Connect two buttons with just two wires
Use diodes to steer current in the right direction.
Fikret Yilmaz, Mobil Elektronik, Istanbul, Turkey; Edited by Martin Rowe and Fran Granville -- EDN, September 17, 2009
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.
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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.
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The EN35 Optocouplers have an absolute maximum reverse voltage rating on the LED of 6 volts. To prevent damage to the IC the input LED diodes should be protected by an external diode that will conduct during the half cycle of the feed voltage that applies a reverse voltage to the LED.
Dick Schumaker - 2009-13-11 13:33:00 PST
