IC provides versatile toggle functions
Using Schmitt-trigger inverters as flip-flops, you can realize a number of novel toggle functions.
Louis Vlemincq, Belgacom, Evere, Belgium; Edited by Martin Rowe and Fran Granville -- EDN, August 21, 2008
The circuit in Figure 1 offers not only as many as six channels in a single IC package, but also a high level of additional flexibility. The configuration of Output 1 is a “plain-vanilla” toggle. A resistive divider comprising R1 and R2 provides a midsupply bias to all the channels through resistors R3, R6, R7, R10, and R12. Because the bias voltage of R1/R2 is within the hysteresis range of the gates, they behave as flip-flops, retaining their high or low state in a stable manner.
Debouncing capacitors C2, C3, C4, and C5 charge to the level of the output. Pushing switch S1 inverts the output state because of the inverting action of the gate. This state remains stable because, in the first gate’s circuit, for example, R4’s value is larger than that of R3, and R4 cannot overcome the hysteresis threshold of the gate. Only the discharge of C2 can accomplish that task. When you release the pushbutton, C2 fully charges after the debouncing delay, and the circuit is ready for another inversion. C1 provides a general power-on-reset feature to all the channels. If your circuit requires only one channel, you can directly connect R1 and R2 to the input of the gate, omitting R3.
Output 2 has the same toggle function as Output 1 but also includes a direct reset. Output 3 works only in a set/reset mode; the position of R8 determines the priority state. Output 4 also has a toggle action, but you can set or reset it to a state opposite that of Output 3. Output 5 works in a similar manner, except it allows only a conditional reset because of the position of D1. Output 5 also includes a forced, nonpriority set. You can mix and match all these functions, providing almost unlimited versatility.
The IC in Figure 1 is a Fairchild Semiconductor CD4000-series circuit, suitable for supplies of 3 to 15V, but it could also be a 74AC14 or 74HC14 from NXP, for example. Any CMOS-input gate having a Schmitt-trigger action is suitable. You must take care to bias the inputs in the middle of their hysteresis range. HCMOS circuits would require an average bias of approximately 1.2V for a 5V supply, for example.
