This simple push-button circuit allows a system to turn on with a short button press and turn off when the button is held down. Essentially, the circuit employs D-type flip-flops with asynchronous clear (U2), inverting (U3), and non-inverting (U1) Schmitt-Trigger buffers.
At the initial state, CLR is set to HIGH, U2's clear data input is HIGH and output Q (POWER ENABLE ) is at Q0, which is latched into the last state as Pin D transferred to the output Pin Q is waiting for CLK rising edge.
To set the output Q, POWER ENABLE HIGH, press the button, B1 which forces the PB signal to go LOW and the CLK signal to go HIGH as U3 inverts the logic and causing the CLK rising edge. At the same time, The CLR signal starts to go LOW, but because the R1 resistor and C1 capacitor create an RC delay, which causes the CLR signal to decay slowly. With this short button press, the CLR signal does not go LOW, which allows for the output Q, POWER ENABLE signal to go HIGH with input D is tied to HIGH. The POWER ENABLE signal remains HIGH even if the button is pressed again after the initial press.
The power-off process begins with holding down the button for an extended period of time. The button must be held for longer than the RC delay specified by R1 and C1. When the button is pressed long enough, the CLR signal goes LOW enough to meet the conditions for the asynchronous clear to occur, causing the POWER ENABLE signal to be low.
Furthermore, D1 is present within the RC circuit, allowing the short time between turnoff and turn-on to bypass R1 and rapidly charge C1.
The delay time can be adjusted using different values for C1 as below.
