitions.
5. When CLR becomes HIGH, if J and K are LOW, Q and Qbar will remain in the previous state and is independant of the state of CLK input.

Why pull up resistors ?.

5V is given to J, K, CLK and CLR through pull up resistors. Pull up resistors helps to pull the inputs ( J, K, CLK and CLR )  to a HIGH value always. When the push button switch turns on, CLK input get grounded through the push button switch. Then, CLK will get LOW voltage.

If the same circuit is done without connecting the pull up resistors of proper resistance, when the swith turns on, power supply get shorted through the switch. High current will flow through the switch due to low resistance of the path. This may damage the power supply and gate operation will not take place.

Why push button switch ?.

Push button switch is connected in between the CLK and ground as shown in the circuit. Push button switch is most suitable for this purpose, because by default it will be in off state. A switch in off state means, CLK gets a HIGH voltage. When the switch is pressed, CLK get grounded and will get a LOW voltage. When press is released, switch will automatically turn off and CLK will get HIGH voltage.

Working of Circuit

Row encircled by a red line in the truth table is used for this circuit. When CLR, J and K becomes HIGH, output will toggle in each negative transitions of the CLK input. Negative transition will occur when the push button switch is pressed. If the switch is pressed, CLK get grounded. Then a transition of CLK input from HIGH to LOW will occur. At this moment, if the output ( Q1 ) is HIGH, it will become LOW. Similarly, if the output ( Q1 ) is LOW, it will become HIGH. If the switch is released, CLK input will change from LOW to HIGH voltage. No change will happen to the output during the positive transition of CLK input.