Sometimes, you need to read the status of pushbuttons that are as much as 5m away from your electronic circuit. That task is easy if you have just one button. You simply design a constant-current source, connect the current line from your button, and measure the current in the line. If you press the button, current flows through it. Otherwise, current does not flow.

Problems occur, however, when you need to read two or more buttons. Several approaches to this problem are available. For example, you could use an RS-485 interface with two wires for communication and two for power. Alternatively, you could use a single-wire connection with one wire for communication and two for power. Another option is to use separate wires for each button. In that case, you would use one more wire than there are buttons. Finally, you could use a POE (power-over-Ethernet) approach, employing four wires for communication and power. All of these approaches require a button reader or a controller, which you must program, adding complexity and cost.

The circuit in Figure 1 shows you how to connect two buttons using diodes. Because the diodes steer the current, the circuit needs just two wires. On a positive cycle from the transformer secondary and with switch S₂ closed, current flows through IC₁, R₂, and D₂. Thus, output pulls low. Conversely, if S₁ closes on a negative cycle, then current flows through D₁ to R₁ and IC₂, which pulls low. The circuit in Figure 2 extends the concept to four pushbutton switches by adding a third wire.