Garage door opener puzzle

I

isw

Guest
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
 
isw <isw@witzend.com> 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?
Click to expand...

it's a pulsed waveform that you can't measure with a regular DVM maybe?
 
In article <nnqsgg$43t$1@usenet.itgate.net>, frank <frank@invalid.net>
wrote:

isw <isw@witzend.com> 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?

it's a pulsed waveform that you can't measure with a regular DVM maybe?
Click to expand...

Hmm. Could be. Time to limber up the ol' wiggle tube.

Isaac
 
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
Click to expand...


Reconnect the LED/resistor and remeasure.
 
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