This Design Idea shows how you can use a single-pole 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. As 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 15 µA 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 IC_1B always sets the CH₁ output to a logic-low level.

When the outputs of IC₃, pins 2 and 14, both go to logic zeros, the feedback chain comprising IC_2A, IC_2B, IC_2C, and IC_4A pulls Q₁'s base to a logic-high level, which in turn pulls one input of IC_1B 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, IC_1A, and normally open momentary-contact switch S₁constitute 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, IC_4B decodes the counter's 01 state and pulls CH₁ low, IC_4C decodes the counter's 10 state and pulls CH₂low, and IC_4D 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 V_CC as appropriate.