Latching D-type CMOS power switch: A “Flip ON Flop OFF” alternative
The venerable Stephen Woodward recently published the design idea (DI) “Flip ON flop OFF” that converts a momentary push button to a classic push-on, push-off switch. Figure 1 is an attempt to go further still in terms of economy.
The circuit shown in Figure 1 utilizes only one half of a dual D-type package and one more capacitor to the original parts count. It also incorporates an RC power on set (or reset), to guarantee the initial state of the switch when power is applied.
Figure 1 U1A debounces SW1 via R1 & C2 so U1A can reliably toggle.
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The initial state of the switch is determined by the Set pin of U1A following the rising voltage on the power input due to the initial discharged state of C1. Capacitor C1 then charges towards ground leaving the flip-flop with the Q output high and the PMOS off.
Alternatively, this RC power on Set circuit can be wired to the Reset pin to change the initial power on state of the switch. The device ESD clamping diodes provide the capacitor discharge path when power is turned off.
The D-type flip-flop is essentially connected in the familiar way of Q-bar to D-input to form a bistable with each clock rising edge toggling the output state. However, in this case the combination of R1 and C2 form a delay network which prevents rapid changes on the D-input, thus effectively de-bouncing the switch by inhibiting state changes until C2 has charged/discharged to the state on the Q-bar output.
—Chris Nother built a discrete Tx/Rx for model aircraft at an early age, later discovering the dreaded “Mains Hum” in a home built “Dinsdale” Hi-Fi amplifier. Employed in R&D using the then newly available available CMOS logic from Motorola and Nat-Semi, career changes to Mainframe Computers, design of disk drive automated test equipment and storage solutions, finally turning full circle in retirement to the hobby that started it all.
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