This Design Idea shows how you can use a singlepole momentary-contact switch to select one of three signal sources by scrolling through three output states.
The circuit in Figure 1 comprises commonly available components from the CD4000 CMOS-logic series, along with a general-purpose NPN transistor. The total cost of the components doesn’t exceed $1. Only one of circuit’s three outputs, CH1, CH2, or CH3, goes low at any given time, and you can use these outputs to control analog switches, relays, or the gates of JFET switches.
sAs long as you apply power, the selected output does not change, making the circuit a good choice for applications requiring nonvolatile operation. Quiescent-current consumption averages only about 15μA at room temperature, 25°C, a low value even for battery-powered applications.
The heart of the circuit comprises a dual JK flip-flop, IC3, that’s configured as a 2-bit ripple counter. Without additional circuitry, the counter would allow selection of four signal sources. Upon initial application of power, a reset circuit comprising R1, C1, and IC1B always sets the CH1 output to a logic-low level.
When the Q outputs of IC3, pins 2 and 14, both go to logic zeros, the feedback chain comprising IC2A, IC2B, IC2C, and IC4A pulls Q1’s base to a logic-high level, which in turn pulls one input of IC1B to a logic low. This action causes the counter to skip the 00 state and advances the count to the 01 state. Components R5, C2, IC1A, and normally open momentary contact switch S1 constitute a debounced switch that provides clock pulses for both sections of the counter, IC3. When a user pushes S1, the counter advances to the 10 state, and a subsequent push advances the counter to the 11 state. A third push restarts the cycle.
To summarize, IC4B decodes the counter’s 01 state and pulls CH1 low, IC4C decodes the counter’s 10 state and pulls CH2 low, and IC4D decodes the counter’s 11 state and pulls CH3 low. The layout of the circuit should be noncritical, but use a low-leakage capacitor for C1. Connect unused logic inputs to ground or VCC as appropriate.
