On Tuesday, August 2, 2016 at 1:52:56 PM UTC-4, isw wrote:
Trying to find out why operation is erratic, I measured the voltage
across the hard-wired pushbutton in the garage.

The button is illuminated by a red LED, which is in series with a 1k
resistor across the switch. The LED is always on. Pressing the switch
shorts the resistor-LED pair and operates the door.

The puzzle is that when I disconnect the LED-switch assembly and measure
the voltage across the wires from the opener unit, I get only a couple
hundred millivolts DC, and less AC.

How is it possible for the LED to light with only 200 mV across it?

Isaac



Reconnect the LED/resistor and remeasure.