The push switch S1 is connected between the capacitor C2 and the resistor R2. Initially, the voltage across C2 will be almost equal to zero because the output is low. The capacitor C2 is connected to the output pin through the resistor R3, C2 charges if the output is high, and remains discharged if it is low. At this voltage level the IC neither trigger nor switches to LOW state, it will remain in the same output state in the initial condition, it remains in the LOW state. When the switch S1 is open both pins will have this Vcc/2 voltage which is a value between the trigger and threshold values. Here the pin2 and 6 are connected together to a resistor voltage divider which gives a voltage of Vcc/2 at both the pins. Once it is triggered the output remains high even the voltage at pin 2 is above Vcc/3, it can be switched to LOW state only by applying a voltage above 2Vcc/3 at the threshold pin 6. The output of the 555 IC switches to high when the voltage at trigger pin 2 is below vcc/3. Here, the circuit is using IC 555 for the push on push off operation. In the circuit, the output is arranged to drive an SPST relay for ON & OFF the loads that need to be connected with an external supply. So, in order to use the toggle circuit with high power loads, it needs to be driven with a transistor. The output current of an IC 4017 has just a few milliamperes. And this shifting repeats for each pulse, thus the circuit obtains an output at pin2 that can repeatedly ON and OFF with each switch press or positive pulse at the pin14. So when the output is shifted to pin4 it instantly resets the IC to the initial state with a HIGH state at pin3 and a LOW state at Pin 2.
Here the output 2 which is at pin4 is connected to the reset pin 15 of the IC. When we press the switch again the HIGH state shifts to output 2 at pin4 and the output at pin 2 switches to a LOW state. Now the High state shifts from output 0 to output 1 and Pin 2 will have a high state at the output. When we press the switch S1 the counter gets a positive edge triggering at the clock input pin 14. In the initial state output 0 is high and output 1 at pin 2 is in a low state. The output for ON/OFF is taken from output 1 at pin2. The IC has 10 outputs, here only two outputs are used to switch ON and OFF by shifting the HIGH state between these two outputs. This circuit is using a decade counter IC 4017, which counts or shifts the output for each rising edge of applied clock signal. A Push On Push off latching switch can use to ON and OFF the load alternatively with the same push action.
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