A signal gate you can measure
Use one bilateral switch to connect and disconnect a small DC signal.
Difficulty: Breadboard. Estimated build time: about 30 minutes. 6-line bill of materials. Compare supplier offers when available. A CD4066BE contains four independently controlled analog switches, not a multiplexer.
Prepare the bench
Use an isolated, regulated 5 V source with a 20 mA current limit and a digital voltmeter with approximately 10 MΩ input resistance. These are bench tools, not included in the shopping list. Disconnect power before changing connections. Confirm breadboard rail continuity and check for supply shorts before powering up.
Power the 14-pin package
Use CD4066BE PDIP-14, top view. Pin 14 goes to +5 V, pin 7 to ground, with 100 nF between them close to the IC. Tie unused controls 5, 6 and 12 to ground. Leave unused signal pins 3, 4, 8, 9, 10 and 11 unconnected.
Wire one channel
Make Vin with 10 kΩ from +5 V to Vin and 10 kΩ from Vin to ground. Connect Vin to signal pin 1 and a 100 kΩ load from signal pin 2 to ground. Add 10 kΩ from control pin 13 to ground. A jumper from pin 13 to +5 V turns the switch on; removing it turns the switch off.
Measure both states
Measure pin 2 relative to ground. Off should be near 0 V because of the load resistor. On should be somewhat below 2.5 V due to divider loading and switch on-resistance. Keep all signals between ground and +5 V. This is not an isolated relay or a motor switch.